Here's what I was thinking about. There is only a limited space inside these things for wiring and separate cables would mean separate Neutrals would have to be run and really cram up the inside. If there was only one neutral to the unit then it would have to be inside the same cable or raceway with all the others. If the Neutrals in the unit for the 3 functions are tied together it can't be fed from more than 1 cable without being a violation. I don't know that the average person wiring these things would be aware of that.
I don't see why it couldn't be on 1 20A circuit with a 12/4+Gr MC feed to it though.
Bill
[This message has been edited by Bill Addiss (edited 10-24-2001).]
