For a 3-phase, 208-volt, continuous-duty motor of 10 horsepower, the conductor ampacity is approximately which value?

For a 3-phase, 208-volt, continuous-duty motor of 10 horsepower, the conductor ampacity is approximately which value?

• A. 38.5 amps
• B. 46.2 amps
• C. 32.2 amps
• D. 30.8 amps

Answer: (A)

The key idea is sizing conductors for a continuous-duty motor by using 125% of the motor’s full-load current. For a 10-horsepower, three-phase motor at 208 volts, you first find the motor’s full-load current from the NEC motor data tables. That full-load current is about 30.8 amps. Since the motor runs continuously, you multiply by 1.25 to get the required conductor ampacity: 30.8 × 1.25 = 38.5 amps. So the conductor ampacity should be approximately 38.5 amps. The other values don’t fit because they either use the motor current without the 125% allowance, or they misestimate the motor’s full-load current. The 38.5-amp result aligns with the 125% rule for continuous-duty motor circuits.

The key idea is sizing conductors for a continuous-duty motor by using 125% of the motor’s full-load current. For a 10-horsepower, three-phase motor at 208 volts, you first find the motor’s full-load current from the NEC motor data tables. That full-load current is about 30.8 amps. Since the motor runs continuously, you multiply by 1.25 to get the required conductor ampacity: 30.8 × 1.25 = 38.5 amps. So the conductor ampacity should be approximately 38.5 amps.

The other values don’t fit because they either use the motor current without the 125% allowance, or they misestimate the motor’s full-load current. The 38.5-amp result aligns with the 125% rule for continuous-duty motor circuits.