Installing a garage mini split with AC and heat. Running <25 ft of 12 gauge to disconnect and 10 gauge whip to condenser. Unit specs say rated for 8.7 Amp with max Amps in heat mode at 13.0 amps. Should I use a 20 Amp breaker due to the 80% breaker rule or is a 15 Amp ok because the 13.0 amps are split between legs?
The unit is a Daizuki DXTH18X426A-20
To maintain code, UL et al compliance, and insurance coverage, you have to follow the manufacturer's instructions.
If the unit is 240V, the 13 amps are not split between the legs. 13 amps comes in, 13 amps goes out. (Actually, it's the same for 120V, but only one leg.)
The model number posted has an instruction manual, which says:
1）Must follow the electric safety regulations when installing the unit.
2）According to the local safety regulations, use qualified power supply circuit and air switch.
3）Make sure the power supply matches with the requirement of air conditioner.
Though the instructions do not apparently state the circuit breaker size, and the table for wire size does say a 13-amp load should have a 1.5 sq mm wire size (a little less than 14 gauge) point 1 says electrical code should be followed.
NEC requires circuits feeding long-term heavy-draw devices to be de-rated 80%:
15-amp circuit = 12 amps max de-rated
20-amp circuit = 16 amps max de-rated
Since the unit has 13 Amps draw, a 20 amp breaker would be used, with 12-gauge wire to the unit.
(NEC requirements are deduced from "KM DIY Home"'s mention of split-leg power supply and the 80% rule. If this deduction is not accurate, the local authority should be followed.)
Per your comment on this answer:
Service Install Instructions don't specify circuit protection level. Unit say minimum circuit ampacity 13A / Maximum Fuse 20 A. My original thought when I selected the 15 Amp was driven by safety. After purchase I started to question if the 20Amp was a better choice.
If you ran the appropriate wire gauge then you can go up to a 20 amp breaker.
15A fuse is acceptable, it's greater than min circ ampacity, greater than 8.7A.
20A fuse is the maximum acceptable.
Wire size and overcurrent protection for Air Conditioning are calculated independently. NEC 440.6 says conductors are sized per 310.16, so if unit minimum ampacity is 13A, then #14 will be good since rated for 15A. (The "small conductor" overcurrent rules in 240.4(D)(4) do not apply per NEC 240.3, 240.4(D), 240.4(G)).
Also if the label on the unit says "maximum fuse size" the then you must use a fused disconnect in addition to the breaker in your panel. If it says "maximum overcurrent protection" then breaker protection at the panel is adequate and the local service disconnect can be fused, unfused, or additional breaker.
