Industrial Plant Design/System Planning/Determination of Loads

Determination of Loads

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This is a Design Guide is based on IEEE standards.

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General

Determination of the load is the Electrical Engineer's first problem and may be difficult to solve. The size and number of primary and secondary substations, the size, number, and arrangement of primary feeders, and the type of secondary distribution are largely dependent on the amount and nature of the load and its distribution.

The plant distribution system usually must be designed before all loads are known. This is at a time when the equipment layout itself is only in the formative stage. Equipment may be bought piecemeal during which time changes in machines are taking place either in number or size. Ideas are changed by the impact of what is commercially available, or manufacturers’ recommendations for improved models, better ways of securing a given result, or competitive conditions. Manufacturing processes are being changed as available equipment is fitted into the prospective production schedule.

Many plants are built to manufacture new products, which adds to the difficulty of establishing power requirements. Plant layouts are subject to considerable modification of the original scheme. Entire plant rearrangement may be necessary in the middle of a job; air and refrigerating compressors, fans, blowers, and pumps may come into the picture or shift position rapidly oil-fired annealing furnaces may become electrically heated as a result of laboratory tests that prove a controlled atmosphere necessary, thus adding hundreds if not thousands of kilowatts to the plant load.

Even after a plant is in operation, loads may change in size and location. New models, new products, and production methods call for continual change in the distribution system, bu. These changes can be minimized by careful planning.

Preliminary Loads

Preliminary estimates of loads is a problem deserving the closest study. These estimates may have to be used as the basis for major decisions. At this stage in the plant design, the Electrical Engineer often has available only a few building layout drawings or perhaps a plant map. The general locations of the major pieces of equipment will usually be roughly indicated and their power requirements may or may not be known. Starting with this information the Electrical Engineer must call on all his knowledge and experience as well as on that of other plant engineers and designers to enable him to arrive at an estimate which will stand up as the loads become better defined. In most cases, it is better to consider the lighting and power loads separately and combine them later to determine the demand in any one area, since present practice is usually to supply these loads from a load-center substation.

The factors most frequently used in determining distribution system loads are as follows:

Demand Factor

The ratio of the maximum demand on a system to the total connected load of the system. The maximum demand is usually the integrated maximum kilowatt demand over a 15 or 30-minute interval, rather than the instantaneous or peak demand.

Diversity Factor

The ratio of the sum of the individual maximum demands of the various parts of a system to the maximum demand of the whole system.

Load Factor

The ratio of the average load over a designated period of time to the peak load occurring in that period.

Information on the demand and the diversity factors for the various loads and groups of loads is needed to design the system. For example, the sum of the connected loads on a branch load circuit, multiplied by the demand factor of these loads, will give the maximum demand that the branch circuit must carry.

The sum of the maximum demands of the branch circuits associated with a sub-load center or panelboard divided by the diversity factor of those branch circuits will give the maximum demand at the sub-load center and on the circuit supplying it.

The sum of the maximum demands of the circuits radiating from a load center, divided by the diversity factor of those circuits, will give the maximum demand on the transformer at the load center. The sum of the maximum demands of the load-center transformers divided by the diversity factor of the transformer loads will give the maximum demand on their primary feeder. With the use of the proper demand and diversity factors as outlined above, the maximum demands on the various parts of the system from the branch load circuits to the power source can be determined.