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The Official Magazine of the Bluewater Cruising Association
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Fusing Electric Motor Circuits

Jeff Cote

Pacific Yacht Systems
May 8th, 2018

An electrical circuit of the utmost importance on the boat is for the bilge pumps.

So checking that these circuits are adequately run and protected is a matter of safety. Generally, electric motors have their own over-current device to protect the motor from drawing too much current. 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 of current that a motor pulls as it gets going or when it encounters difficulty in performing its function.

There can also be a problem if something like a piece of trash gets stuck in the bilge pump. It can create what is called a locked rotor condition where the motor doesn’t turn. The current drawn in these situations may not be high enough to trip the over-current, fuse or breaker protection installed in the circuit, and left unattended, it can become a fire hazard.

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

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