Never use the ground for neutral. In re: the 120 outlet wired with 12 gauge wire, I see no reason why you couldn't convert it to 240V use by using the white as hot (marked with black tape at each end to mark it hot). No neutral would be required with a straight 240V circuit, though a ground is always required. If your AC unit is 120/240V (I've never seen one, but they may exist for all I know) then you can't use the 120V circuit as there is no way to provide a neutral with 12-2.Thank you.
ok let me expain more ,i am planning to install an AC unit which requries 220v single phase.I have a 120v outlet right there,with 2 wires and a ground AWG12,what is the best way to get what i need to get that unit running ,can i use the ground for neutral,and the black and white for hot.
Excellent point. A 120V circuit probably has several receptacles or lights, etc. connected to it, thus rendering it unsuitable for easy conversion to 240V.This idea will work provided the following conditions are met.
1) The outlet in question is dedicated. Meaning there are no other outlets on the circuit.