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Code Q&A: Protection of Conductors

Feb. 28, 2019
Many different Code sections must be understood to properly size conductor overcurrent protection.

Q. Per the NEC, conductors must be protected at what amperage?

See the answer below.

This answer is based on the 2017 NEC.

A. Except as permitted by Sec. 240.4(A) through (G), conductors must be protected against overcurrent in accordance with their ampacity after ampacity correction and adjustment as specified in Sec. 310.15.

Table 310.15(B)(16) contains the most commonly used conductor ampacities based on up to three current carrying conductors in a raceway or directly buried in an ambient temperature of 86˚F. If any other conditions apply, such as more than three current carrying conductors, or a different ambient temperature, the conductor ampacities found on this table must be corrected and/or adjusted accordingly.

Section 240.4 uses the term “overcurrent.” According to the Art. 100 definition, overcurrent is any current in excess of the rated current of equipment or conductor ampacity. Overcurrent can be from an overload condition, a short-circuit, or a ground fault.

One example of where this definition is important is in Sec. 240.4(A). Where a power loss hazard exists, conductor “overload” protection isn’t required, but short-circuit overcurrent protection is required where the interruption of the circuit will create a hazard; such as in a material-handling electromagnet circuit or fire pump circuit [Sec. 240.4(A)].

Keep in mind that different applications (such as conductors for motors) have their own set of requirements [Sec. 240.4(G)]. Many different Code sections must be understood to properly size conductor overcurrent protection.

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.

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