200amp 4 wire feeder

[tim89s]

Why can you use 3/0 cu for a 4 wire 200 amp panel, 3 phase commercial install? One answer I got was "There is something in the code that states you dont have to derate feeders". Another answer was "I have always used 3/0 for a 200 amp panel." Is it because you derate the 3/0 using the 90 degree table, which is 180 amps after derate. Then you step up to the next highest standard overcurrent proctection which is 200 amps? Please help me to understand this a little better.

Thank you
Tim

 

[augie47]

There are a couple of reasons it is normally acceptable.
1st, the 75? rating is 200 amps. I am assuming the question you have deals with the derating due to the 4 conductors. You will note in 310.15(B)(4) the neutral only counts as a current carrying conductor when the major portion of the load is non linear which is normally not the situation.
Secondly if you do derate, the derated value on 90? conductors is 180 amps and per 240.4 you are allowed to go to the next higher OCP (unless your load exceed 180 amps)

 

[infinity]

The likelihood that the load is more than 50% of harmonic current is slim, therefore in most cases the neutral is not considered a CCC so no derating is required at all. As stated even if you do derate you'll end up with a derated value of 180 amps which can be protected up to the next standard size 200 amp OCPD along as the connected load is 180 amps or less.