ECN Forum Forums Code Related Discussion NEC & other Code issues Breaker size for 30 hp motor

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

It looks like your caculations are correct.
The 100 amp is the max size breaker you can use. You may not go up the the next size.
You may go to an 80 amp breaker if it will
allow the motor to start.
i don't know why Uglys would list a 70 amp.
That seems to be too small.

430.52(C)(1) Exception 1 allows going to the next higher size the way I read it.

Exception No. 1: Where the values for branch-circuit short-circuit and ground-fault protective devices determined by Table 430.52 do not correspond to the standard sizes or ratings of fuses, nonadjustable circuit breakers,thermal protective devices, or possible settings of adjustable circuit breakers, a higher size, rating, or possible setting that does not exceed the next higher standard ampere rating shall be permitted.

Table 430-152 lists the maximum setting of 250%. This can be changed per 430-52(c)(1)(c)to 400% for loads of 100A or less. I think Uglies is looking at the breaker as disconnecting means per 430-110. This gives a minimum 115%. Where I work we use mimimum 150% on motor circuit breakers with a maximum of 400%. I think the only other consideration would be given to the locked rotor amp rating.

Still, 150% of 40 would be a 60 amp breaker. Are they using 175%? I can't think of any reason in particular they would use that. Edit from here, I looked at the wrong column.

[This message has been edited by rad74ss (edited 03-16-2005).]