I did find a website that apparently references the NEC:
“Sizing the neutral: Sec. 220-22. You must size the neutral conductor to carry the maximum unbalanced current in the circuit (i.e. the largest load between the neutral and anyone ungrounded phase conductor)...”
The website also says...
A transformer supplies 240VAC to a 20A circuit breaker as shown in the attached image. I believe I can use 12AWG wire for wires X1, X4, 2 & 3. However, I'm not sure what gauge wire the neutral should be. Maybe 12AWG or something larger?