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 pester 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.