Conductor protection for fire pump motors: the protective device shall be rated at not less than what percent of the motor's full-load current?

Conductor protection for fire pump motors: the protective device shall be rated at not less than what percent of the motor's full-load current?

• A. 80
• B. 125
• C. 100
• D. 150

Answer: (B)

The main idea is to size the protective device so it can tolerate the motor’s starting surge while still protecting the conductors from faults. A fire pump motor draws a much higher current when starting than it does in normal running, so using a protection rating at only the running (full-load) current would cause nuisance trips. Setting the protective device at 125% of the motor’s full-load current provides enough headroom for start-up inrush without compromising protection against actual faults. While a higher rating (like 150%) would still meet the requirement, 125% is the minimum commonly specified. That’s why 125% is the correct choice.

The main idea is to size the protective device so it can tolerate the motor’s starting surge while still protecting the conductors from faults. A fire pump motor draws a much higher current when starting than it does in normal running, so using a protection rating at only the running (full-load) current would cause nuisance trips. Setting the protective device at 125% of the motor’s full-load current provides enough headroom for start-up inrush without compromising protection against actual faults. While a higher rating (like 150%) would still meet the requirement, 125% is the minimum commonly specified. That’s why 125% is the correct choice.