I'm buying a used AC condenser for a rental property. The unit says max amps = 15 (see below), however the HVAC guy tells me I want a 20AMP double pole breaker with 10 AWG? I would have though 15 AMP double pole with 12 AWG would be fine. Is he correct? The wire length is probably 20 feet max.
It says specifically that you must use a 15A breaker max. Max means not 20.
It also says you must use a HACR breaker. All new breakers are HACR.
You are required to comply with a component's labeling and instructions. NEC 110.3. You cannot go "la la la, I think I'll do something else because reasons". Unless you can convince your AHJ of the reasons. (AHJ is your local government which issues permits and does inspections.)
Why is the wire 10 AWG? Being Future-proof. The cost of wire ($5) is next to nothing compared to the cost of tearing out 12 AWG wire to replace it with 10 AWG (hundred$). The last guy installed the heavier wire so you'd be "good to go". You do still need to change the breaker to correct size, but that's cheap.
Here is a good article and presentation about 100% vs. 80% rated breakers.
In this case, I trust your HVAC guy even though it appears to me as well that he is oversizing the wire and the breaker.
I am an engineer who has worked on testing breaker and relay products for electricians in the past. Thermal-magnetic breakers can trip at levels below even their 80% rating if temperature in the breaker panel is high. Also, a brown out or saggy mains voltage can cause increased current needs to your device and result in breaker trips. Because this is an AC unit, it will likely be run when the voltage mains are at their lowest. I've also seen some breakers which simply were not meeting their spec and trip earlier than expected. As for the 10AWG wire, I suspect he chose this not to meet code, but to eliminate voltage drop in your wire run and hopefully stop any nuisance tripping.
So your guy might be just playing it safe, but I think he may have reasons to suspect you will be getting nuisance tripping on that unit unless you do as he said. You wouldn't want your tenants to be calling over and over because you cheaped out on this. And you would likely then call to complain to him that he messed up. :)
Typically in an ideal situation I would use the next size up breaker and size the wire accordingly. Then at the disconnect use the size fuse required by the unit. The fuses are slow blow so they won’t pop with the high startup current but the breakers blow quickly. Doing it this way tends to have fewer nuisance trips. Wiring everything for 15 amps would be perfectly acceptable and realistically should not have any problems. The important thing is to have something that will trip at 15 amps to avoid damage to the air conditioner.