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Demand factors may be considered for calculating loads or portions of loads that may not all be on at the same time (noncoincident loads).
Simply stated, the NEC defines demand factor as the ratio of the maximum demand of a system (or part of a system) to the total connected load on the system (or part of the system). Demand factor is always one (100%) or less.
A connected load can be multiplied by the demand factor to calculate the load used to determine the size of the components of the system. With a noncoincident load, multiplying the total connected load by a demand factor will yield a lower number, indicating that the circuit may be sized smaller than if the load were coincident (all on at the same time). In general, a branch circuit must be sized for the entire load connected to it, but a feeder may be reduced in size based on how much of the load is actually on at the same time. No demand factor should be applied to the portion of a load that is considered continuous.
For example, suppose that a load such as lighting or receptacles would require 2,000 volt-amperes if it all operated at the same time. If only half the load is expected to operate at any given time, its demand factor might be 50%. So the circuit supplying it would only need to be sized to accommodate 1,000 volt-amperes.
If two loads are entirely noncoincident, the smaller of the two loads can be be omitted from calculations according to Section 220.20. The concept of noncoincident loads is the basis of demand factors, but the NEC does not use the words coincident and noncoincident in defining demand factors.
© Hughes Associates,Inc. 2012