What is the required copper THW branch-circuit conductor size for a 3-phase, 75-horsepower, 240-volt motor with a nameplate current of 188 A and a 75°C terminal rating?

What is the required copper THW branch-circuit conductor size for a 3-phase, 75-horsepower, 240-volt motor with a nameplate current of 188 A and a 75°C terminal rating?

• A. 200 kcmil
• B. 250 kcmil
• C. 350 kcmil
• D. 500 kcmil

Answer: (B)

The main idea is sizing motor branch-circuit conductors using 125% of the motor’s full-load current. The motor’s full-load current is 188 A, so you need conductor ampacity of at least 188 × 1.25 = 235 A. With copper THW and a 75°C terminal rating, you consult the 75°C ampacity table. Among the given options, 200 kcmil does not meet 235 A, while 250 kcmil exceeds it, making 250 kcmil the smallest size that satisfies the requirement. Therefore, the required copper THW conductor size is 250 kcmil.

The main idea is sizing motor branch-circuit conductors using 125% of the motor’s full-load current. The motor’s full-load current is 188 A, so you need conductor ampacity of at least 188 × 1.25 = 235 A. With copper THW and a 75°C terminal rating, you consult the 75°C ampacity table. Among the given options, 200 kcmil does not meet 235 A, while 250 kcmil exceeds it, making 250 kcmil the smallest size that satisfies the requirement. Therefore, the required copper THW conductor size is 250 kcmil.