Code Q&A: Motor Protection

Feb. 4, 2016

NEC requirements for protecting motors from short circuits, ground faults, and overload

Q. What is the Code requirement for protecting a motor from short circuit, ground fault, and overload with a single device?

Find the Answer

A. A motor can be protected against overload as well as short circuit and ground fault by a single overcurrent device sized to the overload requirements contained in Sec. 430.32 [430.55].

For example, let’s say you have a 5-hp, 230V, single-phase motor with a service factor of 1.15 and a nameplate current rating of 28A. As noted in Sec. 430.32(A)(1), motors rated more than 1 hp with a service factor of 1.15 or greater shall be selected to trip or shall be rated at no more than 125% of the motor nameplate full-load current rating.

Therefore, the fuse rating is calculated as follows: 28A x 1.25 = 35A.

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.