Why would an air conditioning unit specify a maximum breaker size?

Question

A ductless mini split unit I am interested in specifies a maximum breaker size of 15 amps. Why would this be the case?

The unit is going to be installed on the opposite side of the house from where the breaker is, so for a 220v system that runs at about 5 amps, a 70 foot run of electrical wire calls for 8 gauge wire.

I was going to throw a 20 amp circuit breaker for this run until I came across the spec sheet for the unit. I found a similar post where the top answer states that you can always use bigger wire which only reassures me that 8 gauge wire is fine in a 15 amp breaker, but I can't seem to find why the AC unit would call for no bigger than a 15 amp breaker.

In other words, why would a device on a circuit be sensitive to the potential max amperage of its circuit

Answer 1

The unit won't be protected properly by a larger breaker

While hermetic compressors and fan motors are protected against overload by internal means, or by their drive electronics if you have an inverter-based system, the wiring of the unit still needs to be protected in case it shorts out internally. As a result of this and the fact that air conditioners universally are complex, multimotor loads, UL has taken pity on installers and inspectors and requires the manufacturer to do the breaker sizing math for us. (Otherwise, you'd have to dig into Articles 430 and 440 of the NEC and size the breaker yourself, which is a bit fiddly for a multimotor load.)

So, simply use the 15A breaker the manufacturer calls for and call it a day, although I doubt you need 8AWG wire for a unit that draws less than 10A at 240VAC. In fact, for that run, 14AWG is perfectly fine. (The chart you found is probably for low-voltage DC circuits, where voltage drop is a far greater limiting factor than at 240V.) However, there is some merit to future-proofing an air conditioner circuit, and to that end, I'd run ½" ENT as that'll get you up to 50A with THHN wire and the assumption that your panel supports 75°C terminations, then pull the 3 14AWG wires needed through it.

Answer 2

It's to limit the time it takes for the breaker to trip in the case a failure less than a dead short. The time curve for all breakers aren't identical, but suppose the unit fails and it draws 30A, which is 5x rated current. A 15A thermal-magnetic breaker that trips on an inverse time curve (see below) should trip in less than a minute, a 20A breaker could hold for 5 minutes. That heat could be the difference between replacing components and replacing the whole thing.

The unit itself may also have some internal protection less than 15A, but 15A is the smallest size the NEC recognizes in electrical panels.

I've seen the chart you referenced above, I think it still had the source, but it is completely inaccurate for AC circuits and modern wire insulation. I suggest you consult some online voltage drop calculators (1 2 3) to see the actual loss, less than 5% is good, 3% is better.

Answer 3

According to Southwire voltage drop chart 70 ft run at 240 volts ac with 9 and 1/2 amps you would have a voltage drop of 1.49 volts ac.14 gauge wire would be fine that is within the 3% voltage drop.

Answer 4

Probably you got 1 hp aircon,at Nec table it is 8 amps. Multiplied by 1.75 equals 14 amps so ,15 At CB make sense. For wire 8 x 1.25 equals 10 amps only so #12 wire can be used.