Code Q&A: Sizing Circuit Conductors for a Single Continuous-Duty Motor
Q. What is the Code required method of sizing the conductors to a single continuous-duty motor?
See answer below.
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 [Three-Phase Motors] [Sec. 430.22].
Example: What size circuit conductor is required for a 3 hp, 230V, single-phase motor?
Solution: Determine the branch-circuit conductor size based on the motor’s FLC [Sec. 430.6(A)(1), Sec. 430.22(A) and Table 310.15(B)(16)].
3 hp, 230V Motor FLC = 17A [Table 430.248]
17A × 1.25 = 21.25A
Answer: 10 AWG rated 30A at 60°C [Table 310.15(B)(16)]
The branch-circuit circuit breaker overcurrent protection is based on motor’s FLC [240.6(A), 430.6(A)(1), 430.52(C)(1), and Table 430.248].
The circuit equipment grounding conductor must be sized to the 45A overcurrent protection device — 8 AWG [Table 250.122], but it’s not required to be sized larger than the circuit conductors — 10 AWG.
Example: What size branch-circuit conductor and overcurrent protection device is required for a 7½ hp, 230V, three-phase motor with 60ºC terminals?
Solution: Motor FLC = 22A [Table 430.250]
Conductor Size = 22A × 1.25 = 27.50A
Note: The branch-circuit short-circuit and ground-fault protection device using an inverse time breaker is sized at 60A in accordance with Sec. 430.52(C)(1), Exception No. 1.
Circuit Overcurrent Protection = 22A × 2.50 = 55A
Answer: A 10 AWG conductor rated 30A at 60°C [Sec. 110.14(C)(1), Table 310.15(B)(16)] and a 60A (next size up from 55A) overcurrent protection device [Sec. 240.6(A)].
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