A 1000 sq ft house with a full basement, a 12000 watt range and a 6000 watt dryer has a demand of 13500 watts, or 56 amps. It gets a 100 amp service with #6 neutral.

If the same house has an 18 kw electric furnace, it's demand is 31500 watts, or 132 amps. It gets a 200 amp service fed with 2/0 hots. The neutral load is still 56 amps. The ampacity of a 2/0 in a house is 200 amps (note to table 2) and table 17 requires a grounding conductor of 6 awg.

Therefore, this 200 amp service should get a 6 awg neutral.

However, the inspection department has a rule of thumb where they only allow a reduction of two trade sizes, so we must install the larger 1 awg neutral.

I want to do the calculation and install even smaller neutrals. It's in the code.

http://www.codemath.com/cgi-bin/Run.pl?script=Cec8_200