Generator Feeder Size Between Upstream and Downstream CB

[faresos]

Hello everyone:

I have a client wants to use an existing generator 400KW @ 480/277V. The total load we are trying to feed is about 200A. As you see the generator is way oversized for that load which most likely will experience a wet stacking condition. I'm not sure what is the generator output circuit breaker but most likely will be 800A for that size. Anyhow, my question is, if I provide a 300A circuit breaker downstream (in series) of the 800A (generator circuit breaker), can I size my wire based on downstream 300A CB or it has to be based on the upstream breaker which is 800A?

Thanks,

 

[raider1]

What you are describing would be a feeder tap. Provided that you comply with one of the feeder tap methods specified by 240.21(B) you would be fine to do what you are suggesting.

A new section in the 2017 NEC Specifically addresses this. Check out 445.13(B)

445.13 Ampacity of Conductors.
(B) Overcurrent Protection Provided. Where the generator
set is equipped with a listed overcurrent protective device or a
combination of a current transformer and overcurrent relay,
conductors shall be permitted to be tapped from the load side
of the protected terminals in accordance with 240.21(B).
Tapped conductors shall not be permitted for portable
generators rated 15 kW or less where field wiring connection
terminals are not accessible.

Chris