there should be some method of calculating the size of the neutral conductor Brian
The 12 wire should be adequate for the load
(cheap shot I know)
I think everyone is just site engineering these as they go along but when I've done it we used a #10 with 3 #12 hot.
But it may all be moot anyway since paired MC is being used in most jobs now anyway.
Right from the panel no conduit on the job at all. awful.