Article 100 defines “demand factor” as the ratio of the maximum demand of a system (or part of a system) to the total connected load of a system (or part of a system). Using demand factors is a way to account for actual usage versus available usage, and thus arrive at more realistic load values and thus less costly installations.
So, for example, you have 10 welding receptacles in a shop. The shop owns 2 welders. At any given time, only 20% of those receptacles will be in use. The demand factor is 20%.
Table 220.42 provides the demand factors to apply to that portion of the total branch circuit load calculated for general illumination [220.42]. It may sound a bit tricky, but you must apply these only to a portion of that portion.
Suppose you have a warehouse and 70% of your total branch circuit load is for general lighting. According to this table, the first 12,500VA (or less) of that 70% has a demand factor of 100. Anything over that gets a 50% demand factor.
The demand factors do not apply where all the lighting is expected to be used all the time.
