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Code Q&A: Ampacity Adjustment Factors ― Metal Wireways

Sept. 6, 2018

What ampacity adjustment factors apply to conductors installed in metal wireways?

Q. What ampacity adjustment factors apply to conductors installed in metal wireways?

See the answer below.

This answer is based on the 2017 NEC.

A. When more than 30 current‑carrying conductors are installed in any cross‑sectional area of the wireway, the conductor ampacity, as listed in Table 310.15(B)(16), must be adjusted per Table 310.15(B)(3)(a) [376.22(B)]. Signaling and motor‑control conductors between a motor and its starter used only for starting duty aren’t considered current carrying for conductor ampacity adjustment.

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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.