Fusing Electric Motor Circuits

Generally electric motors have their own over-current device. When this is not the case the fuse or breaker should be rated to the full load potential of the motor (provided that the wires run for the circuit are adequate). These breakers or fuses need to be time delayed to withstand the inrush current that a motor pulls as it gets going or when it encounters difficulty in performing its function.

One circuit that is of outmost importance on the boat is the bilge pumps. In some cases these circuits are run with small wires and long runs. If something like a piece of trash gets stuck in the bilge pump it can create what is called a locked rotor condition. The pull of current in these situations may not be enough to trip the overcurrent protection device installed in the circuit and left unattended it can become a fire hazard. So checking that these circuits are adequately run and protected is a matter of safety.

ABYC standards require that all motors and motor circuits withstand locked rotor conditions for 7 hours without creating a fire hazard.