Tap conductor assemblies & Neutral/EGC sizing

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Carultch

Senior Member
Location
Massachusetts
I have a situation with a 400A Fused disconnect tapped to a 1200A feeder.


The load side feeder of the 400A disconnect extends a significant distance, so it is of interest to reduce its wiring as much as practical.

I understand that where the EGC is routed with the tap conductors, it has to be sized to the 1200A feeder, rather than the 400A disconnect. Thus #3/0 instead of #3. 250.122(G)
I also understand that when the neutral carries negligible current, its minimum size is the size of the EGC. 215.2(A)(2)

In this example, neutral is insulated from ground in the 400A fused disconnect.

See my example in the attachment.
Would I need to run a 3/0 neutral with the tap conductors, and then splice in the disconnect to a 2/0 neutral for the distant feeder?
Or would a 2/0 neutral suffiice everywhere.
 

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  • 400A tap from 1200A feeder.jpg
    400A tap from 1200A feeder.jpg
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Carultch

Senior Member
Location
Massachusetts
Run emt and skip the wire egc.

PVC is already in place.

If I did have metal conduit, I'd imagine that I'd still need a neutral to match what the hypothetical EGC would need to be.

Does it make sense that I would have to splice the 3/0 neutral on the tap, to the 2/0 neutral on the feeder?
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
PVC is already in place.

If I did have metal conduit, I'd imagine that I'd still need a neutral to match what the hypothetical EGC would need to be.

Does it make sense that I would have to splice the 3/0 neutral on the tap, to the 2/0 neutral on the feeder?

I think the idea of matching the EGC size is that if, somehow, any which way, the neutral ends up carrying fault current, it should be able to carry enough current to trip the breaker. So it does sort of make sense that if the fault is on the line side of the disco then it may need to carry more current than on the load side.

EDIT: I see now that shortcircuit said about as much in his last sentence.
 
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