Code Q&A: Finding Highest Rated Motor in Group

Feb. 18, 2016

NEC requirements for determining the highest rated motor

Q. When sizing conductors for more than one motor, what does the Code consider to be the highest rated motor?

Find the Answer

A. When sizing motor circuit conductors, the highest rated motor is the motor with the highest rated full-load current (FLC) rating [430.17].

For example, let’s say you have this group of three motors:

10-hp, 3-phase, 208V

3-hp, single-phase, 208V

3-hp, single-phase, 120V

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Which one of these has the highest FLC rating?

A quick check of the tables in Sec. 430 reveals the following:

The 10-hp motor has a FLC value of 30.8A [Table 430.250].

The 3-hp, 208V motor has a FLC value of 18.7A [Table 430.248].

The 3-hp, 115V motor has a FLC value of 34.0A [Table 430.248].

Therefore, the highest rated motor in this group is the 3-hp, single-phase, 120V motor.

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.