Construction

Code Q&A

March 28, 2003

When you find yourself stumped by the Code, just e-mail your question to [email protected] or visit www.mikeholt.com. Your question and Mike's answer may be posted in a future Code Q&A column. Q. I’m working on a job that includes a raceway with nine 12 AWG current-carrying conductors. What’s the ampacity of these conductors—after conductor ampacity adjustment—as required by Table 310.15(B)(2)(a)

Mike Holt

When you find yourself stumped by the Code, just e-mail your question to [email protected] or visit www.mikeholt.com. Your question and Mike's answer may be posted in a future Code Q&A column.

Q. I’m working on a job that includes a raceway with nine 12 AWG current-carrying conductors. What’s the ampacity of these conductors—after conductor ampacity adjustment—as required by Table 310.15(B)(2)(a) in the Code? It’s my understanding that you must take the amperage of 12 THHN as listed in Table 310.16 under the 90°C column, which is 30A. I multiplied this value by 70%, yielding an ampacity of 21A, which may be placed on a 20A breaker.

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My boss instructed me to use the 20A rating of the circuit breaker, which results in the conductor having an ampacity of 14A. Based on this interpretation, this conductor can’t be placed on a 20A circuit. Who is correct?

A. You are. The ampacity after conductor adjustment for 12 AWG THHN conductors in a dry location is based on 30A rating as listed in Table 310.16 under the 90C column [110.14(C)]. Therefore, you can place this 21A conductor on a 20A protection device.

About the Author

Mike Holt

Mike Holt is the owner of Mike Holt Enterprises (www.MikeHolt.com), one of the largest electrical publishers in the United States. He earned a master's degree in the Business Administration Program (MBA) from the University of Miami. He earned his reputation as a National Electrical Code (NEC) expert by working his way up through the electrical trade. Formally a construction editor for two different trade publications, Mike started his career as an apprentice electrician and eventually became a master electrician, an electrical inspector, a contractor, and an educator. Mike has taught more than 1,000 classes on 30 different electrical-related subjects — ranging from alarm installations to exam preparation and voltage drop calculations. He continues to produce seminars, videos, books, and online training for the trade as well as contribute monthly Code content to EC&M magazine.