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200A cable neutral is 3/0, hots are 4/0. 100A is 2 #2s, and a #4 for the neutral. Why is this allowed in service cable, but not in the interior wiring in a home?
 

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If you had perfectly balanced 120 loads between both phases there would be no current on the service neutral. So they figure in most cases there will be less current in general on the neutral. The branch circuits are not as cut and dry (can be different combinations) so full size on that wiring.

Edit: Like if you had say a few 120 loads going all on the same phase the branch neutrals would be running full current but the reduced service neutral is still big enough to handle the loads.
 

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200A cable neutral is 3/0, hots are 4/0. 100A is 2 #2s, and a #4 for the neutral. Why is this allowed in service cable, but not in the interior wiring in a home?
Load diversity.

On a branch circuit, you can have full-load on one side of the MWBC, and zero on the other. This means the neutral must be capable of carrying the full load current.

It's a different story with a service. First, it's rare that the service runs at 100% of it's rating, and highly improbable that that 100% is all on one hot leg.
 

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Load diversity.

On a branch circuit, you can have full-load on one side of the MWBC, and zero on the other. This means the neutral must be capable of carrying the full load current.

It's a different story with a service. First, it's rare that the service runs at 100% of it's rating, and highly improbable that that 100% is all on one hot leg.
Even then, one some services the predominate loads are 240 which means they force the hots to be well upsized to what the 120s need. So even with everything 120 running on one leg (near impossible) it still might not approach the undersized neutral rating.

Some code articles actually allow you to have a neutral only as big as the EGC be loading permits it.
 

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Discussion Starter #10
480sparky said:
Load diversity.

On a branch circuit, you can have full-load on one side of the MWBC, and zero on the other. This means the neutral must be capable of carrying the full load current.

It's a different story with a service. First, it's rare that the service runs at 100% of it's rating, and highly improbable that that 100% is all on one hot leg.
What's MWBC?
 

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Can be two sizes smaller than ungrounded conductors. I thought I remember seeing that it can even be as small as the grounding electrode for water for residential.

Our local utility wants full size neutral for commercial services.
 
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