Neutral size?

Please settle a bet I have with my brother in law.

We have 100A. Use #3CU I believe. Can the neutral service wire be sized smaller?

 

Neutral is same size. Grounding conductor can be one size smaller in certain instances. The neutral must be sized to handle any unbalanced load.

 

Just to add to that..... any unbalanced load and right up to 100A.

 

The answer is YES! You may have #3 copper conductors in your 100 amp residential service, but #4 copper is all that is required by the NEC. The neutral conductor could be a #4 and it would be smaller and still be legal. That being said, I am not a fan of reducing neutrals and wouldn't do it even on a 200 amp service.

 

hmmm; 220.61 provides for reduced neutral size, but ONLY IF you can prove the maximum possible unbalanced load is accommodated by the ampacity of the so-called neutral conductor. For instance, if you have several 240V and 120V loads, you may calculate the worst case load on one side of the service using only the 120V loads. So, I take this to mean that if you have 5, 20 AMP 120V branch breakers on one side of your panel you MAY NOT reduce the neutral size on a 100A service. If you have 2 x 15A and 2 x 20A breakers on the worst case side then you may size for 70 AMP, since that would be your worse case unbalanced load.
However, Table 310.15 B 6 starts service ampacity at 100Amps and #4 copper; calling for a #6 neutral may raise questions from the inspector to prove this case, and I would not find this risk acceptable given the $ savings. Larger services have more value in doing the neutral reduction.

The OP needs to know the insulation of the #3, in any case. I've seen in the field a #4 Cu TW feeding a 100A breaker, which is not correct with a non-motor load.