Philippine Electrical Code Part 1/Chapter 2. Wiring and Protection/Article 2.50 - Grounding And Bonding

Article 2.50 - Grounding And Bonding

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This is not an official copy of the Philippine Electrical Code. This is a Design Guide according to the provisions of the Philippine Electrical Code. It also provides interpretation and annotations of the clauses of the Philippine Electrical Code. Any information, interpretation and/or annotation presented herein are based on personal opinion of the authors and does not represent the official interpretation of the Philippine Electrical Code.

While the authors have used good faith and efforts to ensure that the information and instructions contained in this work are accurate, the authors disclaim all responsibility for errors or omissions, including without limitation responsibility for damages resulting from the use of or reliance on this work. Use of the information and instructions contained in this work is at your own risk. If any contents or other technology this work contains or describes is subject to open source licenses or the intellectual property rights of others, it is your responsibility to ensure that your use thereof complies with such licenses and/or rights

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2.50.1 General

2.50.1.1 Scope.

This article covers general requirements for grounding and bonding of electrical installations, and specific requirements in (1) through (6). (1) Systems, circuits, and equipment required, permitted, or not permitted to be grounded

(2) Circuit conductor to be grounded on grounded systems

(3) Location of grounding connections

(4) Types and sizes of grounding and bonding conductors and electrodes

(5) Methods of grounding and bonding

(6) Conditions under which guards, isolation, or insulation may be substituted for grounding

2.50.1.2 Definitions.

Effective Ground-Fault Current Path. An intentionally constructed, permanent, low-impedance electrically conductive path designed and intended to carry current under ground-fault conditions from the point of a ground fault on a wiring system to the electrical supply source and that facilitates the operation of the overcurrent protective device or ground fault detectors on high-impedance grounded systems.

Ground Fault. An unintentional, electrically conducting connection between an ungrounded conductor of an electrical circuit and the normally non–current-carrying conductors, metallic enclosures, metallic raceways, metallic equipment, or earth.

Ground-Fault Current Path. An electrically conductive path from the point of a ground fault on a wiring system through normally non– current-carrying conductors, equipment, or the earth to the electrical supply source.

FPN: Examples of ground-fault current paths could consist of any combination of equipment grounding conductors, metallic raceways, metallic cable sheaths, electrical equipment, and any other electrically conductive material such as metal water and gas piping, steel framing members, stucco mesh, metal ducting, reinforcing steel, shields of communications cables, and the earth itself.

2.50.1.3 Application of Other Articles.

In other articles applying to particular cases of installation of conductors and equipment, requirements are identified in Table 2.50.1.3 that are in addition to, or modifications of, those of this article.

2.50.1.4 General Requirements for Grounding and Bonding.

The following general requirements identify what grounding and bonding of electrical systems are required to accomplish. The prescriptive methods contained in Article 2.50 shall be followed to comply with the performance requirements of this section.

(a) Grounded Systems.

(1) Electrical System Grounding. Electrical systems that are grounded shall be connected to earth in a manner that will limit the voltage imposed by lightning, line surges, or unintentional contact with higher-voltage lines and that will stabilize the voltage to earth during normal operation.

(2) Grounding of Electrical Equipment. Non–current-carrying conductive materials enclosing electrical conductors or equipment, or forming part of such equipment, shall be connected to earth so as to limit the voltage to ground on these materials.

(3) Bonding of Electrical Equipment. Non–current-carrying conductive materials enclosing electrical conductors or equipment, or forming part of such equipment, shall be connected together and to the electrical supply source in a manner that establishes an effective ground-fault current path.

Conductor/Equipment Article Section
Agricultural buildings 5.47.1.9 and 5.47.1.10
Audio signal processing, amplification, and reproduction equipment 6.40.1.7
Branch circuits 2.10.1.5, 2.10.1.6, 4.6.1.3
Cablebus 3.70.1.9
Cable trays 3.92 3.92.1.3(c), 3.92.1.7
Capacitors 4.60.1.10, 4.60.2.4
Circuits and equipment operating at less than 50 volts 7.20
Closed-loop and programmed power distribution 7.80.1.3
Communications circuits 8.0
Community antenna television and radio distribution systems 8.20.3.1, 8.20.4.1, 8.20.4.4
Conductors for general wiring 3.10
Cranes and hoists 6.10
Electrically driven or controlled irrigation machines 6.75.1.11(c), 6.75.1.12, 6.75.1.13, 6.75.1.14,6.75.1.15
Electric signs and outline lighting 6.0
Electrolytic cells 6.68
Elevators, dumbwaiters, escalators, moving walks, wheelchair lifts, and stairway chair lifts 6.20
Fire alarm systems 7.60.1.9
Fixed electric heating equipment for pipelines and vessels 4.27.4.5, 4.27.6.4
Fixed outdoor electric deicing and snow- melting equipment 4.26.3.8
Flexible cords and cables 4.0.2.3, 4.0.2.4
Floating buildings 5.53.3.1, 5.53.3.3, 5.53.3.4
Grounding-type receptacles, adapters, cord connectors, and attachment plugs 4.6.1.9
Hazardous (classified) locations 5.0–5.17
Health care facilities 5.17
Induction and dielectric heating equipment 6.65
Industrial machinery 6.70
Information technology equipment 6.45.1.15
Intrinsically safe systems 5.4.1.50
Luminaires (lighting fixtures) and lighting equipment 4.10.5.1, 4.10.5.2, 4.10.5.4, 4.10.5.5,4.10.15.6(b)
Luminaires (fixtures), lampholders, and lamps 4.10
Marinas and boatyards 5.55.1.15
Mobile homes and mobile home park 5.50
Motion picture and television studios and similar locations 4.30 5.30.2.10, 5.30.6.4(b)
Motors, motor circuits, and controllers 6.50
Outlet, device, pull, and junction boxes; conduit bodies; and fittings 8.10 3.14.1.4, 3.14.2.11
Over 600 volts, nominal, underground wiring methods 5.51 3.0.2.20(b)
Panelboards 2.30 4.8.3.11
Pipe organs 6.80
Receptacles and cord connectors 4.6.1.3
Recreational vehicles and recreational vehicle parks
Services
Solar photovoltaic systems 6.90.5.1, 6.90.5.2, 6.90.5.3, 6.90.5.5, 6.90.5.7
Swimming pools, fountains, and similar installations
Switchboards and panelboards 4.8.1.3(d)
Switches 4.4.1.12
Theaters, audience areas of motion picture and television studios, and similar locations 5.20.7.1
Transformers and transformer vaults 4.50.1.10
Use and identification of grounded conductors 2.0
X-ray equipment 6.60 5.17.5.8

(4) Bonding of Electrically Conductive Materials and Other Equipment. Electrically conductive materials that are likely to become energized shall be connected together and to the electrical supply source in a manner that establishes an effective ground-fault current path.

(5) Effective Ground-Fault Current Path. Electrical equipment and wiring and other electrically conductive material likely to become energized shall be installed in a manner that creates a permanent, low- impedance circuit facilitating the operation of the overcurrent device or ground detector for high-impedance grounded systems. It shall be capable of safely carrying the maximum ground-fault current likely to be imposed on it from any point on the wiring system where a ground fault may occur to the electrical supply source. The earth shall not be considered as an effective ground-fault current path.

(b) Ungrounded Systems.

(1) Grounding Electrical Equipment. Non–current-carrying conductive materials enclosing electrical conductors or equipment, or forming part of such equipment, shall be connected to earth in a manner that will limit the voltage imposed by lightning or unintentional contact with higher-voltage lines and limit the voltage to ground on these materials.

(2) Bonding of Electrical Equipment. Non–current-carrying conductive materials enclosing electrical conductors or equipment, or forming part of such equipment, shall be connected together and to the supply system grounded equipment in a manner that creates a permanent, low-impedance path for ground-fault current that is capable of carrying the maximum fault current likely to be imposed on it.

(3) Bonding of Electrically Conductive Materials and Other Equipment. Electrically conductive materials that are likely to become energized shall be connected together and to the supply system grounded equipment in a manner that creates a permanent, low- impedance path for ground-fault current that is capable of carrying the maximum fault current likely to be imposed on it.

(4) Path for Fault Current. Electrical equipment, wiring, and other electrically conductive material likely to become energized shall be installed in a manner that creates a permanent, low-impedance circuit from any point on the wiring system to the electrical supply source to facilitate the operation of overcurrent devices should a second fault occur on the wiring system. The earth shall not be considered as an effective fault-current path.

FPN No. 1: A second fault that occurs through the equipment enclosures and bonding is considered a ground fault.

FPN No. 2: See Figure 2.50.1.4 for information on the organization of Article 2.50.

2.50.1.6 Objectionable Current over Grounding Conductors.

(a) Arrangement to Prevent Objectionable Current. The grounding of electrical systems, circuit conductors, surge arresters, and conductive non–current-carrying materials and equipment shall be installed and arranged in a manner that will prevent objectionable current over the grounding conductors or grounding paths.

(b) Alterations to Stop Objectionable Current. If the use of multiple grounding connections results in objectionable current, one or more of the following alterations shall be permitted to be made, provided that the requirements of 2.50.1.4(a)(5) or (b)(4) are met:

(1) Discontinue one or more but not all of such grounding connections.
(2) Change the locations of the grounding connections.
(3) Interrupt the continuity of the conductor or conductive path interconnecting the grounding connections.
(4) Take other suitable remedial and approved action.

(c) Temporary Currents Not Classified as Objectionable Currents. Temporary currents resulting from accidental conditions, such as ground-fault currents, that occur only while the grounding conductors are performing their intended protective functions shall not be classified as objectionable current for the purposes specified in 2.50.1.6(a) and (b).

(d) Limitations to Permissible Alterations. The provisions of this section shall not be considered as permitting electronic equipment from being operated on ac systems or branch circuits that are not grounded as required by this article. Currents that introduce noise or data errors in electronic equipment shall not be considered the objectionable currents addressed in this section.

(e) Isolation of Objectionable Direct-Current Ground Currents. Where isolation of objectionable dc ground currents from cathodic protection systems is required, a listed ac coupling/dc isolating device shall be permitted in the equipment grounding path to provide an effective return path for ac ground-fault current while blocking dc current.

2.50.1.8 Connection of Grounding and Bonding Equipment.

Grounding conductors and bonding jumpers shall be connected by exothermic welding, listed pressure connectors, listed clamps, or other listed means. Connection devices or fittings that depend solely on solder shall not be used. Sheet metal screws shall not be used to connect grounding conductors or connection devices to enclosures.

2.50.1.10 Protection of Ground Clamps and Fittings. =

Ground clamps or other fittings shall be approved for general use without protection or shall be protected from physical damage as indicated in (1) or (2) as follows:

(1) In installations where they are not likely to be damaged
(2) Where enclosed in metal, wood, or equivalent protective covering 2.50.1.12 Clean Surfaces. Nonconductive coatings (such as paint, lacquer, and enamel) on equipment to be grounded shall be removed from threads and other contact surfaces to ensure good electrical continuity or be connected by means of fittings designed so as to make such removal unnecessary.

2.50.2 System Grounding

2.50.2.1 Alternating-Current Systems to Be Grounded.

Alternating-current systems shall be grounded as provided for in 2.50.2.1(a), (b), (c), or (d). Other systems shall be permitted to be grounded. If such systems are grounded, they shall comply with the applicable provisions of this article.

FPN: An example of a system permitted to be grounded is a corner-grounded delta transformer connection. See 2.50.2.7(4) for conductor to be grounded.

(a) Alternating-Current Systems of Less Than 50 Volts. Alternating-current systems of less than 50 volts shall be grounded under any of the following conditions:

(1) Where supplied by transformers, if the transformer supply system exceeds 150 volts to ground
(2) Where supplied by transformers, if the transformer supply system is ungrounded
(3) Where installed as overhead conductors outside of buildings

(b) Alternating-Current Systems of 50 Volts to 1000 Volts.

Alternating-current systems of 50 volts to 1000 volts that supply premises wiring and premises wiring systems shall be grounded under any of the following conditions:

(1) Where the system is 1-phase and can be grounded so that the maximum voltage to ground on the ungrounded conductors does not exceed 250 volts
(2) Where the system is 3-phase, 4-wire, wye connected in which the neutral is used as a circuit conductor
(3) Where the system is 3-phase, 4-wire, delta connected in which the midpoint of one phase winding is used as a circuit conductor

(c) Alternating-Current Systems of 1 kV and Over. Alternating- current systems supplying mobile or portable equipment shall be grounded as specified in 2.50.10.9. Where supplying other than mobile or portable equipment, such systems shall be permitted to be grounded.

(d) Separately Derived Systems. Separately derived systems, as covered in 2.50.2.1(a) or (b), shall be grounded as specified in 2.50.2.11.

FPN No. 1: An alternate ac power source such as an on-site generator is not a separately derived system if the neutral is solidly interconnected to a service- supplied system neutral.

FPN No. 2: For systems that are not separately derived and are not required to be grounded as specified in 2.50.2.11, see 4.45.1.13 for minimum size of conductors that must carry fault current.

(e) Impedance Grounded Neutral Systems. Impedance grounded neutral systems shall be grounded in accordance with 2.50.2.17 or 2.50.10.7.

2.50.2.2 Alternating-Current Systems of 50 Volts to 1000 Volts Not Required to Be Grounded.

The following ac systems of 50 volts to 1000 volts shall be permitted to be grounded but shall not be required to be grounded:

(1) Electric systems used exclusively to supply industrial electric furnaces for melting, refining, tempering, and the like

(2) Separately derived systems used exclusively for rectifiers that supply only adjustable-speed industrial drives

(3) Separately derived systems supplied by transformers that have a primary voltage rating less than 1000 volts, provided that all the following conditions are met:

a. The system is used exclusively for control circuits.
b. The conditions of maintenance and supervision ensure that only licensed electrical practitioner or non licensed electrical practitioner under the supervision of a licensed electrical practitioner service the installation.
c. Continuity of control power is required.
d. Ground detectors are installed on the control system.

(4) Other systems that are not required to be grounded in accordance with the requirements of 2.50.2.1(b). Where an alternating-current system is not grounded as permitted in 2.50.2.2(1) through (4), ground detectors shall be installed on the system.

Exception: Systems of less than 120 volts to ground as permitted by this Code shall not be required to have ground detectors. 2.50.2.3 Circuits Not to Be Grounded. The following circuits shall not be grounded:

(1) Circuits for electric cranes operating over combustible fibers in Class III locations, as provided in 5.3.3.56.
(2) Circuits in health care facilities as provided in 5.17.4.2 and 5.17.7.1
(3) Circuits for equipment within electrolytic cell working zone as provided in Article 6.68
(4) Secondary circuits of lighting systems as provided in 4.11.1.5(a)

2.50.2.5 Grounding Service-Supplied Alternating-Current Systems.

(a) System Grounding Connections. A premises wiring system supplied by a grounded ac service shall have a grounding electrode conductor connected to the grounded service conductor, at each service, in accordance with 2.50.2.5(a)(1) through (a)(5).

(1) General. The connection shall be made at any accessible point from the load end of the service drop or service lateral to and including the terminal or bus to which the grounded service conductor is connected at the service disconnecting means.

FPN: See definitions of Service Drop and Service Lateral in Article 1.0.

(2) Outdoor Transformer. Where the transformer supplying the service is located outside the building, at least one additional grounding connection shall be made from the grounded service conductor to a grounding electrode, either at the transformer or elsewhere outside the building.

Exception: The additional grounding connection shall not be made on high-impedance grounded neutral systems. The system shall meet the requirements of 2.50.2.17.

(3) Dual Fed Services. For services that are dual fed (double ended) in a common enclosure or grouped together in separate enclosures and employing a secondary tie, a single grounding electrode connection to the tie point of the grounded conductor(s) from each power source shall be permitted.

(4) Main Bonding Jumper as Wire or Busbar. Where the main bonding jumper specified in 2.50.2.9 is a wire or busbar and is installed from the grounded conductor terminal bar or bus to the equipment grounding terminal bar or bus in the service equipment, the grounding electrode conductor shall be permitted to be connected to the equipment grounding terminal, bar, or bus to which the main bonding jumper is connected.

(5) Load-Side Grounding Connections. A grounding connection shall not be made to any grounded conductor on the load side of the service disconnecting means except as otherwise permitted in this article.

FPN: See 2.50.2.11(a) for separately derived systems, 2.50.2.13 for connections at separate buildings or structures, and 2.50.7.13 for use of the grounded circuit conductor for grounding equipment.

(b) Main Bonding Jumper. For a grounded system, an unspliced main bonding jumper shall be used to connect the equipment grounding conductor(s) and the service-disconnect enclosure to the grounded conductor within the enclosure for each service disconnect in accordance with 2.50.2.9.

Exception No. 1: Where more than one service disconnecting means is located in an assembly listed for use as service equipment, an unspliced main bonding jumper shall bond the grounded conductor(s) to the assembly enclosure.

Exception No. 2: Impedance grounded neutral systems shall be permitted to be connected as provided in 2.50.2.17 and 2.50.10.7.

(c) Grounded Conductor Brought to Service Equipment. Where an ac system operating at less than 1000 volts is grounded at any point, the grounded conductor(s) shall be run to each service disconnecting means and shall be bonded to each disconnecting means enclosure. The grounded conductor(s) shall be installed in accordance with 2.50.2.5(c)(1) through (c)(3).

Exception: Where more than one service disconnecting means are located in an assembly listed for use as service equipment, it shall be permitted to run the grounded conductor(s) to the assembly, and the conductor(s) shall be bonded to the assembly enclosure.

(1) Routing and Sizing. This conductor shall be routed with the phase conductors and shall not be smaller than the required grounding electrode conductor specified in Table 2.50.3.17 but shall not be required to be larger than the largest ungrounded service-entrance phase conductor. In addition, for service-entrance phase conductors larger than 1100 kcmil copper or 850 mm2 aluminum, the grounded conductor shall not be smaller than 12½ percent of the area of the largest service-entrance phase conductor. The grounded conductor of a 3-phase, 3-wire delta service shall have an ampacity not less than that of the ungrounded conductors.
(2) Parallel Conductors. Where the service-entrance phase conductors are installed in parallel, the size of the grounded conductor shall be based on the total circular mil area of the parallel conductors as indicated in this section. Where installed in two or more raceways, the size of the grounded conductor in each raceway shall be based on the size of the ungrounded service-entrance conductor in the raceway but not smaller than 50 mm2.

FPN: See 3.10.1.4 for grounded conductors connected in parallel.

(3) High Impedance. The grounded conductor on a high- impedance grounded neutral system shall be grounded in accordance with 2.50.2.17.

(d) Grounding Electrode Conductor. A grounding electrode conductor shall be used to connect the equipment grounding conductors, the service-equipment enclosures, and, where the system is grounded, the grounded service conductor to the grounding electrode(s) required by Part 2.50.3. High-impedance grounded neutral system connections shall be made as covered in 2.50.2.17.

FPN: See 2.50.2.5(a) for ac system grounding connections.

(e) Ungrounded System Grounding Connections. A premises wiring system that is supplied by an ac service that is ungrounded shall have, at each service, a grounding electrode conductor connected to the grounding electrode(s) required by Part 2.50.3. The grounding electrode conductor shall be connected to a metal enclosure of the service conductors at any accessible point from the load end of the service drop or service lateral to the service disconnecting means.

2.50.2.7 Conductor to Be Grounded — Alternating-Current Systems.

For ac premises wiring systems, the conductor to be grounded shall be as specified in the following:

(1) Single-phase, 2-wire — one conductor

(2) Single-phase, 3-wire — the neutral conductor

(3) Multiphase systems having one wire common to all phases — the common conductor

(4) Multiphase systems where one phase is grounded — one phase conductor

(5) Multiphase systems in which one phase is used as in (2) — the neutral conductor

2.50.2.9 Main Bonding Jumper and System Bonding Jumper.

For a grounded system, main bonding jumpers and system bonding jumpers shall be installed as follows:

(a) Material. Main bonding jumpers and system bonding jumpers shall be of copper or other corrosion-resistant material. A main bonding jumper and a system bonding jumper shall be a wire, bus, screw, or similar suitable conductor.

(b) Construction. Where a main bonding jumper or a system bonding jumper is a screw only, the screw shall be identified with a green finish that shall be visible with the screw installed.

(c) Attachment. Main bonding jumpers and system bonding jumpers shall be attached in the manner specified by the applicable provisions of 2.50.1.8.

(d) Size. Main bonding jumpers and system bonding jumpers shall not be smaller than the sizes shown in Table 2.50.3.17. Where the supply conductors are larger than 1100 kcmil copper or 850 mm2 aluminum, the bonding jumper shall have an area that is not less than 12½ percent of the area of the largest phase conductor except that, where the phase conductors and the bonding jumper are of different materials (copper or aluminum), the minimum size of the bonding jumper shall be based on the assumed use of phase conductors of the same material as the bonding jumper and with an ampacity equivalent to that of the installed phase conductors.

2.50.2.11 Grounding Separately Derived Alternating-Current Systems.

(a) Grounded Systems. A separately derived ac system that is grounded shall comply with 2.50.2.11(a)(1) through (a)(8). A grounding connection shall not be made to any grounded circuit conductor on the load side of the point of grounding of the separately derived system except as otherwise permitted in this article.

FPN: See 2.50.2.13 for connections at separate buildings or structures, and 2.50.7.13 for use of the grounded circuit conductor for grounding equipment.

Exception: Impedance grounded neutral system grounding connections shall be made as specified in 2.50.2.17 or 2.50.10.7.

(1) System Bonding Jumper. An unspliced system bonding jumper in compliance with 2.50.2.9(a) through (d) that is sized based on the derived phase conductors shall be used to connect the equipment grounding conductors of the separately derived system to the grounded conductor. This connection shall be made at any single point on the separately derived system from the source to the first system disconnecting means or overcurrent device, or it shall be made at the source of a separately derived system that has no disconnecting means or overcurrent devices.

Exception No. 1: For separately derived systems that are dual fed (double ended) in a common enclosure or grouped together in separate enclosures and employing a secondary tie, a single system bonding jumper connection to the tie point of the grounded circuit conductors from each power source shall be permitted.

Exception No. 2: A system bonding jumper at both the source and the first disconnecting means shall be permitted where doing so does not establish a parallel path for the grounded conductor. Where a grounded conductor is used in this manner, it shall not be smaller than the size specified for the system bonding jumper but shall not be required to be larger than the ungrounded conductor(s). For the purposes of this exception, connection through the earth shall not be considered as providing a parallel path.

Exception No. 3: The size of the system bonding jumper for a system that supplies a Class 1, Class 2, or Class 3 circuit, and is derived from a transformer rated not more than 1000 volt-amperes, shall not be smaller than the derived phase conductors and shall not be smaller than 2.0 mm2 (1.6 mm dia.) copper or 3.5 mm2 (2.0 mm dia.) aluminum.

(2) Equipment Bonding Jumper Size. Where a bonding jumper of the wire type is run with the derived phase conductors from the source of a separately derived system to the first disconnecting means, it shall be sized in accordance with 2.50.5.13(c), based on the size of the derived phase conductors.

(3) Grounding Electrode Conductor, Single Separately Derived System. A grounding electrode conductor for a single separately derived system shall be sized in accordance with 2.50.3.17 for the derived phase conductors and shall be used to connect the grounded conductor of the derived system to the grounding electrode as specified in 2.50.2.11(a)(7). This connection shall be made at the same point on the separately derived system where the system bonding jumper is installed.

Exception No. 1: Where the system bonding jumper specified in 2.50.2.11(a)(1) is a wire or busbar, it shall be permitted to connect the grounding electrode conductor to the equipment grounding terminal, bar, or bus, provided the equipment grounding terminal, bar, or bus is of sufficient size for the separately derived system.

Exception No. 2: Where a separately derived system originates in listed equipment suitable as service equipment, the grounding electrode conductor from the service or feeder equipment to the grounding electrode shall be permitted as the grounding electrode conductor for the separately derived system, provided the grounding electrode conductor is of sufficient size for the separately derived system. Where the equipment ground bus internal to the equipment is not smaller than the required grounding electrode conductor for the separately derived system, the grounding electrode connection for the separately derived system shall be permitted to be made to the bus.

Exception No. 3: A grounding electrode conductor shall not be required for a system that supplies a Class 1, Class 2, or Class 3 circuit and is derived from a transformer rated not more than 1000 volt-amperes, provided the grounded conductor is bonded to the transformer frame or enclosure by a jumper sized in accordance with 2.50.2.11(a)(1), Exception No. 3, and the transformer frame or enclosure is grounded by one of the means specified in 2.50.7.5.

(4) Grounding Electrode Conductor, Multiple Separately Derived Systems. Where more than one separately derived system is installed, it shall be permissible to connect a tap from each separately derived system to a common grounding electrode conductor. Each tap conductor shall connect the grounded conductor of the separately derived system to the common grounding electrode conductor. The grounding electrode conductors and taps shall comply with 2.50.2.11(a)(4)a through (a)(4)c.

Exception No. 1: Where the system bonding jumper specified in 2.50.2.11(a)(1) is a wire or busbar, it shall be permitted to connect the grounding electrode conductor to the equipment grounding terminal, bar, or bus, provided the equipment grounding terminal, bar, or bus is of sufficient size for the separately derived system.

Exception No. 2: A grounding electrode conductor shall not be required for a system that supplies a Class 1, Class 2, or Class 3 circuit and is derived from a transformer rated not more than 1000 volt-amperes, provided the system grounded conductor is bonded to the transformer frame or enclosure by a jumper sized in accordance with 2.50.2.11(a)(1), Exception No. 3 and the transformer frame or enclosure is grounded by one of the means specified in 2.50.7.5.

a. Common Grounding Electrode Conductor Size. The common grounding electrode conductor shall not be smaller than 80 mm2 copper or 125 mm2 aluminum.

b. Tap Conductor Size. Each tap conductor shall be sized in accordance with 2.50.3.17 based on the derived phase conductors of the separately derived system it serves.

Exception: Where a separately derived system originates in listed equipment suitable as service equipment, the grounding electrode conductor from the service or feeder equipment to the grounding electrode shall be permitted as the grounding electrode conductor for the separately derived system, provided the grounding electrode conductor is of sufficient size for the separately derived system. Where the equipment ground bus internal to the equipment is not smaller than the required grounding electrode conductor for the separately derived system, the grounding electrode connection for the separately derived system shall be permitted to be made to the bus.

c. Connections. All tap connections to the common grounding electrode conductor shall be made at an accessible location by one of the following methods:

1. A listed connector.
2. Listed connections to aluminum or copper busbars not less than 6 mm × 50 mm. Where aluminum busbars are used, the installation shall comply with 2.50.3.15(a).
3. By the exothermic welding process.

Tap conductors shall be connected to the common grounding electrode conductor in such a manner that the common grounding electrode conductor remains without a splice or joint.

(5) Installation. The installation of all grounding electrode conductors shall comply with 2.50.3.15(a), (b), (c), and (e).

(6) Bonding. Structural steel and metal piping shall be bonded in accordance with 2.50.5.15(d).

(7) Grounding Eletrode. The grounding electrode shall be as near as practicable to and preferably in the same area as the grounding electrode conductor connection to the system. The grounding electrode shall be the nearest one of the following:

a. Metal water pipe grounding electrode as specified in 2.50.3.3(a)(1)
b. Structural metal grounding electrode as specified in 2.50.3.3(a)(2)

Exception No. 1: Any of the other electrodes identified in 2.50.3.3(a) shall be used where the electrodes specified by 2.50.2.11(a)(7) are not available.

Exception No. 2 to (1) and (2): Where a separately derived system originates in listed equipment suitable for use as service equipment, the grounding electrode used for the service or feeder equipment shall be permitted as the grounding electrode for the separately derived system.

FPN: See 2.50.5.15(d) for bonding requirements of interior metal water piping in the area served by separately derived systems.

(8) Grounded Conductor. Where a grounded conductor is installed and the system bonding jumper is not located at the source of the separately derived system, 2.50.2.11(a)(8)a, (a)(8)b, and (a)(8)c shall apply.

a. Routing and Sizing. This conductor shall be routed with the derived phase conductors and shall not be smaller than the required grounding electrode conductor specified in Table 2.50.3.17 but shall not be required to be larger than the largest ungrounded derived phase conductor. In addition, for phase conductors larger than 1100 kcmil copper or 850 mm2 aluminum, the grounded conductor shall not be smaller than 12½ percent of the area of the largest derived phase conductor. The grounded conductor of a 3-phase, 3-wire delta system shall have an ampacity not less than that of the ungrounded conductors.
b. Parallel Conductors. Where the derived phase conductors are installed in parallel, the size of the grounded conductor shall be based on the total circular mil area of the parallel conductors, as indicated in this section. Where installed in two or more raceways, the size of the grounded conductor in each raceway shall be based on the size of the ungrounded conductors in the raceway but not smaller than 50 mm2 .

FPN: See 3.10.1.4 for grounded conductors connected in parallel.

c. Impedance Grounded System. The grounded conductor of an impedance grounded neutral system shall be installed in accordance with 2.50.2.17 or 2.50.10.7.

(b) Ungrounded Systems. The equipment of an ungrounded separately derived system shall be grounded as specified in 2.50.2.11(b)(1) and (b)(2).

(1) Grounding Electrode Conductor. A grounding electrode conductor, sized in accordance with 2.50.3.17 for the derived phase conductors, shall be used to connect the metal enclosures of the derived system to the grounding electrode as specified in 2.50.2.11(b)(2). This connection shall be made at any point on the separately derived system from the source to the first system disconnecting means.
(2) Grounding Electrode. Except as permitted by 2.50.2.15 for portable and vehicle-mounted generators, the grounding electrode shall comply with 2.50.2.11(a)(7).

2.50.2.13 Buildings or Structures Supplied by Feeder(s) or Branch Circuit(s).

(a) Grounding Electrode. Building(s) or structure(s) supplied by feeder(s) or branch circuit(s) shall have a grounding electrode or grounding electrode system installed in accordance with 2.50.3.1. The grounding electrode conductor(s) shall be connected in accordance with 2.50.2.13(b) or (c). Where there is no existing grounding electrode, the grounding electrode(s) required in 2.50.3.1 shall be installed.

Exception: A grounding electrode shall not be required where only a single branch circuit supplies the building or structure and the branch circuit includes an equipment grounding conductor for grounding the conductive non–current-carrying parts of equipment. For the purpose of this section, a multiwire branch circuit shall be considered as a single branch circuit.

(b) Grounded Systems. For a grounded system at the separate building or structure, the connection to the grounding electrode and grounding or bonding of equipment, structures, or frames required to be grounded or bonded shall comply with either 2.50.2.13(b)(1) or (b)(2).

(1) Equipment Grounding Conductor. An equipment grounding conductor as described in 2.50.6.9 shall be run with the supply conductors and connected to the building or structure disconnecting means and to the grounding electrode(s). The equipment grounding conductor shall be used for grounding or bonding of equipment, structures, or frames required to be grounded or bonded. The equipment grounding conductor shall be sized in accordance with 2.50.6.13. Any installed grounded conductor shall not be connected to the equipment grounding conductor or to the grounding electrode(s).
(2) Grounded Conductor. Where (1) an equipment grounding conductor is not run with the supply to the building or structure, (2) there are no continuous metallic paths bonded to the grounding system in each building or structure involved, and (3) ground-fault protection of equipment has not been installed on the supply side of the feeder(s), the grounded conductor run with the supply to the building or structure shall be connected to the building or structure disconnecting means and to the grounding electrode(s) and shall be used for grounding or bonding of equipment, structures, or frames required to be grounded or bonded. The size of the grounded conductor shall not be smaller than the larger of either of the following:
a. That required by 2.20.3.22
b. That required by 2.50.6.13

(c) Ungrounded Systems. The grounding electrode(s) shall be connected to the building or structure disconnecting means.

(d) Disconnecting Means Located in Separate Building or Structure on the Same Premises. Where one or more disconnecting means supply one or more additional buildings or structures under single management, and where these disconnecting means are located remote from those buildings or structures in accordance with the provisions of 2.25.2.3, Exception Nos. 1 and 2, all of the following conditions shall be met:

(1) The connection of the grounded conductor to the grounding electrode at a separate building or structure shall not be made.
(2) An equipment grounding conductor for grounding any non– current-carrying equipment, interior metal piping systems, and building or structural metal frames is run with the circuit conductors to a separate building or structure and bonded to existing grounding electrode(s) required in Part 2.50.3, or, where there are no existing electrodes, the grounding electrode(s) required in Part 2.50.3 shall be installed where a separate building or structure is supplied by more than one branch circuit.
(3) Bonding the equipment grounding conductor to the grounding electrode at a separate building or structure shall be made in a junction box, panelboard, or similar enclosure located immediately inside or outside the separate building or structure.
(e) Grounding Electrode Conductor. The size of the grounding electrode conductor to the grounding electrode(s) shall not be smaller than given in 2.50.3.17, based on the largest ungrounded supply conductor. The installation shall comply with Part 2.50.3.

2.50.2.15 Portable and Vehicle-Mounted Generators.

(a) Portable Generators. The frame of a portable generator shall not be required to be connected to a grounding electrode as defined in 2.50.3.3 for a system supplied by the generator under the following conditions:

(1) The generator supplies only equipment mounted on the generator, cord-and-plug-connected equipment through receptacles mounted on the generator, or both, and
(2) The non–current-carrying metal parts of equipment and the equipment grounding conductor terminals of the receptacles are bonded to the generator frame.

(b) Vehicle-Mounted Generators. The frame of a vehicle shall not be required to be connected to a grounding electrode as defined in 2.50.3.3 for a system supplied by a generator located on this vehicle under the following conditions:

(1) The frame of the generator is bonded to the vehicle frame, and
(2) The generator supplies only equipment located on the vehicle or cord-and-plug-connected equipment through receptacles mounted on the vehicle, or both equipment located on the vehicle and cord-and- plug-connected equipment through receptacles mounted on the vehicle or on the generator, and
(3) The non–current-carrying metal parts of equipment and the equipment grounding conductor terminals of the receptacles are bonded to the generator frame.

(c) Grounded Conductor Bonding. A system conductor that is required to be grounded by 2.50.2.7 shall be bonded to the generator frame where the generator is a component of a separately derived system.

FPN: For grounding portable generators supplying fixed wiring systems, see 2.50.2.1(d).

2.50.2.17 High-Impedance Grounded Neutral Systems.

High- impedance grounded neutral systems in which a grounding impedance, usually a resistor, limits the ground-fault current to a low value shall be permitted for 3-phase ac systems of 250 volts to 1000 volts where all the following conditions are met:

(1) The conditions of maintenance and supervision ensure that only licensed electrical practitioner or non licensed electrical practitioner under the supervision of a licensed electrical practitioner service the installation.

(2) Continuity of power is required.

(3) Ground detectors are installed on the system.

(4) Line-to-neutral loads are not served.

High-impedance grounded neutral systems shall comply with the provisions of 2.50.2.17(a) through (g).

(a) Grounding Impedance Location. The grounding impedance shall be installed between the grounding electrode conductor and the system neutral. Where a neutral is not available, the grounding impedance shall be installed between the grounding electrode conductor and the neutral derived from a grounding transformer.

(b) Neutral Conductor. The neutral conductor from the neutral point of the transformer or generator to its connection point to the grounding impedance shall be fully insulated.

The neutral conductor shall have an ampacity of not less than the maximum current rating of the grounding impedance. In no case shall the neutral conductor be smaller than 8.0 mm2 (3.2 mm dia.) copper or 14 mm2 aluminum or copper-clad aluminum. (c) System Neutral Connection. The system neutral conductor shall not be connected to ground except through the grounding impedance.

FPN: The impedance is normally selected to limit the ground-fault current to a value slightly greater than or equal to the capacitive charging current of the system. This value of impedance will also limit transient overvoltages to safe values. For guidance, refer to criteria for limiting transient overvoltages in ANSI/IEEE 142-1991, Recommended Practice for Grounding of Industrial and Commercial Power Systems.

(d) Neutral Conductor Routing. The conductor connecting the neutral point of the transformer or generator to the grounding impedance shall be permitted to be installed in a separate raceway. It shall not be required to run this conductor with the phase conductors to the first system disconnecting means or overcurrent device.

(e) Equipment Bonding Jumper. The equipment bonding jumper (the connection between the equipment grounding conductors and the grounding impedance) shall be an unspliced conductor run from the first system disconnecting means or overcurrent device to the grounded side of the grounding impedance.

(f) Grounding Electrode Conductor Location. The grounding electrode conductor shall be attached at any point from the grounded side of the grounding impedance to the equipment grounding connection at the service equipment or first system disconnecting means.

(g) Equipment Bonding Jumper Size. The equipment bonding jumper shall be sized in accordance with (1) or (2) as follows:

(1) Where the grounding electrode conductor connection is made at the grounding impedance, the equipment bonding jumper shall be sized in accordance with 2.50.3.17, based on the size of the service entrance conductors for a service or the derived phase conductors for a separately derived system.
(2) Where the grounding electrode conductor is connected at the first system disconnecting means or overcurrent device, the equipment bonding jumper shall be sized the same as the neutral conductor in 2.50.2.17(b).

2.50.3 Grounding Electrode System and Grounding Electrode Conductor

2.50.3.1 Grounding Electrode System.

All grounding electrodes as described in 2.50.3.3(a)(1) through (a)(6) that are present at each building or structure served shall be bonded together to form the grounding electrode system. Where none of these grounding electrodes exist, one or more of the grounding electrodes specified in 2.50.3.3(a)(4) through (a)(7) shall be installed and used.

Exception: Concrete-encased electrodes of existing buildings or structures shall not be required to be part of the grounding electrode system where the steel reinforcing bars or rods are not accessible for use without disturbing the concrete.

2.50.3.3 Grounding Electrodes.

(a) Electrodes Permitted for Grounding.

(1) Metal Underground Water Pipe. A metal underground water pipe in direct contact with the earth for 3 000 mm or more (including any metal well casing effectively bonded to the pipe) and electrically continuous (or made electrically continuous by bonding around insulating joints or insulating pipe) to the points of connection of the grounding electrode conductor and the bonding conductors. Interior metal water piping located more than 1 500 mm from the point of entrance to the building shall not be used as a part of the grounding electrode system or as a conductor to interconnect electrodes that are part of the grounding electrode system.

Exception: In industrial and commercial buildings or structures where conditions of maintenance and supervision ensure that only licensed electrical practitioner or non licensed electrical practitioner under the supervision of a licensed electrical practitioner service the installation, interior metal water piping located more than 1 500 mm from the point of entrance to the building shall be permitted as a part of the grounding electrode system or as a conductor to interconnect electrodes that are part of the grounding electrode system, provided that the entire length, other than short sections passing perpendicular through walls, floors, or ceilings, of the interior metal water pipe that is being used for the conductor is exposed.

(2) Metal Frame of the Building or Structure. The metal frame of the building or structure, where any of the following methods are used to make an earth connection:
a. 3 000 mm or more of a single structural metal member in direct contact with the earth or encased in concrete that is in direct contact with the earth
b. The structural metal frame is bonded to one or more of the grounding electrodes as defined in 2.50.3.3(a)(1), (a)(3), or (a)(4)
c. The structural metal frame is bonded to one or more of the grounding electrodes as defined in 2.50.3.3(a)(5) or (a)(6) that comply with 2.50.3.7, or
d. Other approved means of establishing a connection to earth.
(3) Concrete-Encased Electrode. An electrode encased by at least 50 mm of concrete, located within and near the bottom of a concrete foundation or footing that is in direct contact with the earth, consisting of at least 6 000 mm of one or more bare or zinc galvanized or other electrically conductive coated steel reinforcing bars or rods of not less than 13 mm in diameter, or consisting of at least 6 000 mm of bare copper conductor not smaller than 22 mm2 . Reinforcing bars shall be permitted to be bonded together by the usual steel tie wires or other effective means.
(4) Ground Ring. A ground ring encircling the building or structure, in direct contact with the earth, consisting of at least 6 000 mm of bare copper conductor not smaller than 30 mm2 . (5) Rod and Pipe Electrodes. Rod and pipe electrodes shall not be less than 2 400 mm in length and shall consist of the following materials.
a. Electrodes of pipe or conduit shall not be smaller than metric designator 21 (trade size ¾) and, where of iron or steel, shall have the outer surface galvanized or otherwise metal-coated for corrosion protection.
b. Electrodes of rods of iron or steel shall be at least 16 mm in diameter. Stainless steel rods less than 16 mm in diameter, nonferrous rods, or their equivalent shall be listed and shall not be less than 13 mm in diameter.
(6) Plate Electrodes. Each plate electrode shall expose not less than 0.186 m 2 (2 ft 2 ) of surface to exterior soil. Electrodes of iron or steel plates shall be at least 6 mm in thickness. Electrodes of nonferrous metal shall be at least 1.5 mm in thickness.
(7) Other Local Metal Underground Systems or Structures. Other local metal underground systems or structures such as piping systems, underground tanks, and underground metal well casings that are not effectively bonded to a metal water pipe.

(b) Electrodes Not Permitted for Grounding. The following shall not be used as grounding electrodes:

(1) Metal underground gas piping system
(2) Aluminum electrodes

FPN: See 2.50.5.15(b) for bonding requirements of gas piping.

2.50.3.4 Grounding Electrode System Installation.

FPN: See 5.47.1.9 and 5.47.1.10 for special grounding and bonding requirements for agricultural buildings.

(a) Rod, Pipe, and Plate Electrodes. Where practicable, rod, pipe, and plate electrodes shall be embedded below permanent moisture level. Rod, pipe, and plate electrodes shall be free from nonconductive coatings such as paint or enamel.

(b) Electrode Spacing. Where more than one of the electrodes of the type specified in 2.50.3.3(a)(5) or (a)(6) are used, each electrode of one grounding system (including that used for air terminals) shall not be less than 1 800 mm from any other electrode of another grounding system. Two or more grounding electrodes that are effectively bonded together shall be considered a single grounding electrode system.

(c) Bonding Jumper. The bonding jumper(s) used to connect the grounding electrodes together to form the grounding electrode system shall be installed in accordance with 2.50.3.15(a), (b), and (e), shall be sized in accordance with 2.50.3.17, and shall be connected in the manner specified in 2.50.3.21.

(d) Metal Underground Water Pipe. Where used as a grounding electrode, metal underground water pipe shall meet the requirements of 2.50.3.4(d)(1) and (d)(2).

(1) Continuity. Continuity of the grounding path or the bonding connection to interior piping shall not rely on water meters or filtering devices and similar equipment.
(2) Supplemental Electrode Required. A metal underground water pipe shall be supplemented by an additional electrode of a type specified in 2.50.3.3(a)(2) through (a)(7). Where the supplemental electrode is a rod, pipe, or plate type, it shall comply with 2.50.3.7. The supplemental electrode shall be permitted to be bonded to the grounding electrode conductor, the grounded service-entrance conductor, the nonflexible grounded service raceway, or any grounded service enclosure.

Exception: The supplemental electrode shall be permitted to be bonded to the interior metal water piping at any convenient point as covered in 2.50.3.3(a)(1), Exception.

(e) Supplemental Electrode Bonding Connection Size. Where the supplemental electrode is a rod, pipe, or plate electrode, that portion of the bonding jumper that is the sole connection to the supplemental grounding electrode shall not be required to be larger than 14 mm2 copper wire or 22 mm2 aluminum wire.

(f) Ground Ring. The ground ring shall be buried at a depth below the earth’s surface of not less than 750 mm.

(g) Rod and Pipe Electrodes. The electrode shall be installed such that at least 2 400 mm of length is in contact with the soil. It shall be driven to a depth of not less than 2 400 mm except that, where rock bottom is encountered, the electrode shall be driven at an oblique angle not to exceed 45 degrees from the vertical or, where rock bottom is encountered at an angle up to 45 degrees, the electrode shall be permitted to be buried in a trench that is at least 750 mm deep. The upper end of the electrode shall be flush with or below ground level unless the aboveground end and the grounding electrode conductor attachment are protected against physical damage as specified in 2.50.1.10.

(h) Plate Electrode. Plate electrodes shall be installed not less than 750 mm below the surface of the earth.

2.50.3.5 Supplementary Grounding Electrodes.

Supplementary grounding electrodes shall be permitted to be connected to the equipment grounding conductors specified in 2.50.6.9 and shall not be required to comply with the electrode bonding requirements of 2.50.3.1 or 2.50.3.4(c) or the resistance requirements of 2.50.3.7, but the earth shall not be used as an effective ground-fault current path as specified in 2.50.1.4(a)(5) and 2.50.1.4(b)(4).

2.50.3.7 Resistance of Rod, Pipe, and Plate Electrodes.

A single electrode consisting of a rod, pipe, or plate that does not have a resistance to ground of 25 ohms or less shall be augmented by one additional electrode of any of the types specified by 2.50.3.3(a)(2) through (a)(7). Where multiple rod, pipe, or plate electrodes are installed to meet the requirements of this section, they shall not be less than 1 800 mm apart.

FPN: The paralleling efficiency of rods longer than 2 400 mm is improved by spacing greater than 1 800 mm.

2.50.3.9 Common Grounding Electrode.

Where an ac system is connected to a grounding electrode in or at a building or structure, the same electrode shall be used to ground conductor enclosures and equipment in or on that building or structure. Where separate services, feeders, or branch circuits supply a building and are required to be connected to a grounding electrode(s), the same grounding electrode(s) shall be used.

Two or more grounding electrodes that are effectively bonded together shall be considered as a single grounding electrode system in this sense.

2.50.3.11 Use of Air Terminals.

Air terminal conductors and driven pipes, rods, or plate electrodes used for grounding air terminals shall not be used in lieu of the grounding electrodes required by 2.50.3.1 for grounding wiring systems and equipment. This provision shall not prohibit the required bonding together of grounding electrodes of different systems.

FPN No. 1: See 2.50.5.17 for spacing from air terminals. See 8.0.4.1(d), 8.10.2.11(j), and 8.20.4.1(d) for bonding of electrodes.

FPN No. 2: Bonding together of all separate grounding electrodes will limit potential differences between them and between their associated wiring systems. 2.50.3.13 Grounding Electrode Conductor Material. The grounding electrode conductor shall be of copper, aluminum, or copper-clad aluminum. The material selected shall be resistant to any corrosive condition existing at the installation or shall be suitably protected against corrosion. The conductor shall be solid or stranded, insulated, covered, or bare.

2.50.3.15 Grounding Electrode Conductor Installation.

Grounding electrode conductors shall be installed as specified in 2.50.3.15(a) through (f).

(a) Aluminum or Copper-Clad Aluminum Conductors. Bare aluminum or copper-clad aluminum grounding conductors shall not be used where in direct contact with masonry or the earth or where subject to corrosive conditions. Where used outside, aluminum or copper-clad aluminum grounding conductors shall not be terminated within 450 mm of the earth.

(b) Securing and Protection Against Physical Damage. Where exposed, a grounding electrode conductor or its enclosure shall be securely fastened to the surface on which it is carried. A 22 mm2 or larger copper or aluminum grounding electrode conductor shall be protected where exposed to physical damage. A 14 mm2 grounding electrode conductor that is free from exposure to physical damage shall be permitted to be run along the surface of the building construction without metal covering or protection where it is securely fastened to the construction; otherwise, it shall be in rigid metal conduit, intermediate metal conduit, rigid nonmetallic conduit, electrical metallic tubing, or cable armor. Grounding electrode conductors smaller than 14 mm2 shall be in rigid metal conduit, intermediate metal conduit, rigid nonmetallic conduit, electrical metallic tubing, or cable armor.

(c) Continuous. Grounding electrode conductor(s) shall be installed in one continuous length without a splice or joint except as permitted in (1) through (4):

(1) Splicing shall be permitted only by irreversible compression- type connectors listed as grounding and bonding equipment or by the exothermic welding process.
(2) Sections of busbars shall be permitted to be connected together to form a grounding electrode conductor.
(3) Bonding jumper(s) from grounding electrode(s) and grounding electrode conductor(s) shall be permitted to be connected to an aluminum or copper busbar not less than 6 mm × 50 mm. The busbar shall be securely fastened and shall be installed in an accessible location. Connections shall be made by a listed connector or by the exothermic welding process.
(4) Where aluminum busbars are used, the installation shall comply with 2.50.3.15(a).

(d) Grounding Electrode Conductor Taps. Where a service consists of more than a single enclosure as permitted in 2.30.6.2(a), it shall be permitted to connect taps to the common grounding electrode conductor. Each such tap conductor shall extend to the inside of each such enclosure. The common grounding electrode conductor shall be sized in accordance with 2.50.3.17, based on the sum of the circular mil area of the largest ungrounded service entrance conductors. Where more than one set of service entrance conductors as permitted by 2.30.4.1, Exception No. 2 connect directly to a service drop or lateral, the common grounding electrode conductor shall be sized in accordance with Table 2.50.3.17 Note 1. The tap conductors shall be permitted to be sized in accordance with the grounding electrode conductors specified in 2.50.3.17 for the largest conductor serving the respective enclosures. The tap conductors shall be connected to the common grounding electrode conductor in such a manner that the common grounding electrode conductor remains without a splice or joint.

(e) Enclosures for Grounding Electrode Conductors. Ferrous metal enclosures for grounding electrode conductors shall be electrically continuous from the point of attachment to cabinets or equipment to the grounding electrode and shall be securely fastened to the ground clamp or fitting. Nonferrous metal enclosures shall not be required to be electrically continuous. Ferrous metal enclosures that are not physically continuous from cabinets or equipment to the grounding electrode shall be made electrically continuous by bonding each end of the raceway or enclosure to the grounding electrode conductor. Bonding shall apply at each end and to all intervening ferrous raceways, boxes, and enclosures between the service equipment and the grounding electrode. The bonding jumper for a grounding electrode conductor raceway or cable armor shall be the same size as, or larger than, the required enclosed grounding electrode conductor. Where a raceway is used as protection for a grounding electrode conductor, the installation shall comply with the requirements of the appropriate raceway article.

(f) To Electrode(s). A grounding electrode conductor shall be permitted to be run to any convenient grounding electrode available in the grounding electrode system, or to one or more grounding electrode(s) individually, or to the aluminum or copper busbar as permitted in 2.50.3.15(c). The grounding electrode conductor shall be sized for the largest grounding electrode conductor required among all the electrodes connected to it.

2.50.3.17 Size of Alternating-Current Grounding Electrode Conductor.

The size of the grounding electrode conductor of a grounded or ungrounded ac system shall not be less than given in Table 2.50.3.17, except as permitted in 2.50.3.17(a) through (c).

FPN: See 2.50.2.5(c) for size of ac system conductor brought to service equipment.

(a) Connections to Rod, Pipe, or Plate Electrodes. Where the grounding electrode conductor is connected to rod, pipe, or plate electrodes as permitted in 2.50.3.3(a)(5) or (a)(6), that portion of the conductor that is the sole connection to the grounding electrode shall not be required to be larger than 14 mm2 copper wire or 22 mm2 aluminum wire.

(b) Connections to Concrete-Encased Electrodes. Where the grounding electrode conductor is connected to a concrete-encased electrode as permitted in 2.50.3.3(a)(3), that portion of the conductor that is the sole connection to the grounding electrode shall not be required to be larger than 22 mm2 copper wire.

(c) Connections to Ground Rings. Where the grounding electrode conductor is connected to a ground ring as permitted in 2.50.3.3(a)(4), that portion of the conductor that is the sole connection to the grounding electrode shall not be required to be larger than the conductor used for the ground ring.

Table 2.50.3.17 Grounding Electrode Conductor for Alternating-Current Systems
Size of Largest Ungrounded Service- Entrance Conductor or Equivalent Area for Parallel Conductorsa mm2 Size of Grounding Electrode Conductor mm2 (mm dia)
30 or smaller 50 or smaller 8.0(3.2) 14
38 or 50 60 or 80 14 22
60 or 80 100 or 125 22 30
Over 80 through 175 Over 125 through 250 30 50
Over 175 through 600 Over 250 through 900 50 80
Over 600 through 1100 Over 900 through 1750 60 100
Over 1100 Over 1750 80 125
Notes:
1. Where multiple sets of service-entrance conductors are used as permitted in 2.30.4.1, Exception No. 2, the equivalent size of the largest service-entrance conductor shall be determined by the largest sum of the areas of the corresponding conductors of each set.
2. Where there are no service-entrance conductors, the grounding electrode conductor size shall be determined by the equivalent size of the largest service-entrance conductor required for the load to be served.
a This table also applies to the derived conductors of separately derived ac systems.
b See installation restrictions in 2.50.3.15(a).

2.50.3.19 Grounding Electrode Conductor and Bonding Jumper Connection to Grounding Electrodes.

(a) Accessibility. The connection of a grounding electrode conductor or bonding jumper to a grounding electrode shall be accessible.

Exception No. 1: An encased or buried connection to a concrete- encased, driven, or buried grounding electrode shall not be required to be accessible.

Exception No. 2: An exothermic or irreversible compression connection to fire-proofed structural metal shall not be required to be accessible.

(b) Effective Grounding Path. The connection of a grounding electrode conductor or bonding jumper to a grounding electrode shall be made in a manner that will ensure a permanent and effective grounding path. Where necessary to ensure the grounding path for a metal piping system used as a grounding electrode, effective bonding shall be provided around insulated joints and around any equipment likely to be disconnected for repairs or replacement. Bonding conductors shall be of sufficient length to permit removal of such equipment while retaining the integrity of the bond.

2.50.3.21 Methods of Grounding and Bonding Conductor Connection to Electrodes.

The grounding or bonding conductor shall be connected to the grounding electrode by exothermic welding, listed lugs, listed pressure connectors, listed clamps, or other listed means. Connections depending on solder shall not be used. Ground clamps shall be listed for the materials of the grounding electrode and the grounding electrode conductor and, where used on pipe, rod, or other buried electrodes, shall also be listed for direct soil burial or concrete encasement. Not more than one conductor shall be connected to the grounding electrode by a single clamp or fitting unless the clamp or fitting is listed for multiple conductors. One of the following methods shall be used:

(1) A pipe fitting, pipe plug, or other approved device screwed into a pipe or pipe fitting

(2) A listed bolted clamp of cast bronze or brass, or plain or malleable iron

(3) For indoor telecommunications purposes only, a listed sheet metal strap-type ground clamp having a rigid metal base that seats on the electrode and having a strap of such material and dimensions that it is not likely to stretch during or after installation

(4) An equally substantial approved means

2.50.4 Enclosure, Raceway, and Service Cable Grounding

2.50.4.1 Service Raceways and Enclosures.

Metal enclosures and raceways for service conductors and equipment shall be grounded. Exception: A metal elbow that is installed in an underground installation of rigid nonmetallic conduit and is isolated from possible contact by a minimum cover of 450 mm to any part of the elbow shall not be required to be grounded.

2.50.4.5 Underground Service Cable or Raceway.

(a) Underground Service Cable. The sheath or armor of a continuous underground metal-sheathed or armored service cable system that is bonded to the grounded underground system shall not be required to be grounded at the building or structure. The sheath or armor shall be permitted to be insulated from the interior metal raceway conduit or piping.

(b) Underground Service Raceway Containing Cable. An underground metal service raceway that contains a metal-sheathed or armored cable bonded to the grounded underground system shall not be required to be grounded at the building or structure. The sheath or armor shall be permitted to be insulated from the interior metal raceway or piping.

2.50.4.7 Other Conductor Enclosures and Raceways.

Except as permitted by 2.50.6.3(i), metal enclosures and raceways for other than service conductors shall be grounded.

Exception No. 1: Metal enclosures and raceways for conductors added to existing installations of open wire, knob and tube wiring, and nonmetallic-sheathed cable shall not be required to be grounded where these enclosures or wiring methods comply with (1) through (4) as follows:

(1) Do not provide an equipment ground
(2) Are in runs of less than 7 600 mm
(3) Are free from probable contact with ground, grounded metal, metal lath, or other conductive material
(4) Are guarded against contact by persons

Exception No. 2: Short sections of metal enclosures or raceways used to provide support or protection of cable assemblies from physical damage shall not be required to be grounded.

Exception No. 3: A metal elbow shall not be required to be grounded where it is installed in a nonmetallic raceway and is isolated from possible contact by a minimum cover of 450 mm to any part of the elbow or is encased in not less than 50 mm of concrete.

2.50.5 Bonding

2.50.5.1 General.

Bonding shall be provided where necessary to ensure electrical continuity and the capacity to conduct safely any fault current likely to be imposed.

2.50.5.3 Services.

(a) Bonding of Services. The non–current-carrying metal parts of equipment indicated in 2.50.5.3(a)(1), (a)(2), and (a)(3) shall be effectively bonded together.

(1) The service raceways, cable trays, cablebus framework, auxiliary gutters, or service cable armor or sheath except as permitted in 2.50.4.5.
(2) All service enclosures containing service conductors, including meter fittings, boxes, or the like, interposed in the service raceway or armor.
(3) Any metallic raceway or armor enclosing a grounding electrode conductor as specified in 2.50.3.15(b). Bonding shall apply at each end and to all intervening raceways, boxes, and enclosures between the service equipment and the grounding electrode.

(b) Method of Bonding at the Service. Electrical continuity at service equipment, service raceways, and service conductor enclosures shall be ensured by one of the following methods:

(1) Bonding equipment to the grounded service conductor in a manner provided in 2.50.1.8
(4) Other listed devices, such as bonding-type locknuts, bushings, or bushings with bonding jumpers

Bonding jumpers meeting the other requirements of this article shall be used around concentric or eccentric knockouts that are punched or otherwise formed so as to impair the electrical connection to ground. Standard locknuts or bushings shall not be the sole means for the bonding required by this section.

2.50.5.5 Bonding for Other Systems.

An accessible means external to enclosures for connecting intersystem bonding and grounding electrode conductors shall be provided at the service equipment and at the disconnecting means for any additional buildings or structures by at least one of the following means:

(1) Exposed nonflexible metallic raceways

(2) Exposed grounding electrode conductor

(3) Approved means for the external connection of a copper or other corrosion-resistant bonding or grounding conductor to the grounded raceway or equipment

FPN No. 1: A 14 mm2 copper conductor with one end bonded to the grounded nonflexible metallic raceway or equipment and with 150 mm or more of the other end made accessible on the outside wall is an example of the approved means covered in 2.50.5.5(3).

FPN No. 2: See 8.0.4.1, 8.10.2.11, and 8.20.4.1 for bonding and grounding requirements for communications circuits, radio and television equipment, and CATV circuits.

2.50.5.7 Bonding Other Enclosures.

(a) General. Metal raceways, cable trays, cable armor, cable sheath, enclosures, frames, fittings, and other metal non–current-carrying parts that are to serve as grounding conductors, with or without the use of supplementary equipment grounding conductors, shall be effectively bonded where necessary to ensure electrical continuity and the capacity to conduct safely any fault current likely to be imposed on them. Any nonconductive paint, enamel, or similar coating shall be removed at threads, contact points, and contact surfaces or be connected by means of fittings designed so as to make such removal unnecessary.

(b) Isolated Grounding Circuits. Where required for the reduction of electrical noise (electromagnetic interference) on the grounding circuit, an equipment enclosure supplied by a branch circuit shall be permitted to be isolated from a raceway containing circuits supplying only that equipment by one or more listed nonmetallic raceway fittings located at the point of attachment of the raceway to the equipment enclosure. The metal raceway shall comply with provisions of this article and shall be supplemented by an internal insulated equipment grounding conductor installed in accordance with 2.50.7.17(d) to ground the equipment enclosure.

FPN: Use of an isolated equipment grounding conductor does not relieve the requirement for grounding the raceway system.

2.50.5.8 Bonding for Over 250 Volts.

For circuits of over 250 volts to ground, the electrical continuity of metal raceways and cables with metal sheaths that contain any conductor other than service conductors shall be ensured by one or more of the methods specified for services in 2.50.5.3(b), except for (b)(1).

Exception: Where oversized, concentric, or eccentric knockouts are not encountered, or where a box or enclosure with concentric or eccentric knockouts listed to provide a permanent, reliable electrical bond, the following methods shall be permitted:

(1) Threadless couplings and connectors for cables with metal sheaths

(2) Two locknuts, on rigid metal conduit or intermediate metal conduit, one inside and one outside of boxes and cabinets

(3) Fittings with shoulders that seat firmly against the box or cabinet, such as electrical metallic tubing connectors, flexible metal conduit connectors, and cable connectors, with one locknut on the inside of boxes and cabinets

(4) Listed fittings

2.50.5.9 Bonding Loosely Jointed Metal Raceways.

Expansion fittings and telescoping sections of metal raceways shall be made electrically continuous by equipment bonding jumpers or other means.

2.50.5.11 Bonding in Hazardous (Classified) Locations.

Regardless of the voltage of the electrical system, the electrical continuity of non– current-carrying metal parts of equipment, raceways, and other enclosures in any hazardous (classified) location as defined in Article 500 shall be ensured by any of the methods specified in 2.50.5.3(b)(2) through (b)(4) that are approved for the wiring method used. One or more of these bonding methods shall be used whether or not supplementary equipment grounding conductors are installed.

2.50.5.13 Equipment Bonding Jumpers.

(a) Material. Equipment bonding jumpers shall be of copper or other corrosion-resistant material. A bonding jumper shall be a wire, bus, screw, or similar suitable conductor.

(b) Attachment. Equipment bonding jumpers shall be attached in the manner specified by the applicable provisions of 2.50.1.8 for circuits and equipment and by 2.50.3.21 for grounding electrodes.

(c) Size — Equipment Bonding Jumper on Supply Side of Service. The bonding jumper shall not be smaller than the sizes shown in Table 2.50.3.17 for grounding electrode conductors. Where the service-entrance phase conductors are larger than 1100 kcmil copper or 850 mm2 aluminum, the bonding jumper shall have an area not less than 12½ percent of the area of the largest phase conductor except that, where the phase conductors and the bonding jumper are of different materials (copper or aluminum), the minimum size of the bonding jumper shall be based on the assumed use of phase conductors of the same material as the bonding jumper and with an ampacity equivalent to that of the installed phase conductors. Where the service-entrance conductors are paralleled in two or more raceways or cables, the equipment bonding jumper, where routed with the raceways or cables, shall be run in parallel. The size of the bonding jumper for each raceway or cable shall be based on the size of the service-entrance conductors in each raceway or cable.

(d) Size — Equipment Bonding Jumper on Load Side of Service. The equipment bonding jumper on the load side of the service overcurrent devices shall be sized, as a minimum, in accordance with the sizes listed in Table 2.50.6.13, but shall not be required to be larger than the largest ungrounded circuit conductors supplying the equipment and shall not be smaller than 2.0 mm2 (1.6 mm dia.). A single common continuous equipment bonding jumper shall be permitted to bond two or more raceways or cables where the bonding jumper is sized in accordance with Table 2.50.6.13 for the largest overcurrent device supplying circuits therein.

(e) Installation. The equipment bonding jumper shall be permitted to be installed inside or outside of a raceway or enclosure. Where installed on the outside, the length of the equipment bonding jumper shall not exceed 1 800 mm and shall be routed with the raceway or enclosure. Where installed inside of a raceway, the equipment bonding jumper shall comply with the requirements of 2.50.6.10 and 2.50.7.19.

Exception: An equipment bonding jumper longer than 1 800 mm shall be permitted at outside pole locations for the purpose of bonding or grounding isolated sections of metal raceways or elbows installed in exposed risers of metal conduit or other metal raceway.

2.50.5.15 Bonding of Piping Systems and Exposed Structural Steel.

(a) Metal Water Piping. The metal water piping system shall be bonded as required in (a)(1), (a)(2), or (a)(3) of this section. The bonding jumper(s) shall be installed in accordance with 2.50.3.15(a), (b), and (e). The points of attachment of the bonding jumper(s) shall be accessible.

(1) General. Metal water piping system(s) installed in or attached to a building or structure shall be bonded to the service equipment enclosure, the grounded conductor at the service, the grounding electrode conductor where of sufficient size, or to the one or more grounding electrodes used. The bonding jumper(s) shall be sized in accordance with Table 2.50.3.17 except as permitted in 2.50.5.15(a)(2) and (a)(3).
(2) Buildings of Multiple Occupancy. In buildings of multiple occupancy where the metal water piping system(s) installed in or attached to a building or structure for the individual occupancies is metallically isolated from all other occupancies by use of nonmetallic water piping, the metal water piping system(s) for each occupancy shall be permitted to be bonded to the equipment grounding terminal of the panelboard or switchboard enclosure (other than service equipment) supplying that occupancy. The bonding jumper shall be sized in accordance with Table 2.50.6.13.
(3) Multiple Buildings or Structures Supplied by a Feeder(s) or Branch Circuit(s). The metal water piping system(s) installed in or attached to a building or structure shall be bonded to the building or structure disconnecting means enclosure where located at the building or structure, to the equipment grounding conductor run with the supply conductors, or to the one or more grounding electrodes used. The bonding jumper(s) shall be sized in accordance with 2.50.3.17, based on the size of the feeder or branch circuit conductors that supply the building. The bonding jumper shall not be required to be larger than the largest ungrounded feeder or branch circuit conductor supplying the building.

(b) Other Metal Piping. Where installed in or attached to a building or structure, metal piping system(s), including gas piping, that is likely to become energized shall be bonded to the service equipment enclosure, the grounded conductor at the service, the grounding electrode conductor where of sufficient size, or to the one or more grounding electrodes used. The bonding jumper(s) shall be sized in accordance with 2.50.6.13, using the rating of the circuit that is likely to energize the piping system(s). The equipment grounding conductor for the circuit that is likely to energize the piping shall be permitted to serve as the bonding means. The points of attachment of the bonding jumper(s) shall be accessible.

FPN: Bonding all piping and metal air ducts within the premises will provide additional safety.

(c) Structural Metal. Exposed structural metal that is interconnected to form a metal building frame and is not intentionally grounded and is likely to become energized shall be bonded to the service equipment enclosure, the grounded conductor at the service, the grounding electrode conductor where of sufficient size, or the one or more grounding electrodes used. The bonding jumper(s) shall be sized in accordance with Table 2.50.3.17 and installed in accordance with 2.50.3.15(a), (b), and (e). The points of attachment of the bonding jumper(s) shall be accessible.

(d) Separately Derived Systems. Metal water piping systems and structural metal that is interconnected to form a building frame shall be bonded to separately derived systems in accordance with (d)(1) through (d)(3).

(1) Metal Water Piping System(s). The grounded conductor of each separately derived system shall be bonded to the nearest available point of the metal water piping system(s) in the area served by each separately derived system. This connection shall be made at the same point on the separately derived system where the grounding electrode conductor is connected. Each bonding jumper shall be sized in accordance with Table 2.50.3.17 based on the largest ungrounded conductor of the separately derived system.

Exception No. 1: A separate bonding jumper to the metal water piping system shall not be required where the metal water piping system is used as the grounding electrode for the separately derived system.

Exception No. 2: A separate water piping bonding jumper shall not be required where the metal frame of a building or structure is used as the grounding electrode for a separately derived system and is bonded to the metal water piping in the area served by the separately derived system.

(2) Structural Metal. Where exposed structural metal that is interconnected to form the building frame exists in the area served by the separately derived system, it shall be bonded to the grounded conductor of each separately derived system. This connection shall be made at the same point on the separately derived system where the grounding electrode conductor is connected. Each bonding jumper shall be sized in accordance with Table 2.50.3.17 based on the largest ungrounded conductor of the separately derived system.

Exception No. 1: A separate bonding jumper to the building structural metal shall not be required where the metal frame of a building or structure is used as the grounding electrode for the separately derived system.

Exception No. 2: A separate bonding jumper to the building structural metal shall not be required where the water piping of a building or structure is used as the grounding electrode for a separately derived system and is bonded to the building structural metal in the area served by the separately derived system.

(3) Common Grounding Electrode Conductor. Where a common grounding electrode conductor is installed for multiple separately derived systems as permitted by 2.50.2.11(a)(4), and exposed structural metal that is interconnected to form the building frame or interior metal piping exists in the area served by the separately derived system, the metal piping and the structural metal member shall be bonded to the common grounding electrode conductor.

Exception: A separate bonding jumper from each derived system to metal water piping and to structural metal members shall not be required where the metal water piping and the structural metal members in the area served by the separately derived system are bonded to the common grounding electrode conductor.

2.50.5.17 Lightning Protection Systems.

The lightning protection system ground terminals shall be bonded to the building or structure grounding electrode system.

FPN No. 1: See 2.50.3.11 for use of air terminals. For further information, see NFPA 780-2004, Standard for the Installation of Lightning Protection Systems, which contains detailed information on grounding, bonding, and spacing from lightning protection systems.

FPN No. 2: Metal raceways, enclosures, frames, and other non–current-carrying metal parts of electric equipment installed on a building equipped with a lightning protection system may require bonding or spacing from the lightning protection conductors in accordance with NFPA 780-2004, Standard for the Installation of Lightning Protection Systems. Separation from lightning protection conductors is typically 1 800 mm through air or 900 mm through dense materials such as concrete, brick, or wood.

2.50.6 Equipment Grounding and Equipment Grounding Conductors

2.50.6.1 Equipment Fastened in Place or Connected by Permanent Wiring Methods (Fixed).

Exposed non–current-carrying metal parts of fixed equipment likely to become energized shall be grounded under any of the following conditions:

(1) Where within 2 400 mm vertically or 1 500 mm horizontally of ground or grounded metal objects and subject to contact by persons

(2) Where located in a wet or damp location and not isolated

(3) Where in electrical contact with metal

(4) Where in a hazardous (classified) location as covered by Articles 5.0 through 5.17

(5) Where supplied by a metal-clad, metal-sheathed, metal- raceway, or other wiring method that provides an equipment ground, except as permitted by 2.50.4.7, Exception No. 2, for short sections of metal enclosures

(6) Where equipment operates with any terminal at over 150 volts to ground

Exception No. 1: Metal frames of electrically heated appliances, exempted by special permission, in which case the frames shall be permanently and effectively insulated from ground.

Exception No. 2: Distribution apparatus, such as transformer and capacitor cases, mounted on wooden poles, at a height exceeding 2 400 mm above ground or grade level.

Exception No. 3: Listed equipment protected by a system of double insulation, or its equivalent, shall not be required to be grounded. Where such a system is employed, the equipment shall be distinctively marked.

2.50.6.3 Fastened in Place or Connected by Permanent Wiring Methods (Fixed) — Specific.

Exposed, non–current-carrying metal parts of the kinds of equipment described in 2.50.6.3(a) through (k), and non–current-carrying metal parts of equipment and enclosures described in 2.50.6.3(l) and (m), shall be grounded regardless of voltage.

(a) Switchboard Frames and Structures. Switchboard frames and structures supporting switching equipment, except frames of 2-wire dc switchboards where effectively insulated from ground.

(b) Pipe Organs. Generator and motor frames in an electrically operated pipe organ, unless effectively insulated from ground and the motor driving it.

(c) Motor Frames. Motor frames, as provided by 4.30.13.2.

(d) Enclosures for Motor Controllers. Enclosures for motor controllers unless attached to ungrounded portable equipment.

(e) Elevators and Cranes. Electric equipment for elevators and cranes.

(f) Garages, Theaters, and Motion Picture Studios. Electric equipment in commercial garages, theaters, and motion picture studios, except pendant lampholders supplied by circuits not over 250 volts to ground.

(g) Electric Signs. Electric signs, outline lighting, and associated equipment as provided in Article 6.0.

(h) Motion Picture Projection Equipment. Motion picture projection equipment.

(i) Power-Limited Remote-Control, Signaling, and Fire Alarm Circuits. Equipment supplied by Class 1 power-limited circuits and Class 1, Class 2, and Class 3 remote-control and signaling circuits, and by fire alarm circuits, shall be grounded where system grounding is required by Part 2.50.2 or Part 2.50.8.

(j) Luminaires (Lighting Fixtures). Luminaires (lighting fixtures) as provided in Part 4.10.5.

(k) Skid Mounted Equipment. Permanently mounted electrical equipment and skids shall be grounded with an equipment bonding jumper sized as required by 2.50.6.13.

(l) Motor-Operated Water Pumps. Motor-operated water pumps, including the submersible type.

(m) Metal Well Casings. Where a submersible pump is used in a metal well casing, the well casing shall be bonded to the pump circuit equipment grounding conductor.

2.50.6.5 Equipment Connected by Cord and Plug.

Under any of the conditions described in 2.50.6.5(1) through (4), exposed non–current- carrying metal parts of cord-and-plug-connected equipment likely to become energized shall be grounded.

Exception: Listed tools, listed appliances, and listed equipment covered in 2.50.6.5(2) through (4) shall not be required to be grounded where protected by a system of double insulation or its equivalent. Double insulated equipment shall be distinctively marked.

(1) In hazardous (classified) locations (see Articles 5.0 through 5.17)

(2) Where operated at over 150 volts to ground

Exception No. 1: Motors, where guarded, shall not be required to be grounded.

Exception No. 2: Metal frames of electrically heated appliances, exempted by special permission, shall not be required to be grounded, in which case the frames shall be permanently and effectively insulated from ground.

(3) In residential occupancies:

a. Refrigerators, freezers, and air conditioners
b. Clothes-washing, clothes-drying, dish-washing machines; kitchen waste disposers; information technology equipment; sump pumps and electrical aquarium equipment
c. Hand-held motor-operated tools, stationary and fixed motor- operated tools, light industrial motor-operated tools
d. Motor-operated appliances of the following types: hedge clippers, lawn mowers, snow blowers, and wet scrubbers
e. Portable handlamps

(4) In other than residential occupancies:

a. Refrigerators, freezers, and air conditioners
b. Clothes-washing, clothes-drying, dish-washing machines; information technology equipment; sump pumps and electrical aquarium equipment
c. Hand-held motor-operated tools, stationary and fixed motor- operated tools, light industrial motor-operated tools
d. Motor-operated appliances of the following types: hedge clippers, lawn mowers, snow blowers, and wet scrubbers
e. Portable handlamps
f. Cord-and-plug-connected appliances used in damp or wet locations or by persons standing on the ground or on metal floors or working inside of metal tanks or boilers
g. Tools likely to be used in wet or conductive locations

Exception: Tools and portable handlamps likely to be used in wet or conductive locations shall not be required to be grounded where supplied through an isolating transformer with an ungrounded secondary of not over 50 volts.

2.50.6.7 Nonelectric Equipment.

The metal parts of nonelectric equipment described in this section shall be grounded.

(1) Frames and tracks of electrically operated cranes and hoists

(2) Frames of nonelectrically driven elevator cars to which electric conductors are attached

(3) Hand-operated metal shifting ropes or cables of electric elevators

FPN: Where extensive metal in or on buildings may become energized and is subject to personal contact, adequate bonding and grounding will provide additional safety.

2.50.6.9 Types of Equipment Grounding Conductors.

The equipment grounding conductor run with or enclosing the circuit conductors shall be one or more or a combination of the following:

(1) A copper, aluminum, or copper-clad aluminum conductor. This conductor shall be solid or stranded; insulated, covered, or bare; and in the form of a wire or a busbar of any shape.

(2) Rigid metal conduit.

(3) Intermediate metal conduit.

(4) Electrical metallic tubing.

(5) Listed flexible metal conduit meeting all the following conditions:

a. The conduit is terminated in fittings listed for grounding.
b. The circuit conductors contained in the conduit are protected by overcurrent devices rated at 20 amperes or less.
c. The combined length of flexible metal conduit and flexible metallic tubing and liquidtight flexible metal conduit in the same ground return path does not exceed 1 800 mm.
d. Where used to connect equipment where flexibility is necessary after installation, an equipment grounding conductor shall be installed.

(6) Listed liquidtight flexible metal conduit meeting all the following conditions:

a. The conduit is terminated in fittings listed for grounding.
b. For metric designators 12 through 16 (trade sizes through ½), the circuit conductors contained in the conduit are protected by overcurrent devices rated at 20 amperes or less.
c. For metric designators 21 through 35 (trade sizes ¾ through 1¼), the circuit conductors contained in the conduit are protected by overcurrent devices rated not more than 60 amperes and there is no flexible metal conduit, flexible metallic tubing, or liquidtight flexible metal conduit in trade sizes metric designators 12 through 16 (trade sizes through ½) in the grounding path.
d. The combined length of flexible metal conduit and flexible metallic tubing and liquidtight flexible metal conduit in the same ground return path does not exceed 1 800 mm.
e. Where used to connect equipment where flexibility is necessary after installation, an equipment grounding conductor shall be installed.

(7) Flexible metallic tubing where the tubing is terminated in fittings listed for grounding and meeting the following conditions:

a. The circuit conductors contained in the tubing are protected by overcurrent devices rated at 20 amperes or less.
b. The combined length of flexible metal conduit and flexible metallic tubing and liquidtight flexible metal conduit in the same ground return path does not exceed 1 800 mm.

(8) Armor of Type AC cable as provided in 3.20.3.9.

(9) The copper sheath of mineral-insulated, metal-sheathed cable.

(10) Type MC cable where listed and identified for grounding in accordance with the following:

a. The combined metallic sheath and grounding conductor of interlocked metal tape–type MC cable
b. The metallic sheath or the combined metallic sheath and grounding conductors of the smooth or corrugated tube type MC cable

(11) Cable trays as permitted in 3.92.1.3(c) and 3.92.1.7.

(12) Cablebus framework as permitted in 3.70.1.3.

(13) Other listed electrically continuous metal raceways and listed auxiliary gutters.

(14) Surface metal raceways listed for grounding.

2.50.6.10 Identification of Equipment Grounding Conductors.

Unless required elsewhere in this Code, equipment grounding conductors shall be permitted to be bare, covered, or insulated. Individually covered or insulated equipment grounding conductors shall have a continuous outer finish that is either green or green with one or more yellow stripes except as permitted in this section.

Conductors with insulation or individual covering that is green, green with one or more yellow stripes, or otherwise identified as permitted by this section shall not be used for ungrounded or grounded circuit conductors.

(a) Conductors Larger Than 14 mm2. Equipment grounding conductors larger than 14 mm2 shall comply with 2.50.6.10(a)(1) and (a)(2).

(1) An insulated or covered conductor larger than 14 mm2 shall be permitted, at the time of installation, to be permanently identified as an equipment grounding conductor at each end and at every point where the conductor is accessible.

Exception: Conductors larger than 14 mm2 shall not be required to be marked in conduit bodies that contain no splices or unused hubs.

(2) Identification shall encircle the conductor and shall be accomplished by one of the following:
a. Stripping the insulation or covering from the entire exposed length
b. Coloring the exposed insulation or covering green
c. Marking the exposed insulation or covering with green tape or green adhesive labels

(b) Multiconductor Cable. Where the conditions of maintenance and supervision ensure that only licensed electrical practitioner or non licensed electrical practitioner under the supervision of a licensed electrical practitioner service the installation, one or more insulated conductors in a multiconductor cable, at the time of installation, shall be permitted to be permanently identified as equipment grounding conductors at each end and at every point where the conductors are accessible by one of the following means:

(1) Stripping the insulation from the entire exposed length
(2) Coloring the exposed insulation green
(3) Marking the exposed insulation with green tape or green adhesive labels

(c) Flexible Cord. An uninsulated equipment grounding conductor shall be permitted, but, if individually covered, the covering shall have a continuous outer finish that is either green or green with one or more yellow stripes.

2.50.6.11 Equipment Grounding Conductor Installation.

An equipment grounding conductor shall be installed in accordance with 2.50.6.11(a), (b), and (c).

(a) Raceway, Cable Trays, Cable Armor, Cablebus, or Cable Sheaths. Where it consists of a raceway, cable tray, cable armor, cablebus framework, or cable sheath or where it is a wire within a raceway or cable, it shall be installed in accordance with the applicable provisions in this Code using fittings for joints and terminations approved for use with the type raceway or cable used. All connections, joints, and fittings shall be made tight using suitable tools.

(b) Aluminum and Copper-Clad Aluminum Conductors. Equipment grounding conductors of bare or insulated aluminum or copper-clad aluminum shall be permitted. Bare conductors shall not come in direct contact with masonry or the earth or where subject to corrosive conditions. Aluminum or copper-clad aluminum conductors shall not be terminated within 450 mm of the earth.

(c) Equipment Grounding Conductors Smaller Than 14 mm2. Equipment grounding conductors smaller than 14 mm2 shall be protected from physical damage by a raceway or cable armor except where run in hollow spaces of walls or partitions, where not subject to physical damage, or where protected from physical damage.

2.50.6.13 Size of Equipment Grounding Conductors.

(a) General. Copper, aluminum, or copper-clad aluminum equipment grounding conductors of the wire type shall not be smaller than shown in Table 2.50.6.13 but shall not be required to be larger than the circuit conductors supplying the equipment. Where a raceway or a cable armor or sheath is used as the equipment grounding conductor, as provided in 2.50.6.9 and 2.50.7.5(a), it shall comply with 2.50.1.4(a)(5) or (b)(4).

(b) Increased in Size. Where ungrounded conductors are increased in size, equipment grounding conductors, where installed, shall be increased in size proportionately according to circular mil area of the ungrounded conductors.

(c) Multiple Circuits. Where a single equipment grounding conductor is run with multiple circuits in the same raceway or cable, it shall be sized for the largest overcurrent device protecting conductors in the raceway or cable.

(d) Motor Circuits. Where the overcurrent device consists of an instantaneous trip circuit breaker or a motor short-circuit protector, as allowed in 4.30.4.2, the equipment grounding conductor size shall be permitted to be based on the rating of the motor overload protective device but shall not be less than the size shown in Table 2.50.6.13.

(e) Flexible Cord and Fixture Wire. The equipment grounding conductor in a flexible cord with the largest circuit conductor 5.5 mm2 (2.6 mm dia.) or smaller, and the equipment grounding conductor used with fixture wires of any size in accordance with 2.40.1.5, shall not be smaller than 18 AWG copper and shall not be smaller than the circuit conductors. The equipment grounding conductor in a flexible cord with a circuit conductor larger than 5.5 mm2 (2.6 mm dia.) shall be sized in accordance with Table 2.50.6.13.

(f) Conductors in Parallel. Where conductors are run in parallel in multiple raceways or cables as permitted in 3.10.1.4, the equipment grounding conductors, where used, shall be run in parallel in each raceway or cable. One of the methods in 2.50.6.13(f)(1) or (f)(2) shall be used to ensure the equipment grounding conductors are protected.

(1) Based on Rating of Overcurrent Protective Device. Each parallel equipment grounding conductor shall be sized on the basis of the ampere rating of the overcurrent device protecting the circuit conductors in the raceway or cable in accordance with Table 2.50.6.13.
(2) Ground-Fault Protection of Equipment Installed. Where ground-fault protection of equipment is installed, each parallel equipment grounding conductor in a multiconductor cable shall be permitted to be sized in accordance with Table 2.50.6.13 on the basis of the trip rating of the ground-fault protection where the following conditions are met:
a. Conditions of maintenance and supervision ensure that only licensed electrical practitioner or non licensed electrical practitioner under the supervision of a licensed electrical practitioner will service the installation.
b. The ground-fault protection equipment is set to trip at not more than the ampacity of a single ungrounded conductor of one of the cables in parallel.
c. The ground-fault protection is listed for the purpose of protecting the equipment grounding conductor.

(g) Feeder Taps. Equipment grounding conductors run with feeder taps shall not be smaller than shown in Table 2.50.6.13 based on the rating of the overcurrent device ahead of the feeder but shall not be required to be larger than the tap conductors.

2.50.6.15 Equipment Grounding Conductor Continuity.

(a) Separable Connections. Separable connections such as those provided in drawout equipment or attachment plugs and mating connectors and receptacles shall provide for first-make, last-break of the equipment grounding conductor. First-make, last-break shall not be required where interlocked equipment, plugs, receptacles, and connectors preclude energization without grounding continuity.

(b) Switches. No automatic cutout or switch shall be placed in the equipment grounding conductor of a premises wiring system unless the opening of the cutout or switch disconnects all sources of energy.

Table 2.50.6.13 Minimum Size Equipment Grounding Conductors for Grounding Raceway and Equipment
Rating or Setting of Automatic Overcurrent Device in Circuit Ahead of Equipment, Conduit, etc., Not Exceeding (Amperes) Size mm2 (mm dia.)
Copper Copper Aluminum or Copper-Clad Aluminum*
15 2.0(1.6) 3.5(2.0)
20 3.5(2.0) 5.5(2.6)
30 5.5(2.6) 8.0(3.2)
40 5.5(2.6) 8.0(3.2)
60 5.5(2.6) 8.0(3.2)
100 8.0(3.2) 14
200 14 22
300 22 30
400 30 38
500 30 50
600 38 60
800 50 80
1000 60 100
1200 80 125
1600 100 175
2000 125 200
2500 175 325
3000 200 325
4000 250 800
5000 700 1200
6000 800 1200
Note: Where necessary to comply with 2.50.1.4(a)(5) or (b)(4), the equipment grounding conductor shall be sized larger than given in this table.
* See installation restrictions in 2.50.6.11.

b) Switches. No automatic cutout or switch shall be placed in the equipment grounding conductor of a premises wiring system unless the opening of the cutout or switch disconnects all sources of energy.

2.50.6.17 Identification of Wiring Device Terminals.

The terminal for the connection of the equipment grounding conductor shall be identified by one of the following:

(1) A green, not readily removable terminal screw with a hexagonal head.

(2) A green, hexagonal, not readily removable terminal nut.

(3) A green pressure wire connector. If the terminal for the grounding conductor is not visible, the conductor entrance hole shall be marked with the word green or ground, the letters G or GR, a grounding symbol, or otherwise identified by a distinctive green color. If the terminal for the equipment grounding conductor is readily removable, the area adjacent to the terminal shall be similarly marked.

FPN: See FPN Figure 2.50.6.17.

FPN Figure 2.50.6.17 One Example of a Symbol Used to Identify the Grounding Termination Point for an Equipment Grounding Conductor.

2.50.7 Methods of Equipment Grounding

2.50.7.1 Equipment Grounding Conductor Connections.

Equipment grounding conductor connections at the source of separately derived systems shall be made in accordance with 2.50.2.11(a)(1). Equipment grounding conductor connections at service equipment shall be made as indicated in 2.50.7.1(a) or (b). For replacement of non–grounding-type receptacles with grounding-type receptacles and for branch-circuit extensions only in existing installations that do not have an equipment grounding conductor in the branch circuit, connections shall be permitted as indicated in 2.50.7.1(c).

(a) For Grounded Systems. The connection shall be made by bonding the equipment grounding conductor to the grounded service conductor and the grounding electrode conductor.

(b) For Ungrounded Systems. The connection shall be made by bonding the equipment grounding conductor to the grounding electrode conductor.

(c) Nongrounding Receptacle Replacement or Branch Circuit Extensions. The equipment grounding conductor of a grounding-type receptacle or a branch-circuit extension shall be permitted to be connected to any of the following:

(1) Any accessible point on the grounding electrode system as described in 2.50.3.1
(2) Any accessible point on the grounding electrode conductor
(3) The equipment grounding terminal bar within the enclosure where the branch circuit for the receptacle or branch circuit originates
(4) For grounded systems, the grounded service conductor within the service equipment enclosure
(5) For ungrounded systems, the grounding terminal bar within the service equipment enclosure

FPN: See 4.6.1.3(d) for the use of a ground-fault circuit-interrupting type of receptacle.

2.50.7.3 Short Sections of Raceway.

Isolated sections of metal raceway or cable armor, where required to be grounded, shall be grounded in accordance with 2.50.7.5.

2.50.7.5 Equipment Fastened in Place or Connected by Permanent Wiring Methods (Fixed) — Grounding.

Unless grounded by connection to the grounded circuit conductor as permitted by 2.50.2.13, 2.50.7.11, and 2.50.7.13, non–current-carrying metal parts of equipment, raceways, and other enclosures, if grounded, shall be grounded by one of the following methods.

(a) Equipment Grounding Conductor Types. By any of the equipment grounding conductors permitted by 2.50.6.9.

(b) With Circuit Conductors. By an equipment grounding conductor contained within the same raceway, cable, or otherwise run with the circuit conductors.

Exception No. 1: As provided in 2.50.7.1(c), the equipment grounding conductor shall be permitted to be run separately from the circuit conductors.

Exception No. 2: For dc circuits, the equipment grounding conductor shall be permitted to be run separately from the circuit conductors.

FPN No. 1: See 2.50.5.13 and 2.50.8.9 for equipment bonding jumper requirements.

FPN No. 2: See 4.0.1.7 for use of cords for fixed equipment. 2.50.7.7 Equipment Considered Effectively Grounded. Under the conditions specified in 2.50.7.7(a) and (b), the non–current-carrying metal parts of the equipment shall be considered effectively grounded.

(a) Equipment Secured to Grounded Metal Supports. Electrical equipment secured to and in electrical contact with a metal rack or structure provided for its support and grounded by one of the means indicated in 2.50.7.5. The structural metal frame of a building shall not be used as the required equipment grounding conductor for ac equipment.

(b) Metal Car Frames. Metal car frames supported by metal hoisting cables attached to or running over metal sheaves or drums of elevator machines that are grounded by one of the methods indicated in 2.50.7.5.

2.50.7.9 Cord-and-Plug-Connected Equipment.

Non–current- carrying metal parts of cord-and-plug-connected equipment, if grounded, shall be grounded by one of the methods in 2.50.7.9(a) or (b).

(a) By Means of an Equipment Grounding Conductor. By means of an equipment grounding conductor run with the power supply conductors in a cable assembly or flexible cord properly terminated in a grounding-type attachment plug with one fixed grounding contact.

Exception: The grounding contacting pole of grounding-type plug-in ground-fault circuit interrupters shall be permitted to be of the movable, self-restoring type on circuits operating at not over 250 volts between any two conductors or over 250 volts between any conductor and ground

(b) By Means of a Separate Flexible Wire or Strap. By means of a separate flexible wire or strap, insulated or bare, protected as well as practicable against physical damage, where part of equipment.

2.50.7.11 Frames of Ranges and Clothes Dryers.

Frames of electric ranges, wall-mounted ovens, counter-mounted cooking units, clothes dryers, and outlet or junction boxes that are part of the circuit for these appliances shall be grounded in the manner specified by 2.50.7.5 or 2.50.7.9.

Exception: For existing branch circuit installations only where an equipment grounding conductor is not present in the outlet or junction box, the frames of electric ranges, wall-mounted ovens, counter- mounted cooking units, clothes dryers, and outlet or junction boxes that are part of the circuit for these appliances shall be permitted to be grounded to the grounded circuit conductor if all the following conditions are met:

(1) The supply circuit is 230-volt, single-phase, 2-wire; or 400Y/230-volt derived from a 3-phase, 4-wire, wye-connected system.
(2) The grounded conductor is not smaller than 5.5 mm2 (2.6 mm dia.) copper or 8.0 mm2 (3.2 mm dia.) aluminum.
(3) The grounded conductor is insulated, or the grounded conductor is uninsulated and part of a Type SE service-entrance cable and the branch circuit originates at the service equipment.
(4) Grounding contacts of receptacles furnished as part of the equipment are bonded to the equipment.

2.50.7.13 Use of Grounded Circuit Conductor for Grounding Equipment.

(a) Supply-Side Equipment. A grounded circuit conductor shall be permitted to ground non–current-carrying metal parts of equipment, raceways, and other enclosures at any of the following locations:

(1) On the supply side or within the enclosure of the ac service- disconnecting means
(2) On the supply side or within the enclosure of the main disconnecting means for separate buildings as provided in 2.50.2.13(b)
(3) On the supply side or within the enclosure of the main disconnecting means or overcurrent devices of a separately derived system where permitted by 2.50.2.11(a)(1)

(b) Load-Side Equipment. Except as permitted in 2.50.2.11(a)(1) and 2.50.2.13(b), a grounded circuit conductor shall not be used for grounding non–current-carrying metal parts of equipment on the load side of the service disconnecting means or on the load side of a separately derived system disconnecting means or the overcurrent devices for a separately derived system not having a main disconnecting means.

Exception No. 1: The frames of ranges, wall-mounted ovens, counter- mounted cooking units, and clothes dryers under the conditions permitted for existing installations by 2.50.7.11 shall be permitted to be grounded by a grounded circuit conductor.

Exception No. 2: It shall be permissible to ground meter enclosures by connection to the grounded circuit conductor on the load side of the service disconnect where all of the following conditions apply:

(1) No service ground-fault protection is installed.
(2) All meter socket enclosures are located immediately adjacent to the service disconnecting means.
(3) The size of the grounded circuit conductor is not smaller than the size specified in Table 2.50.6.13 for equipment grounding conductors.

Exception No. 3: Direct-current systems shall be permitted to be grounded on the load side of the disconnecting means or overcurrent device in accordance with 2.50.8.5.

Exception No. 4: Electrode-type boilers operating at over 600 volts shall be grounded as required in 4.90.5.3(e)(1) and 4.90.5.5.

2.50.7.15 Multiple Circuit Connections.

Where equipment is required to be grounded and is supplied by separate connection to more than one circuit or grounded premises wiring system, a means for grounding shall be provided for each such connection as specified in 2.50.7.5 and 2.50.7.9.

2.50.7.17 Connecting Receptacle Grounding Terminal to Box.

An equipment bonding jumper shall be used to connect the grounding terminal of a grounding-type receptacle to a grounded box unless grounded as in 2.50.7.17(a) through (d).

(a) Surface Mounted Box. Where the box is mounted on the surface, direct metal-to-metal contact between the device yoke and the box or a contact yoke or device that complies with 2.50.7.17(b) shall be permitted to ground the receptacle to the box. At least one of the insulating washers shall be removed from receptacles that do not have a contact yoke or device that complies with 2.50.7.17(b) to ensure direct metal-to-metal contact. This provision shall not apply to cover- mounted receptacles unless the box and cover combination are listed as providing satisfactory ground continuity between the box and the receptacle.

(b) Contact Devices or Yokes. Contact devices or yokes designed and listed as self-grounding shall be permitted in conjunction with the supporting screws to establish the grounding circuit between the device yoke and flush-type boxes.

(c) Floor Boxes. Floor boxes designed for and listed as providing satisfactory ground continuity between the box and the device shall be permitted.

(d) Isolated Receptacles. Where required for the reduction of electrical noise (electromagnetic interference) on the grounding circuit, a receptacle in which the grounding terminal is purposely insulated from the receptacle mounting means shall be permitted. The receptacle grounding terminal shall be grounded by an insulated equipment grounding conductor run with the circuit conductors. This grounding conductor shall be permitted to pass through one or more panelboards without connection to the panelboard grounding terminal as permitted in 4.8.3.11, Exception, so as to terminate within the same building or structure directly at an equipment grounding conductor terminal of the applicable derived system or service.

FPN: Use of an isolated equipment grounding conductor does not relieve the requirement for grounding the raceway system and outlet box.

2.50.7.19 Continuity and Attachment of Equipment Grounding Conductors to Boxes.

Where circuit conductors are spliced within a box, or terminated on equipment within or supported by a box, any equipment grounding conductor(s) associated with those circuit conductors shall be spliced or joined within the box or to the box with devices suitable for the use in accordance with 2.50.7.19(a) through (e).

Exception: The equipment grounding conductor permitted in 2.50.7.17(d) shall not be required to be connected to the other equipment grounding conductors or to the box.

(a) Connections. Connections and splices shall be made in accordance with 110.14(b) except that insulation shall not be required.

(b) Grounding Continuity. The arrangement of grounding connections shall be such that the disconnection or the removal of a receptacle, luminaire (fixture), or other device fed from the box does not interfere with or interrupt the grounding continuity.

(c) Metal Boxes. A connection shall be made between the one or more equipment grounding conductors and a metal box by means of a grounding screw that shall be used for no other purpose or a listed grounding device.

(d) Nonmetallic Boxes. One or more equipment grounding conductors brought into a nonmetallic outlet box shall be arranged such that a connection can be made to any fitting or device in that box requiring grounding.

(e) Solder. Connections depending solely on solder shall not be used.

2.50.8 Direct-Current Systems

2.50.8.1 General.

Direct-current systems shall comply with Part 2.50.8 and other sections of Article 2.50 not specifically intended for11 ac systems.

2.50.8.3 Direct-Current Circuits and Systems to Be Grounded.

Direct-current circuits and systems shall be grounded as provided for in 2.50.8.3(a) and (b).

(a) Two-Wire, Direct-Current Systems. A 2-wire, dc system supplying premises wiring and operating at greater than 50 volts but not greater than 300 volts shall be grounded.

Exception No. 1: A system equipped with a ground detector and supplying only industrial equipment in limited areas shall not be required to be grounded.

Exception No. 2: A rectifier-derived dc system supplied from an ac system complying with 2.50.2.1 shall not be required to be grounded.

Exception No. 3: Direct-current fire alarm circuits having a maximum current of 0.030 amperes as specified in Part 7.60.3 shall not be required to be grounded.

(b) Three-Wire, Direct-Current Systems. The neutral conductor of all 3-wire, dc systems supplying premises wiring shall be grounded.

2.50.8.5 Point of Connection for Direct-Current Systems.

(a) Off-Premises Source. Direct-current systems to be grounded and supplied from an off-premises source shall have the grounding connection made at one or more supply stations. A grounding connection shall not be made at individual services or at any point on the premises wiring.

(b) On-Premises Source. Where the dc system source is located on the premises, a grounding connection shall be made at one of the following:

(1) The source
(2) The first system disconnection means or overcurrent device
(3) By other means that accomplish equivalent system protection and that utilize equipment listed and identified for the use 2.50.8.7 Size of Direct-Current Grounding Electrode Conductor. The size of the grounding electrode conductor for a dc system shall be as specified in 2.50.8.7(a) through (e).

(a) Not Smaller Than the Neutral Conductor. Where the dc system consists of a 3-wire balancer set or a balancer winding with overcurrent protection as provided in 445.12(d), the grounding electrode conductor shall not be smaller than the neutral conductor and not smaller than 8.0 mm2 (3.2 mm dia.) copper or 14 mm2 aluminum.

(b) Not Smaller Than the Largest Conductor. Where the dc system is other than as in 2.50.8.7(a), the grounding electrode conductor shall not be smaller than the largest conductor supplied by the system, and not smaller than 8.0 mm2 (3.2 mm dia.) copper or 14 mm2 aluminum.

(c) Connected to Rod, Pipe, or Plate Electrodes. Where connected to rod, pipe, or plate electrodes as in 2.50.3.3(a)(5) or 2.50.3.3(a)(6), that portion of the grounding electrode conductor that is the sole connection to the grounding electrode shall not be required to be larger than 14 mm2 copper wire or 22 mm2 aluminum wire.

(d) Connected to a Concrete-Encased Electrode. Where connected to a concrete-encased electrode as in 2.50.3.3(a)(3), that portion of the grounding electrode conductor that is the sole connection to the grounding electrode shall not be required to be larger than 22 mm2 copper wire.

(e) Connected to a Ground Ring. Where connected to a ground ring as in 2.50.3.3(a)(4), that portion of the grounding electrode conductor that is the sole connection to the grounding electrode shall not be required to be larger than the conductor used for the ground ring.

2.50.8.9 Direct-Current Bonding Jumper.

For dc systems, the size of the bonding jumper shall not be smaller than the system grounding electrode conductor specified in 2.50.8.7.

2.50.8.10 Ungrounded Direct-Current Separately Derived Systems.

Except as otherwise permitted in 2.50.2.15 for portable and vehicle-mounted generators, an ungrounded dc separately derived system supplied from a stand-alone power source (such as an engine– generator set) shall have a grounding electrode conductor connected to an electrode that complies with Part 2.50.3 to provide for grounding of metal enclosures, raceways, cables, and exposed non–current-carrying metal parts of equipment. The grounding electrode conductor connection shall be to the metal enclosure at any point on the separately derived system from the source to the first system disconnecting means or overcurrent device, or it shall be made at the source of a separately derived system that has no disconnecting means or overcurrent devices.

The size of the grounding electrode conductor shall be in accordance with 2.50.8.7.

2.50.9 Instruments, Meters, and Relays

2.50.9.1 Instrument Transformer Circuits.

Secondary circuits of current and potential instrument transformers shall be grounded where the primary windings are connected to circuits of 300 volts or more to ground and, where on switchboards, shall be grounded irrespective of voltage.

Exception: Circuits where the primary windings are connected to circuits of less than 1000 volts with no live parts or wiring exposed or accessible to other than licensed electrical practitioner or non licensed electrical practitioner under the supervision of a licensed electrical practitioner.

2.50.9.3 Instrument Transformer Cases.

Cases or frames of instrument transformers shall be grounded where accessible to other than licensed electrical practitioner or non licensed electrical practitioner under the supervision of a licensed electrical practitioner.

Exception: Cases or frames of current transformers, the primaries of which are not over 150 volts to ground and that are used exclusively to supply current to meters.

2.50.9.5 Cases of Instruments, Meters, and Relays Operating at Less Than 1000 Volts.

Instruments, meters, and relays operating with windings or working parts at less than 1000 volts shall be grounded as specified in 2.50.9.5(a), (b), or (c).

(a) Not on Switchboards. Instruments, meters, and relays not located on switchboards, operating with windings or working parts at 300 volts or more to ground, and accessible to other than licensed electrical practitioner or non licensed electrical practitioner under the supervision of a licensed electrical practitioner, shall have the cases and other exposed metal parts grounded.

(b) On Dead-Front Switchboards. Instruments, meters, and relays (whether operated from current and potential transformers or connected directly in the circuit) on switchboards having no live parts on the front of the panels shall have the cases grounded.

(c) On Live-Front Switchboards. Instruments, meters, and relays (whether operated from current and potential transformers or connected directly in the circuit) on switchboards having exposed live parts on the front of panels shall not have their cases grounded. Mats of insulating rubber or other suitable floor insulation shall be provided for the operator where the voltage to ground exceeds 150.

2.50.9.7 Cases of Instruments, Meters, and Relays — Operating Voltage 1 kV and Over.

Where instruments, meters, and relays have current-carrying parts of 1 kV and over to ground, they shall be isolated by elevation or protected by suitable barriers, grounded metal, or insulating covers or guards. Their cases shall not be grounded.

Exception: Cases of electrostatic ground detectors where the internal ground segments of the instrument are connected to the instrument case and grounded and the ground detector is isolated by elevation.

2.50.9.9 Instrument Grounding Conductor.

The grounding conductor for secondary circuits of instrument transformers and for instrument cases shall not be smaller than 3.5 mm2 (2.0 mm dia.) copper or 5.5 mm2 (2.6 mm dia.) aluminum. Cases of instrument transformers, instruments, meters, and relays that are mounted directly on grounded metal surfaces of enclosures or grounded metal switchboard panels shall be considered to be grounded, and no additional grounding conductor shall be required.

2.50.10.1 General.

Where high-voltage systems are grounded, they shall comply with all applicable provisions of the preceding sections of this article and with 2.50.10.3 through 2.50.10.11, which supplement and modify the preceding sections.

2.50.10.3 Derived Neutral Systems.

A system neutral derived from a grounding transformer shall be permitted to be used for grounding high-voltage systems.

2.50.10.5 Solidly Grounded Neutral Systems.

Solidly grounded neutral systems shall be permitted to be either single point grounded or multigrounded neutral.

(a) Neutral Conductor.

(1) Insulation Level. The minimum insulation level for neutral conductors of solidly grounded systems shall be 600 volts.

Exception No. 1: Bare copper conductors shall be permitted to be used for the neutral of service entrances and the neutral of direct- buried portions of feeders.

Exception No. 2: Bare conductors shall be permitted for the neutral of overhead portions installed outdoors. Exception No. 3: The neutral grounded conductor shall be permitted to be a bare conductor if isolated from phase conductors and protected from physical damage.

FPN: See 2.25.1.4 for conductor covering where within 3 000 mm of any building or other structure.

(2) Ampacity. The neutral conductor shall be of sufficient ampacity for the load imposed on the conductor but not less than 33 percent of the ampacity of the phase conductors.

Exception: In industrial and commercial premises under engineering supervision, it shall be permissible to size the ampacity of the neutral conductor to not less than 20 percent of the ampacity of the phase conductor.

(b) Single Point Grounded System. Where a single point grounded neutral system is used, the following shall apply:

(1) A single point grounded system shall be permitted to be supplied from (a) or (b):
a. A separately derived system
b. A multigrounded neutral system with an equipment grounding conductor connected to the multigrounded neutral at the source of the single point grounded system
(2) A grounding electrode shall be provided for the system.
(3) A grounding electrode conductor shall connect the grounding electrode to the system neutral.
(4) A bonding jumper shall connect the equipment grounding conductor to the grounding electrode conductor.
(5) An equipment bonding conductor shall be provided to each building, structure, and equipment enclosure.
(6) A neutral shall only be required where phase to neutral loads are supplied.
(7) The neutral, where provided, shall be insulated and isolated from earth except at one location.
(8) An equipment grounding conductor shall be run with the phase conductors and shall comply with (a), (b), and (c):
a. Shall not carry continuous load
b. May be bare or insulated
c. Shall have sufficient ampacity for fault current duty

(c) Multigrounded Neutral Systems. Where a multigrounded neutral system is used, the following shall apply:

(1) The neutral of a solidly grounded neutral system shall be permitted to be grounded at more than one point. Grounding shall be permitted at one or more of the following locations:
a. Transformers supplying conductors to a building or other structure
b. Underground circuits where the neutral is exposed
(2) The multigrounded neutral conductor shall be grounded at each transformer and at other additional locations by connection to a made or existing electrode.
(3) At least one grounding electrode shall be installed and connected to the multigrounded neutral circuit conductor every 400 m (1300 ft).
(4) The maximum distance between any two adjacent electrodes shall not be more than 400 m (1300 ft).
(5) In a multigrounded shielded cable system, the shielding shall be grounded at each cable joint that is exposed to personnel contact.

2.50.10.7 Impedance Grounded Neutral Systems.

Impedance grounded neutral systems in which a grounding impedance, usually a resistor, limits the ground-fault current, shall be permitted where all of the following conditions are met:

(1) The conditions of maintenance and supervision ensure that only licensed electrical practitioner or non licensed electrical practitioner under the supervision of a licensed electrical practitioner will service the installation.
(2) Ground detectors are installed on the system.
(3) Line-to-neutral loads are not served. Impedance grounded neutral systems shall comply with the provisions of 2.50.10.7(a) through (d).

(a) Location. The grounding impedance shall be inserted in the grounding conductor between the grounding electrode of the supply system and the neutral point of the supply transformer or generator.

(b) Identified and Insulated. The neutral conductor of an impedance grounded neutral system shall be identified, as well as fully insulated with the same insulation as the phase conductors.

(c) System Neutral Connection. The system neutral shall not be connected to ground, except through the neutral grounding impedance.

(d) Equipment Grounding Conductors. Equipment grounding conductors shall be permitted to be bare and shall be electrically connected to the ground bus and grounding electrode conductor.

2.50.10.9 Grounding of Systems Supplying Portable or Mobile Equipment.

Systems supplying portable or mobile high-voltage equipment, other than substations installed on a temporary basis, shall comply with 2.50.10.9(a) through (f).

(a) Portable or Mobile Equipment. Portable or mobile high- voltage equipment shall be supplied from a system having its neutral grounded through an impedance. Where a delta-connected high- voltage system is used to supply portable or mobile equipment, a system neutral shall be derived.

(b) Exposed Non–Current-Carrying Metal Parts. Exposed non– current-carrying metal parts of portable or mobile equipment shall be connected by an equipment grounding conductor to the point at which the system neutral impedance is grounded.

(c) Ground-Fault Current. The voltage developed between the portable or mobile equipment frame and ground by the flow of maximum ground-fault current shall not exceed 100 volts.

(d) Ground-Fault Detection and Relaying. Ground-fault detection and relaying shall be provided to automatically de-energize any high- voltage system component that has developed a ground fault. The continuity of the equipment grounding conductor shall be continuously monitored so as to de-energize automatically the high-voltage circuit to the portable or mobile equipment upon loss of continuity of the equipment grounding conductor.

(e) Isolation. The grounding electrode to which the portable or mobile equipment system neutral impedance is connected shall be isolated from and separated in the ground by at least 6 000 mm from any other system or equipment grounding electrode, and there shall be no direct connection between the grounding electrodes, such as buried pipe and fence, and so forth.

(f) Trailing Cable and Couplers. High-voltage trailing cable and couplers for interconnection of portable or mobile equipment shall meet the requirements of Part 4.0.3 for cables and 4.90.4.5 for couplers.

2.50.10.11 Grounding of Equipment.

All non–current-carrying metal parts of fixed, portable, and mobile equipment and associated fences, housings, enclosures, and supporting structures shall be grounded.

Exception: Where isolated from ground and located so as to prevent any person who can make contact with ground from contacting such metal parts when the equipment is energized.

Grounding conductors not an integral part of a cable assembly shall not be smaller than 14 mm2 copper or 22 mm2 aluminum.

FPN: See 2.50.6.1, Exception No. 2, for pole-mounted distribution apparatus.