I just removed an old mini-split from my son's house (hadn't worked for years). it was a 220v Fujitsu and I am putting in a 12k MrCool unit (110v). I am planning on re-using the circuit breaker, which is a 20a, and the wiring from the box to the outside disconnect, which is 10g. I'm pretty sure the wire can be re-used as it seems silly to run new 12g wire from the box, but my question is this: from the disconnect to the mini-split - can I use 12g wire? ADDED INFO: After seeing some replies, I looked at a pic I have of the disconnect box (house is several miles away). Is it possible for 12g to have an orange jacket?
If you have cable (thus, a white conductor) you can just re-use the cable in place, with appropriate reconnection and removal of any hot marking on the white wire when you move it from the breaker to the neutral bar.
If you have conduit you'll have to pull a white or gray for neutral.
You'll only be using 1/2 of the existing breaker.
Since the breaker is 20A, 12 AWG is fine as the minimum wire size you can use.
12 gauge is the minimum allowed on a 20 amp circuit/breaker. You cannot have 14 gauge.
So using 12 gauge is okay, even if some of the circuit has 10 gauge.
I imagine the old circuit had 10/3 cable,two hot and a neutral(white) plus ground.
With 120 volt circuit you only need one hot and one neutral(must be white or grey) wire plus ground.
The extra hot wire of the old circuit will need to be capped off(wire nut) at both ends.