What is the maximum overcurrent protective device allowed on a 480 volt branch circuit with several small motors connected all of which are under 1 HP and the maximum load of each motor is 5 amps?

What is the maximum overcurrent protective device allowed on a 480 volt branch circuit with several small motors connected all of which are under 1 HP and the maximum load of each motor is 5 amps?

• A. 10 amps
• B. 15 amps
• C. 20 amps
• D. 30 amps

Answer: (B)

Protect motor branches by sizing the short-circuit/ground-fault protection to up to 250% of the motor’s full-load current. If each motor can draw up to 5 A at full load, 250% of that is 12.5 A. The nearest standard protective device rating is 15 A, so the maximum allowed on the branch circuit is 15 A. Using 10 A would be undersized for motor starting currents, while 20 or 30 A would exceed the 250% limit. This sizing applies even with several small motors on the same branch, because the protection is based on each motor’s full-load current, not the total sum of all motors.

Protect motor branches by sizing the short-circuit/ground-fault protection to up to 250% of the motor’s full-load current. If each motor can draw up to 5 A at full load, 250% of that is 12.5 A. The nearest standard protective device rating is 15 A, so the maximum allowed on the branch circuit is 15 A. Using 10 A would be undersized for motor starting currents, while 20 or 30 A would exceed the 250% limit. This sizing applies even with several small motors on the same branch, because the protection is based on each motor’s full-load current, not the total sum of all motors.