Electrical Testing

# Tip of the Week: The Value of Annex D in NFPA 70 (NEC)

The calculation examples in this annex help you learn how to apply many NEC requirements.

After the body of the NEC, concluding with Chapter 9, you’ll find a series of “Informative Annexes.” In previous issues, we looked at Annexes A, B, and C. So what’s in Annex D?

What’s there is an amazing resource. It consists of calculation examples, and these show you exactly how to make various calculations based on NEC requirements. If you’ve got a reasonably good foundation in the NEC, this is a great place to focus your NEC study time because it ties together so many of the individual NEC requirements and shows you how to apply them.

The examples are numbered D1(a) through D13. They are arranged thematically and clearly labeled, so you can readily find an example, if one exists, that applies to your application. If there isn’t one that directly applies, you can still find one that matches your work flow (in most cases).

Annex D starts with residential calculations (D1 through D2(c) and D4(a) through D7). Examples D3 and D3(a) are “required reading” for nearly anyone who’s sizing feeders and branch circuits in commercial or industrial settings. Much to the relief of those who are just starting with motor calculations and come across it, Example D8 illustrates the correct way to size motor circuit conductors, overload protection, and ground-fault protection.

One of the questions that arises in NEC classes and on Code forums is whether you perform branch circuit calculations first or feeder calculations first. You will find the answer just by walking through these examples.

Another question is why the NEC bothers to include a 90°F column in the ampacity tables, since termination devices are almost never rated above 60°F and the ampacity must be for the lowest rated device. The answer is in Example D3(a).

TAGS: Code Basics