Would the following set-up work: 30AMP double-pole breaker in panel, 12-2 NM to single 30AMP 220V receptacle, black and white hot ,with white wire marked with black tape? Ran wire anticipating 110v a/c, lady with the checkbook( my wife) thinks that we will end up with 220V unit. I f that's the case will above set-up work? Or do I need to pull 10-2 to outlet?

12/2 = 20A 120v OR 240v.
10/2 = 30A 120v OR 240v.

General use receptacles (15 & 20A) CANNOT be on a 30A circuit.

All this said, I highly doubt you will need more than a 20A 240v circuit.

12/2 = 20A 120v OR 240v.
10/2 = 30A 120v OR 240v.

General use receptacles (15 & 20A) CANNOT be on a 30A circuit.

All this said, I highly doubt you will need more than a 20A 240v circuit.Thanks, Stanfortyman.very helpful. By the way this circuit will have just the a/c single outlet only. Thanks, again.