on single phase you can share a common neutral with 2 circuits of the same ampacity as the neutral. for example, two 20 amp circuits can share one 12 gauge neutral. The catch is that the 2 circuits must be on OPPOSITE legs of the 240 volt breaker panel. The neutral will only carry the differnece in current between the two circuits. for exampl, if one circuit has a 12 amp toaster on it and the other has a 9 amp coffee maker, the neutral will be carrying 3 amps
