Single-phase generators supplying three-phase systems

[kwired]

If you provide neutral 1/2 or 1/3 rating of the phase conductor, then what you say is correct. But normally same size neutral is provided as that of phase conductor for small sizes of cables..............

Which is why several people keep saying not to supply traditional multiwire circuits with a non multiwire source, or in OP's case not to supply a MWBC using all three phases with only the single phase multiwire source.

 

[GoldDigger]

If you provide neutral 1/2 or 1/3 rating of the phase conductor, then what you say is correct. But normally same size neutral is provided as that of phase conductor for small sizes of cables..............

You are missing the point, I fear.
Suppose I use a full sized neutral in a three phase MWBC.
Each phase and the neutral are sized for 20A and they have a three pole 20A breaker.
With a balanced set of line to neutral loads of 16A each the neutral current will be zero. But I have a full sized neutral since I could turn on only one of the three loads, putting 16A on the neutral.
This fits nicely with what you describe.
Now feed all three hots from a single 120V 100A generator.
The generator will supply 48A and the neutral will also carry 48A.
Do you now see the problem?
Or were you saying that a *full sized* neutral for a 20A three phase MWBC should always be sized for 60A?