What is the maximum overload protection trip rating for a 3-phase, 75-horsepower, 240-volt motor with a nameplate current of 188 A and a service factor of 1.15 when starting current is not a problem?

What is the maximum overload protection trip rating for a 3-phase, 75-horsepower, 240-volt motor with a nameplate current of 188 A and a service factor of 1.15 when starting current is not a problem?

• A. 230 A
• B. 235 A
• C. 240 A
• D. 250 A

Answer: (B)

Thermal overload protection is designed to allow a motor to run a bit above its rated current without tripping, but not so high that overheating occurs. The maximum permissible setting for an overload trip, when starting current is not a problem, is 125% of the motor’s full-load current. With a nameplate full-load current of 188 A, the highest safe trip rating is 1.25 × 188 A = 235 A. The service factor of 1.15 indicates the motor can handle somewhat more than its rated load for short periods, but it doesn’t raise the overload-trip ceiling beyond 125% of full-load current. Therefore, 235 A is the correct maximum overload protection trip rating.

Thermal overload protection is designed to allow a motor to run a bit above its rated current without tripping, but not so high that overheating occurs. The maximum permissible setting for an overload trip, when starting current is not a problem, is 125% of the motor’s full-load current. With a nameplate full-load current of 188 A, the highest safe trip rating is 1.25 × 188 A = 235 A. The service factor of 1.15 indicates the motor can handle somewhat more than its rated load for short periods, but it doesn’t raise the overload-trip ceiling beyond 125% of full-load current. Therefore, 235 A is the correct maximum overload protection trip rating.