Fig. 2. Pay close attention to the securing and supporting requirements outlined in Sec. 334.30.

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Stumped by the Code? Rules for Multiple Type MC Cables Installed in Contact with Thermal Insulation

Oct. 7, 2021

Test your Code knowledge with this Q&A.

All questions and answers are based on the 2020 NEC.

Q. What does the Code require when multiple Type MC cables are installed in contact with thermal insulation?

A. Where more than two Type MC cables are installed in contact with thermal insulation or pass through the same wood framing opening that is to be sealed with thermal insulation, caulking, or sealing foam, the ampacity of each conductor must be adjusted in accordance with Table 310.15(C)(1) [Sec. 330.80(C)] (Fig. 1 below).

Fig. 1. This is one Code requirement that can be easily overlooked. — **Fig. 1.** This is one Code requirement that can be easily overlooked.

Q. What is the maximum cable length allowed by the NEC between the cable entry of Type NM cable into cabinets and enclosures and the closest cable support?

A. Type NM cable must be supported and secured by staples or straps; cable ties listed and identified for securing and supporting; hangers, or similar fittings, at intervals not exceeding 4½ ft and within 12 in. of every cable entry termination. The cable length between the cable entry termination and the closest cable support must not exceed 18 in. [Sec. 334.30] (Fig. 2 below).

Author’s comment: Many times, there is a tendency to leave a length of sheathed NM cable in a box or enclosure (such as a panelboard) just to have “extra” cable. While this practice is not prohibited, the length of such sheathed cable cannot exceed 18 in. as measured in a straight line.

Q. In one-and two-family dwellings, Type TC-ER-JP cable is permitted by the Code for what type of use?

A. Per Sec. 336.10(9), Type TC-ER-JP cable is permitted to be used in one- and two-family dwellings for branch circuits and feeders, where installed in accordance with Part II of Art. 334 for interior wiring and Part II of Art. 340 for exterior wiring.

Author’s comment: The “ER” marking on Type TC‑ER cable identifies it as suitable for exposed run use in accordance with UL 1277 and the suffix “-JP” identifies it as being suitable for pulling through framing members.

Exception: Where Type TC cable is used to connect a generator and its associated equipment, the cable ampacity limitations of Sec. 334.80 and Sec. 340.80 do not apply.

These materials are provided to us by Mike Holt Enterprises in Leesburg, Fla. To view Code training materials offered by this company, visit www.mikeholt.com/code.

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.