Code Q&A: Sizing Conductors for Single Motor

Your most pressing National Electrical Code (NEC) questions answered

March 20, 2014

Q. How do I size the conductors and protection device for a single motor?

Find the Answer

A. Conductors to a single motor must be sized not less than 125% of the motor FLC rating as listed in Table 430.247 [Direct-Current Motors], Table 430.248 [Single-Phase Motors], or Table 430.250 [3-Phase Motors (430.22)].

Let’s run through a simple example problem to show you how to make the calculation.

Example: What size branch circuit conductor is required for a 7½-hp, 230V, 3-phase motor?

Motor FLC = 22A [Table 430.250]

Conductor’s Size = 22A x 1.25 = 27.5A

As per Table 310.15(B)(16), a 10 AWG conductor is rated for 30A at 75°C.

Note: The branch circuit short circuit and ground fault protection device using an inverse time breaker is sized at 60A, according to 430.52(C)(1), Exception No. 1:

Circuit Protection = 22A x 2.5 = 55A

Use next size up, which is 60A [240.6(A)].

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.