Thanks Yooper I knew I was missing something but I just could not find it. This is at the end of 215.2(A)(1)
Quote
The size of the feeder circuit grounded conductor shall not be smaller than that required by 250.122, except that 250.122(F) shall not apply where grounded conductors are run in parallel.
So if it cannot be smaller than the equipment ground required according to table 250.122, for a 125 amp breaker the minimum neutral would be #6 copper or #4 aluminum.
