Code Q&A: Adjusting Ampacity of NM Cable

March 12, 2015

NEC rules for adjusting the ampacity of an NM cable

Q. When walls are insulated by filling them with spray-applied polyurethane foam insulation — or other types of insulation — must the ampacity of Type NM cable in that wall be adjusted?

Find the Answer

A. Where more than two NM cables are installed in contact with thermal insulation without maintaining spacing between cables, the allowable ampacity of each conductor must be adjusted in accordance with Table 310.15(B)(3)(a) [334.80].

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.