As pointed out by KJay, 250.66 is the minimum sized grounded conductor (usually a neutral) that is permitted. This size is required to provide adequate capacity to conduct ground fault currents back to the source.
To figure out the the minimum size for a grounded conductor, see 220.61(A) You'll need to determine the net calculated load for each phase and that will be the maximum load on the grounded conductor (pick the largest calculated load of the two phases). Then you can apply 220.61(B)(2) which says you can apply a 70% demand factor to any neutral load in excess of 200 amps. See example D4 in annex D, page 767, left column, near the bottom "feeder neutral" and "further demand factor."
I'm with Hotline, this isn't worth the few $$$ you'll save.
