Q. What is the Code rule for sizing conductors serving a single continuous duty motor?
Find the Answer
A. Conductors to a single motor must be sized not less than 125% of the motor full load current (FLC) rating as listed in Table 430.247 [Direct-Current Motors], Table 430.248 [Single-Phase Motors], or Table 430.250 [Three-Phase Motors (430.22)].
Let’s run through an example problem to show you exactly how to choose a properly sized conductor.
What size branch circuit conductor is required for a 7½-hp, 230V, 3-phase motor with 75°C terminals?
As noted in Table 430.250, the motor FLC = 22A.
Calculate the conductor size as follows:
Conductor size = 22A x 1.25 = 27.50A
A quick check of Table 310.15(B)(16) reveals that a 10 AWG conductor is rated 35A at 75°C.
The branch circuit short circuit and ground fault protection device using an inverse time breaker is sized at 60A, according to Sec. 430.52(C)(1), Exception No. 1.
Circuit protection = 22A x 2.50 = 55A
Using the next size up rule, a 60A protective device would be selected [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.