125 hp motor

[ASK_EDDIE]

Looking for a little help on this one , I've been asked to just supply power for an air compressor in a plant. so far the only information is the FLC is the standard FLC of 156 amps . I know that isnt much to work with, the feed is about 40 feet, Its coming from a Square D I-line panel 480v . Im not sure what type of starting device its coming with either .
430.22 125% x 156 = 195 amps
310.16 75c = 3/0
I guess in asking about the i-line breaker , with a 200 amp breaker will it handle the inrush current, I know im missing a lot of information but if i could get some input. thanks !

 

[Cold Fusion]

There are plenty on here that can give you a positive answer. I can't.

I'd get the trip curve for the CB and lay out a reasonable motor starting curve. See if the starting curve fits under the trip curve.

cf

 

[Cow]

I'd most likely go with a 250 amp breaker, you're allowed to round up from the 150% multiplier. A compressor this large is most likely a rotary on a soft start or vfd, but I'd probably still use the largest breaker code will allow. If it trips on startup, the exceptions to 430.52 will allow you to go larger.

 

[active1]

Let me take a shot at this.

125 HP / 460v 3ph motor is 156a FLC
Table 430.52 Inverse time breaker use 250%.
156a X 250%= 390a
Nixt standard size 400a circuit breaker.
Conductor size with THHN & 75 degree terminals 2/0 cu (175a).
250.122 Equipment ground #3 cu.
Minimum size EMT 1.5" (I would go with 2").
Let me know if I did it right.

 

[Cow]

I always use wound rotor in the table, but I've been taught to use squirrel cage also. I don't really know which one is right....

 

[augie47]

Opinion based on field experience not engineering knowledge: 200 may be pushing your luck depending on the type of starting. Is the breaker existing ?
If not Square D has 225 amp and 250 amp available in the same frame sizes for I-Line breakers. As pointed put, you could normally go 400 (larger frame, of course, which could be a problem)
Compressors that size start "unloaded" and that should help. With luck it is a soft start or some reduced voltage type starting, also that size I-Line breaker normally has an adjustable magnetic trip which will help you.

I have seen 125 hp start/run on 200 amp breakers.

 

[tryinghard]

Most likely squirrel cage motor, it's most common. I?m sure costs matters but why not go to the max on the inverse time breaker 430.52?
Breaker: 400a (430.52)
Circuit conductors: 3/0 cu (430.22(A))

 

[don_resqcapt19]

You really need to know what they are using for the motor controller. If it is a VFD, everything changes as far as the sizing of the conductors and the breaker on the line side of the VFD.

 

[active1]

Most likely squirrel cage motor, it's most common. I?m sure costs matters but why not go to the max on the inverse time breaker 430.52?
Breaker: 400a (430.52)
Circuit conductors: 3/0 cu (430.22(A))

My bad. Forgot the 125% on the conductors.
156a X 125% = 195a 3/0 cu