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.
I might be doing something wrong, but I'm only calculating 12.2 amperes for branch-circuit conductor ampacity. (9.2 amps + 0.7 amps + (9.2 amps * 0.25) = 12.2) (NEC 440.33).– Tester101May 22, 2017 at 16:16
4It says right on the label that the fuse or circuit breaker cannot be above 15A, so based on his statement that a 20A breaker is necessary... I would get another HVAC guy.– statueuphemismMay 22, 2017 at 17:19
Thanks for confirming my suspicion. He mentioned something about an 80% rule? Can someone explain that.– What-About-BobMay 22, 2017 at 17:48
1I suspect your HVAC guy is applying the "80% rule" assuming that the condenser requires 15A (by misreading the label). In fact the label already has the "80% rule" applied to its actual 12A requirement (80% of 15 is 12).– brhansMay 22, 2017 at 18:06
3Bottom line is, there is NO blanket "80% rule". Many people who don't know what they are talking about simply apply it to everything as a one-size-fits-all rule, which is WRONG.– Speedy PeteyMay 23, 2017 at 0:41
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.
The HVAC guy obviously knows more than the manufacturer and engineer(s) who approved that label, apparently. But seriously, folks, max means max. Chances are that a bigger breaker could allow an objectionable amount of current into the condenser, resulting in something bad happening, and leaving you liable if you’re the one who flaunted the label and put too big of a breaker on the circuit. You don’t know what might happen. Presumably the engineer who put the specifications on that label does know, and said the breaker cannot be bigger than 15A. Oct 21, 2017 at 10:59
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. :)
It's probably because the people who wire houses don't take time to do a complete NEC calculation on most houses. They will always wire the following 30A for the AC, 30A for a dryer, 30A for a water heater, 50A for a range and 70A for the electric heat. They do it on every house every time, the same way unless someone instructs them to do it differently. Then they may still do it 50 percent of the time. May 22, 2017 at 20:10
@RetiredMasterElectrician, I agree! You are more qualified to answer than me so if you answered I'd upvote you. As I learn little by little what electricians do at different sites, it seems that just because something is up to code doesn't mean it's right. But also, I've been humbled by the knowledge of other electricians as well.– FufferMay 22, 2017 at 21:01
2@RetiredMasterElectrician, you need to associate with a better class of electrician then. I have never in my life simply followed that silly list of breaker/circuits for those items, except for the dryer and water heater. May 23, 2017 at 0:38
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.