National Electrical Code

Code Q&A

Sept. 23, 2004

Q. I don’t see where 300.4 limits the number of NM cables through a given opening in wood or metal framing members. I don’t want to turn the framing members into Swiss cheese and reduce their structural integrity by boring a hole for every NM cable. So ...

Mike Holt

A.You’re correct, the NEC doesn’t limit the number of cables within an opening, and you must not reduce the structural strength of framing members below the requirements of the building code. However, where multiconductor NM cables are stacked or bundled longer than 24 inches, the allowable conductor ampacity, based on 90°C insulation rating of the conductors as listed in Table 310.16 [334.80], shall be adjusted in accordance with the multiplying factors contained in Table 310.15(B)(2)(a).

So what should you do? You can separate the cables after they pass through the framing members so that they aren’t bundled more than 24 inches. But you don’t have to worry about this if no more than nine current-carrying 14 AWG, 12 AWG, or 10 AWG conductors are bundled together for more than 24 inches.

New Ampacity of 14 AWG = 25A x 0.70 = 17.5A, OK with 15A protection
New Ampacity of 12 AWG = 30A x 0.70 = 21A, OK with 20A protection
New Ampacity of 10 AWG = 40A x 0.70 = 28A, OK with 30A protection [240.4(B)]

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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.