Code Q&A: Motor Protection

May 22, 2014

NEC rules for ptotecing motors against overload, short circuit, and ground fault by a single overcurrent device

Q. Does the Code allow a single overcurrent device to provide overload, short circuit, and ground fault protection for a motor? Can you also show me an example of how to size it? (Answer based on 2011 NEC.)

Find the Answer

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

Here’s a sample calculation:

What size dual-element fuse is permitted to protect a 5-hp, 230V, single-phase motor with a service factor of 1.15 and a nameplate current rating of 28A?

As per 430.32(A)(1), a motor marked with service factor of 1.15 or greater requires you to size the device at 125% of its full load current rating.
Therefore, 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.