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

April 18, 2016
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.

About the Author

Mark Lamendola

Mark is an expert in maintenance management, having racked up an impressive track record during his time working in the field. He also has extensive knowledge of, and practical expertise with, the National Electrical Code (NEC). Through his consulting business, he provides articles and training materials on electrical topics, specializing in making difficult subjects easy to understand and focusing on the practical aspects of electrical work.

Prior to starting his own business, Mark served as the Technical Editor on EC&M for six years, worked three years in nuclear maintenance, six years as a contract project engineer/project manager, three years as a systems engineer, and three years in plant maintenance management.

Mark earned an AAS degree from Rock Valley College, a BSEET from Columbia Pacific University, and an MBA from Lake Erie College. He’s also completed several related certifications over the years and even was formerly licensed as a Master Electrician. He is a Senior Member of the IEEE and past Chairman of the Kansas City Chapters of both the IEEE and the IEEE Computer Society. Mark also served as the program director for, a board member of, and webmaster of, the Midwest Chapter of the 7x24 Exchange. He has also held memberships with the following organizations: NETA, NFPA, International Association of Webmasters, and Institute of Certified Professional Managers.

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