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Based on the 2020 NEC.
Q. What are the Code rules for bundling adjustment to conductor ampacity for short raceways?
See answer below.
A. Conductor ampacity adjustment from Table 310.15(C)(1) does not apply to conductors in raceways having a length not exceeding 24 in. [Sec. 310.15(C)(1)(b)].
For example, let’s say you have five 3/0 THWN‑2 conductors in a raceway that does not exceed 24 in. in length. What is the ampacity of the conductors in this arrangement? To find the answer you simply check Table 310.16 and see that 3/0 THWN-2 is rated 225A.
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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.