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)].