Motor Feeder Cable Sizing

[Flapjack]

I read over this thread (http://forums.mikeholt.com/showthread.php?t=129894) but it sounds like Jraef is saying the cable in the MCC feeding the soft starter should be sized at the same ampacity as the rating of the MCC breaker. Doesn't NEC 430.62(A) allow otherwise?

Let's say I have a 100 HP, 460 V motor (124 A FLC) on a soft starter. The soft starter has 225 A fuses, which would be the branch SCPD. The soft starter is fed by a 300 A breaker in the MCC, which would be the feeder SCPD. Per NEC 430.62(A), I should be able to size the cable from the MCC to the soft starter at 125% of 124 A FLC = 2/0 AWG. Any disagreements?

 

[mayanees]

I think the cable needs to be sized at 125% of the rating of the soft-starter per 430.122, and would be protected by the soft-starts overloads.

 

[Flapjack]

I don't believe 430.122 is applicable to soft starters. Only adjustable speed drives.

 

[Smart $]

Motor feeder???

A motor feeder supplies power to more than one motor or one motor and non-motor equipment.

A soft starter is typically installed in the motor branch circuit... its a motor controller.

 

[Smart $]

On second thought, you can actually have a motor feeder as you described. But for your particular case, the feeder would have to be sized:
1) at an ampacity not less than 250.51A
220.5(B)
240.4(B)
240.6(A)
430.62(B)


2) or meet one of the feeder tap rules of 240.21(B) or 240.92(B)

 

[Smart $]

Oh! ...and I forgot to mention you can have a motor feeder with a specific load of one motor under 430.62(A)... which limits your SC/GF OCP to 250% (1/t breaker*) so 300A<124×250%.... then technically you can size that feeder to the 430.22 size... but the rub is you can't go over on the feeder ampacity. 430.62(B) is not specific on how much over, but I imagine the ampacity of smallest compliant conductor.

*See Table 430.52(B) for other types of OCP.