Calculations:Electrical Design Guideline

Electrical Design Calculations Guideline

Figure 1 Power Line

Electrical design calculations guidelines establishes minimum requirements for generating electrical calculations on projects. Electrical calculations should be made for all projects that include electrical components. Design calculations may be made either manually or by computer programs. At a minimum, the following types of calculations should be made where applicable:

* Load calculations

* Conductor sizing

* Conduit sizing

* Motor branch circuit sizing

* Power factor improvement

* Transformer primary and secondary circuit sizing

* Voltage drop

* Motor starting voltage dip

* Short circuit analysis

* Lighting levels

* Grounding in substations where step potentials are of concern

* Harmonic distortion analysis

* Cable pulling calculations

* Generator capability/motor starting.

Basic Requirements for Electrical Calculations

The following are basic requirements for electrical calculations:

1. Non-computer generated calculations must be on standard project calculations sheets.
2. Calculations generated by computer programs must conform with the following procedures:
   1. Include all heading information on each sheet
   2. Insert comments wherever possibly to clarify concepts and actions taken in the computer input
   3. Provide clear documentation of electrical geometry, support conditions, load application, and load requirements
   4. Provide where practicable sketch of model indicting nodes, materials, connectivity, etc.
   5. Provide electronic copy on CD or other suitable device of analysis input and output with hard copy calculations.
   6. Provide manual checks of pertinent results (e.g. service size, main feeder voltage drop) for computer generated output.

Basic Electrical Engineering Formulas

List of Symbols

V - Voltage (volts)

I - Current (amps)

R - Resistance (ohms)

X - Reactance (ohms)

Z - Impedance (ohms)

W Real Power (watts)

θ - Phase angle whose cosine is the power factor

eff - Efficiency

Direct Current (DC) Formulas

Basic Formulas

Resistance

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle R={V \over I}}

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle R={V^2 \over P}}

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle R={P \over I^2}}

Volts

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle V=I \times R}

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle V={P \over I}}

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle V=\sqrt{P \times R}}

Power

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle P=V \times I}

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle P=I^2 \times R}

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle P={V^2 \over R}}

Current

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle I={P \over V}}

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle I={V \over R}}

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle I=\sqrt{P \over R }}

Alternating Current (AC) - Single Phase

Note: V denotes line to neutral voltage.

Basic Formulas

Impedance Z (ohms)

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle Z={V \over I }= \sqrt{R^2+X^2}}

Volts V (volts)

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle V={I \times Z }}

Real Power P (watts)

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle P=V \times I \times cos \phi}

Power Factor

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle pf=cos \phi}

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle pf={P \over S}}

Apparent Power S (volt-ampere)

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle S=V \times I}

Reactive Power Q (volt-ampere-reactive)

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle Q=V \times I \times sin \phi}

Real Power P (watts)

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle P=V \times I \times cos \phi}

Voltage Drop

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle V_d=2 \times (I \times R \times cos \phi + I \times X \times sin \phi)}

where:

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle V_d} = voltage drop

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle cos \phi} = load power factor

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle sin \phi} = load reactive factor

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle X} = reactance

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle R} = resistance

Alternating Current (AC) - Three Phase

Note: V denotes line to neutral voltage.

Basic Formulas

Apparent Power S

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle S=\sqrt {3} \times V \times I}

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle S=\sqrt { P^2 + Q^2}}

Real Power P

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle P=\sqrt{3} \times V \times I \times cos \phi}

Reactive Power Q

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle Q=\sqrt{3}\times V \times I \times sin \phi}

Power Factor pf

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle pf={P \over S}}

Voltage Drop

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle V_d=\sqrt{3} \times (I \times R \times cos \phi + I \times X \times sin \phi)}

where:

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle V_d} = voltage drop

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle cos \phi} = load power factor

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle sin \phi} = load reactive factor

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle X} = reactance

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle R} = resistance

Motors

1 horsepower (hp) = 746 watts.

Note: Motor hp rating relates to motor mechanical output. To determine motor input kVA requirements, the motor efficiency and power factor must be accounted for. In general, for preliminary or rough load calculations, assume:

1 kVA of electrical input power for 1 hp of motor.

Example

Condition: A motor control center with a total connected horsepower of 337 hp can be assumed to require 337 kVA of input power. This is a conservative value, particularly for larger motors.

Torque = (hp x 5250)/revolutions per minute (rpm)

Fan hp = (cubic feet per minute [cfm] x pressure)/(33000 x eff)

Pump hp = (gallons per minute [gpm] x head x specific gravity)/(3960 x eff)

Motors (Single Phase)

hp = (V x I x eff x pf)/746

Motors (3 phase)

Synchronous Speed: ns = (120)(Frequency)/(# of Poles) ( ) 746

V I 3 eff pf

hp

Sample Calculations-

Under Construction