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)].
