Electrical Testing
Tip of the Week: The Value of Annex B in NFPA 70 (NEC)

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

This Annex provides important guidance that is especially valuable to installing electricians.

After the body of the NEC, concluding with Chapter 9, you’ll find a series of “Informative Annexes.” If you’re an electrician installing equipment in the field, you might look at Annex A and, after letting out a big yawn, conclude the Annexes don’t have much to offer you.

Although Annex A may not be very useful for an installing electrician (it is very useful for people in other roles), other Annexes are extremely useful for an installing electrician. Their usefulness typically extends to the design engineer and others, too.

Annex B is a great example of this. As its title states, this Annex provides application information for ampacity calculations.

You’ll notice many Tables in here. If you look at the Table numbers of the first 10 Tables, you’ll notice they are along the pattern of “B.310.15(B)(2)”. The B in that designation is for Annex B. The 310 comes from the fact that these tables originally were in Art. 310. They were removed several Code cycles ago, in an effort to make this Article easier to use. Prior to the change, people were confused by what was really supplemental information. Using the wrong Table was a common mistake.

But that doesn’t mean you should never use these Tables. With the current arrangement of Art. 310, you must go through a series of adjustments to arrive at the correct ampacity for your application. The Annex B Tables B.310.15(B)(2)(1) through (10) provide typical ampacities for conductors rated 0V to 2,000V under a variety of applications.

For example, Table B.310.15(B)(2)(1) does this for two or three insulated conductors within an overall covering and installed in raceway in free air. What if you’re installing three conductors within an overall covering but instead of raceway you’re using underground electrical ducts? Then use Table B.310.15(B)(2)(6). If they’re not within an overall covering, use Table B.310.15(B)(2)(7).

In addition, there’s more information that is useful after these Tables.

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