20 amp breaker on 10/3 wire
 

Is it acceptable to use a 20 amp breaker on 10/3 wire with a 15 amp load (through the wall a/c-heater)? I have already run the 10/3 wire from the breaker panel, when the new panel was installed, and put it on a 30 amp breaker in anticipation of needing a 30 amp circut for a through the wall A/C-heating unit for a guest suite over the garage I am finishing out. Looking at various A/C-heat units, it looks as though a 20 amp circut is all that is needed. I would prefer not to have to pull the 10/3 and run 12/3. Can I just replace the 30 amp breaker with a 20 amp and use the 10/3 wire? Thanks for your help.

Yes, it is fine to use #10 wire with a 20A OCP device. BTW, there are times when this is done, i.e. using a gauge of wire whose ampacity is greater than the breaker rating, to avoid voltage drop in long circuit runs.

And did you really need the 10/3? Most AC/heating units are 240V, so 10/2 (or 12/2) was all you needed.

Well, now that you mention it, I guess not. I suppose I was thinking two hots and a neutral. Amateur hour:bangin:!! Oh well, too late now. I guess this way, I am ready for anything that comes along!!

You could always make that a multi-wire branch circuit and put in two separate outlets with a double pole 20amp breaker. You already have the neutral and your two hots there so there would be no issue with that. Otherwise if you really don't need or want the second outlet, I would just cap off the Red wire on both ends and just use the black, white, and ground for the one outlet.

Since it's a 240v a/c-heat unit, shouldn't I cap off the white (it's landed on the neutral bar in the breaker panel) and use the red and black (landed on the breaker) and ground wires? I have run new circuts (two 20 amp for outlets and bath outlets and two 15 amp for lights and bath light/fan) already so I have plenty of circuts. I just didn't give any thought to only needing 10/2 for the a/c unit. Thanks for everyone's feedback on this. Let's me know to think things through better next time.

There I go "assuming" again! I "assumed" that with it being only 20 amps that it was likely a 120 volt load and not 240 volt. Yes in the case of a 240 volt load you would use the black, red, bare copper ground and cap off the white on both ends. Sorry for any confusion on that.