Hey Guys,

I am trying to figure out the Fuse and wire size for a 100 H.P. motor. I am an electrician and I dont do sizing on motors this big often. I think I figured it out from my code book with sizing the time delay fuse at 175% of the nameplate. But the charts that I am finding show the wire much larger than the 125% that the code says.

I have a nameplate rating of 112 A, for a 480 v, three phase motor. It looks like 196 amps, which I would use a 200 amp fuse for, and then I get 140 amps for my wire size that puts me at a #1 at 150 amps THHN. That does not seem right to me and the charts that I am finding tell me that I should be at a 2/0 or a 3/0. What am I missing? And what is correct? Also, I have a fused disconnect in my M.C.C. that is rated up to 200 amps that I was hoping I could use instead of ordering and replacing one for a 225A fuse.

Motor full load has to be taken from the tables at the end of 430, not the motor nameplate, part of the reasoning is if the motor is ever replaced (with same Hp anyway) the conductor should still be an acceptable size, those tables generally list the values for the worst efficiency/worst power factor motor you would ever find. Sizing overcurrent devices is not all that simple, but for most general duty motors you often can just go with some published data by the fuse or breaker manufacturer of the equipment you are using and usually be fine, problem is when you have some application where starting current is high, or high for longer than usual duration, but is still acceptable, then you may have a situation where you use higher setting devices as the code does permit when the normally used values will not allow for starting the motor.

The NEC FLC for a 100 HP 480 three phase motor is 124 amps. Your conductor must be at least 125% of 124 which gives you 155 amps, minimum conductor ampacity. Table310.15(B)(16) says a 75C 1/0 copper is only good for 150 amps - so you have to be larger than that as a minimum, you could have other adjustments that make it larger yet like ambient temp or number of conductors in a raceway.

General short circuit and ground fault protection by dual element time delay fuses is sized at 175% (see table 430.52) but that is a maximum value, that can be increased if doesn't allow the motor to start. Many other charts you find will recommend a 175 amp fuse, and I have seen a fair share of 100 hp well motors that only had 150 amp fuses that never trip on starting.