Motor Branch Circuit

[joe1772]

I'm confused about conductor and fuse sizing of motor circuits.
Could some one please tell me the proper fuse size and conductor size for the following motors.

3 phase
208 V
squirrel cage motors

5 hp RLA=16.7

50 hp RLA=143

Also could you tell me if Article 240.4(D) applies to conductors suppling motors.

 

[bphgravity]

Re: Motor Branch Circuit

The F.L.C. of each motor is found at 430.148 for single phase and 430.150 for 3-phase.

Overload protection is sized per 430.32(A)(1) or 430.32(C).

Branch-circuit conductor size per 430.22.

Branch-circuit OCD per 430.52.

Feeder conductor (if motors are sharinga feeder) is sized per 430.24.

Feeder OCD is sized per 430.62.

240.4(D) does not apply to motor circuits, see part (G).

 

[joe1772]

Re: Motor Branch Circuit

Thanks for the reply.

So the overloads, at 115%, should be 19.2A and 164.5A.

Time-delay fuses, 25A And 250A

Conductor sizes, THHN copper, 14 and 2/0

 

[bphgravity]

Re: Motor Branch Circuit

Look pretty good.

The first could be a 30-amp fuse: 16.7 x 175% = 29.2-amps. 240.4(B) allows you to go to the next stabdard size. If the motors trip the OCD on the start, 430.52(C)(EX. 2)(b) allows the percentage be increased to a maximum of 225% of the FLC.

#12 and 3/0 would most likely be needed. Even though you are using THHN 90 degree conductors, they can only be loaded to the 75 degree column.