SERVICE ENTRANCE CABLING FOR GENERATOR AND TRANSFER SWITCH

[PE (always learning)]

Hey everyone,

I have a situation where I have a 250 KW generator at 120/208, 3 phase and it is feeding a service entrance rated 1000 amp transfer switch, which then feeds into a 1000 amp distribution panel. The contractor has sized the feeders at 3 sets of 350 KCMIL copper, which is only good for 930 amps. My question is am I allowed to round up to 1000 amps on my ampacity for my feeders from the generator to the transfer switch or am I pretty much forced to upsize the conductors due to 240.4 in the NEC?

Best Regards

 

[mayanees]

I assume you have a 1000-amp generator output breaker, which if it's not adjustable then yes, the cable needs to be good for 1000 amps. But if it's a restricted-access adjustable trip per 240.6(C) you can use the 930-amp cable and adjust the ltpu to protect it.
This will however limit the output of the generator to 930 amps which is roughly 930/(250/0.8/208/1.732) = 107% of the fla. Typically gens are limited to 115% by 445.13 which specifies the size of the conductors, but the exception would let you run cable as low as 100% of the output.

 

[kingpb]

Run 4 // 350KCMIL AL. They are probably cheaper than 3 // copper.

 

[iceworm]

Hey everyone,

I have a situation where I have a 250 KW generator at 120/208, 3 phase and it is feeding a service entrance rated 1000 amp transfer switch, which then feeds into a 1000 amp distribution panel. The contractor has sized the feeders at 3 sets of 350 KCMIL copper, which is only good for 930 amps. My question is am I allowed to round up to 1000 amps on my ampacity for my feeders from the generator to the transfer switch or am I pretty much forced to upsize the conductors due to 240.4 in the NEC? ...

et -
As I am sure you know:
250KW, 208/120, 3ph, .8pf ==> Gen FLA = 867A

First Question:
Did the generator come with an installed circuit breaker?
If so, what is the rating?

445.13.A allows the conductors from the gen to the first OCP to be 115% of the name plate current ==> 998A
So, if the gen came with a CB, then conductors from the gen CB to the premises are protected per 240. I would guess that a CB supplied with this gen will be 1200A - but you need to check. So, no round up here

There is a new provision in the 2017 code (445.13.B) that allows a tap from the gen CB per 240.21.B. And no round up here either.

If the gen did not come with a CB, then the conductors to the first OCP are at least 115% (998A). The first CB could be higher - 100
A or 1200A would not be uncommon.

So, either way, 3-350kcmil ==> 3 x 310 = 930A too small