Breaker size for 30 hp motor - 03/15/05 02:54 AM

I must be missing something. When I calculate a 30 hp. motor on 460 volts @ 40 amps, using an inverse time breaker, I get:

40 amps X 250 % = 100 amps. I just looked in my "Uglies Electrical References" book, and they calculate it as being a 70 amp. breaker. Any idea what's going on. I have a 400 amp. panel I'm getting ready to order, and I have to have the breaker sizes ready to order it. I don't want to order the wrong size breakers Pretty expensive mistake. Does not 430-52 allow me to go up to the next higher rated breaker if my calculations do not correspond to a standard size?? In this particular case, 100 amp. is a standard size, but I have other ones that will not be. I figure " Ugly is probably right, but where am I going wrong?? I can't afford to calculate this job wrong. Thanks again... Steve

40 amps X 250 % = 100 amps. I just looked in my "Uglies Electrical References" book, and they calculate it as being a 70 amp. breaker. Any idea what's going on. I have a 400 amp. panel I'm getting ready to order, and I have to have the breaker sizes ready to order it. I don't want to order the wrong size breakers Pretty expensive mistake. Does not 430-52 allow me to go up to the next higher rated breaker if my calculations do not correspond to a standard size?? In this particular case, 100 amp. is a standard size, but I have other ones that will not be. I figure " Ugly is probably right, but where am I going wrong?? I can't afford to calculate this job wrong. Thanks again... Steve