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breaker sizing

thread242-333170

breaker sizing

breaker sizing

(OP)
We have an electric motor driven pump controlled by a VFD. The pump's HP rating is about twice what the process requires. So, while we might normally install this pump on a 600 amp breaker, we don't expect to ever draw near that amount. The installation location cannot supply the 600 amp circuit. Can we use a smaller breaker at the main distribution panel? If so, can someone point me to the code section(s) that might point this out?
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RE: breaker sizing

In general, no. The motor feeder must be sized for 125% of the motor full load amps per NEC 450.22. But see also 430.22(E). If you can convince the local inspector, you might be able to get away with something smaller. But it's a really bad idea, in my opinion.

RE: breaker sizing

The code does specify a minumum conductor size for motors, but does not specify a minimum OCPD size. The rules in 430.52 specify the maximum permitted OCPD. You can use any smaller size OCPD that will let you start and run the motor. You might have an issue landing the size of conductors required by the code on the smaller OCPD.

RE: breaker sizing

(OP)
Thanks for the replies. Are there breakers that are rated for higher amps than they trip at so that they might be able to accept larger conductors?

RE: breaker sizing

For additional information about VFD installations, see NEC Article 430.120 thru 128.

There are breakers that have high ratings that accept lower rating trip plugs. It will all depend on the equipment you use.

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