Our house was built in 2003 and it came with a GE 24" microwave oven that uses 120 volts/20 Amp. The microwave stops working so we went out and bought a KitchenAid convection microwave oven that requires 20 Amp, 240 volts, 60 Hertz.

The old microwave oven uses 12/2 w/g romex from a subpanel. In order to use 240 volts, I understand that I'd need to install 2-pole 20 amp breaker, and probably my best bet is to run a new 12/3 w/g romex. Use the black and red wires for hot, white for neutral, and bare wire for ground.

The problem is that it's a two-story house and it's nearly impossible for me to run a new line without ripping out some drywalls which I'm trying to avoid if possible. The house sits on a concrete foundation and the joists on the first floor's ceiling are running vertically from oven to subpanel. I've been on the attic but saw no signs of hope.

The question I have for you is: Can I use the existing 12/2 w/b romex? I'm thinking about using the black and white wires for hot and tape both ends of the white wire with red tape. Use the bare wire as neutral and tape both of its ends with white tape. Finally, share the ground with bare ground wire of the built-in oven sitting below. Just above the microwave oven, there is a built-in oven running 240v/30Amp/60Hz on a 10/3 w/g from a main panel.

So, the wiring would be:

Red wire <---------> white wire taped red
Black wire <---------> black wire
White wire <---------> bare wire taped white
Bare wire <---------> bare wire from built-in oven (10/3 w/g)

Thanks in advance for any help.