Neutral sizing - 3phase, 4-wire feeder

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nhee2

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An existing install has a 1200 feeder (breaker set at 1040A and restricted access) with 3 parallel 500's per phase, a 3/0 EGC and 2/0 neutral. The EGC is sized per 250.122(F), and I had assumed the neutral was undersized because it could not be smaller than the EGC per 215.2(A)(2). However I am now seeing that the 250.122(F) requirements do not apply to the grounded conductor sizing requirements.

Assuming the feeder neutral loads are less than the ampacity of paralleled 2/0's, am I correct in thinking that the 2/0 neutral is ok despite the larger EGC?
 
I believe running 1/0 is compliant, the minimum permitted for parallel runs, as long as the total ampacity of the parallel grounded conductors exceeds the ampacity of a single 250.122-required EGC. However, an AHJ may interpret the requirement of 215.2(A)(2) more literal than I. If so, the breaker set at 1040 pushes you up to the 1001-1200A size of a 3/0 copper EGC... and thus a 3/0 neutral.

That said, the minimum size conductor for 1040A is 3x400's. I believe you are looking at 4/0 EGC's under 250.122(B)... but I didn't do the actual calculation to determine the upsized EGC
 
Smart $ said:
However, an AHJ may interpret the requirement of 215.2(A)(2) more literal than I. If so, the breaker set at 1040 pushes you up to the 1001-1200A size of a 3/0 copper EGC... and thus a 3/0 neutral.
Click to expand...

But, requiring (3) 3/0's would make the statement in 215(A)(2) regarding "250.122(F) shall not apply" meaningless. So your initial thought - that 1/0 would comply, seems more correct.

Thanks.
 
nhee2 said:
But, requiring (3) 3/0's would make the statement in 215(A)(2) regarding "250.122(F) shall not apply" meaningless....
Click to expand...
Not after taking into consideration 250.122(B) is also in play. Like I said, the AHJ may take the requirement more literal than I.

Code does not specify how to transition minimum EGC size to minimum grounded conductor size for parallel sets. Is it based on size alone, or the ampacity of an equivalently sized grounded conductor divided by the number of sets???
 
iwire said:
Sounds like someone thought 3 x 500s = 1,200 and got caught.
Click to expand...

Yes, that's what I assume. I don't know the actual trip setting (I used 1040 as an example in order to ask my question about neutral sizing), but I know there are multiple '1200A' feeders in the facility with parallel 500's, whose trip setting is adjusted down.
 
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