Circuit Protection for a Pump


Thread Starter


Hello guys,

I was hoping you can help me on my problem.

How do I size a Circuit Protection (AT, A ) for a Pump that is 480V, 11Kw, with a full load ampere of 21A. Can you help me on the formula to accurately size a feeder protection for this one?


Bob Peterson

If this is in the US, the conductor ampacity needs to be 125% of the FLC taken from the chart in the NEC.

The motor needs overload protection, typically via an overload relay wired into the contactor.

The conductors need short circuit protection - fuses of circuit breakers. The NEC has a chart for that as well. Generally you can use a CB with a rating that does not exceed 250% of the actual FLC taken from the motor nameplate, or 175% if it is a fuse.

There are some special cases where this is not true but in general that is how it is done here.