Stand-Alone SS Circuit Sizing

[HarrySlother]

Hi guys, having issues sizing the feeder breaker and the wires ahead of a stand-alone soft start. (See picture). SS is going to be provided by others; I'm assuming it will have internal protection. Usually these are in MCCs so I don't have to deal with this.

Couple hang-ups:
1. Is there a way to size the feeder breaker, other than making it "large" enough that it won't cause nuisance tripping?

2. I'm assuming the primary wires need to be protected by the breaker. Is that correct? It just seems silly to have different sizes coming into and out of the soft start.

 

[ptonsparky]

IIRC the SS will limit the current but drag out the time to start.
The feeders to the SS are protected from overload by the SS so you need to worry only about short circuit protection.
The 500 seems fine to me. A 400 would probably work but...

Request information for the SS starter. That's the best method.

 

[Jraef]

Only 430.52 applies for the breaker/fuses, but it is REALLY IMPORTANT to get the soft starter data in advance. Some cheap foreign brands are ONLY listed by UL using fuses to get any sort of reasonable SCCR. You don’t want to install a breaker now, then end up having to find a way to add fuses after it arrives (unless this is a T&M job…)

After that the standard rules on motor conductor sizes apply. The 500kCMil would be correct for a single conductor, but 2x250 seemed excessive to me, unless you are adjusting for VD and temperature or something? 2x 3/0 would be sufficient otherwise. From an NEC standpoint, an RVSS is no different than any other motor starter.

 

[HarrySlother]

Thanks Jraef. I agree with the SCCR - always something to watch out for.

2x250 seemed excessive to me, unless you are adjusting for VD and temperature or something?

I figured these wires on the primary were only protected by the feeder breaker, so they'd have to have 500A of ampacity. Could I really use a single 500K if I have a 500A feeder breaker?

 

[Jraef]

If there is nothing else except the motor starter, and that motor starter has motor thermal overload protection that meets the article 430 requirements, then the conductors are protected by that thermal OL protection curve. The circuit breaker only needs to handle short circuit protection. That’s why motor starter circuits are handled differently in the NEC.