The NEC has nine chapters, but doesn’t have one devoted to calculations. So how can you know that your electrical calculations conform to the NEC?
The answer has two parts. First, the NEC may not have a chapter devoted to calculations, but it does have an annex that contains and explains calculation examples. Second is the content organization of the NEC itself.
We’ve drawn on Annex D in previous issues, illustrating many lessons from just a small part of it. If your work will (or does) involve electrical calculations, it’s worthwhile to set aside time for a thorough and methodical study of Annex D.
But how does the content organization of the NEC help with calculations? The first four chapters apply to all installations. They are arranged in the same order that you would use them if walking through an Annex D example. You could pick any Annex D example to see the steps required.
You begin with load calculations. The requirements are in Chapter 2. The title of Article 220, “Branch-Circuit, Feeder, and Service Calculations” is pretty clear on what to expect. But before you go there, apply the nearby articles for branch-circuits, feeders, and services as appropriate. For example, before doing your feeder calculations per Article 220, apply the feeder requirements of Article 215.
After you’ve done circuit and protection calculations, you need to size your conductors. The requirements are in Article 310, in the order of application.