220.61(A) requires 100% neutrals. It's not so much what *is* on the neutral as what *can* be on the neutral. If you lose a phase, then what? L-N fault insufficient to trip the breaker, then what? 220.61(B) permits some reductions, but you still can't just ignore loads, there's just a demand factor placed on them, and stats flat out that non-linear loads cannot be reduced.
200% neutral comes into play with harmonics and is an engineering decision, not a code-based one. You can have a perfectly balanced load, yet still be putting 170% current onto the neutral simply due to the nature of the load. In fact, the FPN note 2 in 220.61(C) explicitely calls attention to this.
Last edited by SteveFehr; 02/13/08 01:38 PM.
