Requirements for branch circuits, feeders, and services each have their own Articles (210, 215, and 230, respectively). The NEC also contains an Article providing requirements for outside branch circuits and feeders (225).

When performing the load calculations for any of these circuits, you must follow Art. 220. The requirements of Art. 220 are modified or replaced in other Articles, for specific types of loads. Table 220.3, which occupies nearly a whole page, lists these loads and the additional Articles to apply.

The typical industrial or commercial facility has load-side distribution at 480V, with 480V transformer(s) center-tapped to provide 277V for lighting. Branch-circuit panel supply transformers step down 480V to 120V. The NEC lists these and other typical voltages as nominal voltages to use for load calculations (unless otherwise specified) [220.5].

"Nominal voltage" is sometimes a point of confusion. Suppose you want to add loads to an existing panel and must do load calculations. So you measure 115V on a snap-in breaker at the panel. Should you use 115V for your calculations? No; the nominal voltage is 120V. If you look at the transformer nameplate, it will probably say 208Y/120V.

Another point of confusion is what to do with fractions of an ampere. The NEC permits you to round off to the nearest whole number [220.5(B)].