Sizing Feeders

[DMatranga]

Good Morning all - this may be a relatively easy question for most of you to answer, but I am kind of new to the forum, and still fairly green when it comes to 3 phase commercial equipment. My questions is...

I have a 250kw 3phase/4wire generator, 480/277v with two fused outputs. One breaker is a 200amp 3 pole, the other a 225amp 3pole. When sizing my feeders - I believe I am going to refer to table 310.16 as the norm, but being it is 3 phase, I feel I have to refer to 315.b where there are four-six current carrying conductors in a raceway derate 80%.

The reason why I am asking this is because a licensed electrician bid the project utilizing 2/0 copper for both sets of feeders. I believe this is incorrect and want to make sure we are covered.

I think I should be using 250MCM copper for the 200Amp transfer switch - which is 250MCM good for 255Amps at 80% is 204 amps.

I think I should be using 300MCM copper for the 225 transfer switch -
which is 300MCM good for 285 at 80% is 228 amps.

Can someone let me know if I am over engineering this? Are there special instances where I can utilize a smaller feeder such as 2/0? If so, where can I find this in the NEC? Or am I sizing my feeders correctly?

I thank you all in advance. And look forward to your responses. Enjoy the day!

 

[augie47]

Welcome to the Forum:

The key is in 310.15(B)(4) where it tells you the neutral counts as a current carrying conductor where the majority of the load is non-linear.
If you carefully check you will find this is not the case in most situations so de-rating for 4 conductors is not necessary.
That being said, 2/0 THHN is only rated at 175 (75?) and would only be good for 200 amps as allowed in 310.15(B)(6) fro single phase dwelling services and whole house feeders.

In most situations the answer lies between the two scenarios you list. For a 200 amp feeder (other than SF dwelling) a 2/0 Cu would be too small, and a 250 would be over-sized UNLESS is was non-linear loads. 3/0 is what is most common.

 

[DMatranga]

Thank you for the quick reply for what I feel should be second nature - sometimes I get to second guessing, overthinking, and overpricing! Have to keep myself in check, and I have the help of all of you guys to thank.

Thanks Again

Dan
Estimator

 

[Smart $]

I notice you did not state whether the two feeders are run in the same or separate conduits. If they share a conduit for any portion of their runs, derating applies.

 

[DMatranga]

Thank you for your attention to my question, the two sets of feeders will be in separate conduits.