clarification on neutral in disconnect

[motobeart50]

Good evening

A new company with different ideas that confuse me.
I'm told to feed a fused disconnect from a 100amp ( 240v) breaker in the msp with 2, 2awg.
Return to the msp with 2, 2awg and connect to the sub panel feeders with Polaris connectors.
All good. From a pv disconnect I will tap to those conductors before the fuses.
But they also want a 2awg in and out of the neutral bar of the disconnect from the msp.
I would think I could just pass the #8 neutral of the inverter output through the disconnect straight to the msp neutral bar.
This would satisfy the conditions of 215.a(2) and table 250.122.
This would allow me to pull 4 #2s and 2 #8s , one neutral, one egc rather than 6 #2s and 2 #8s .
Am I right or wrong.?
Sorry for the length of the question. But I appreciate any answers.
Barry

 

[jaggedben]

I hope I'm understanding your description and question correctly. (Got a diagram? That would help.) Are they asking you to use 2 disconnects to tie an inverter output into a feeder? Before I answer your question I have to say that I think it's a bad choice to use two fused disconnects instead of a single subpanel! (Or another option, just tap the conductors and put a main breaker in the panel downstream.) But that barely affects the answer to your question...

The concern is complying with 300.20. The load on the feeder will rarely be perfectly balanced, which means you cannot run just the black and red alone through a raceway or knockout for either the feed to the disconnect or the return. In order to be compliant you need to do one of the following:

• Run all the ungrounded conductors, both feed and return, through the same raceway and/or knockouts. In which case you can do as you propose with the smaller neutral to serve the inverter.
• Run the neutral for the feed with the feed, and the neutral for the return with the return, as on your plans, if they are through different raceways/knockouts.
• Use entirely non-ferrous raceways and enclosures, which is no doubt impossible. (Your MSP is gonna be steel, and so is your disconnect I'm sure.)

As a practical matter, it's highly likely your inspector won't really understand 300.20 him/herself well enough to accept that your installation could be compliant without running the neutrals as you've been asked to. I've run into this enough to believe that it's not worth cutting that corner. Also, if you were to do the subpanel instead of the two discos, then you really do need to run a full size neutral to-and-back from the new sub, since someone could come stick a new load in there.