Cooling BTUs already take the coefficient of performance of the vapor-compression cycle into account. 4w of heat removed for each 1w of input power is around the max COP for an air cooled condenser, but adding an evaporative cooling tower can raise that up to ~7.

I just looked at a spec sheet for a 230V single-phase 12k BTU mini-split and the minimum circuit ampacity was 3A for the air handler and 12A for the condenser, add those together for 15A, divide by .8 is 18.75A, next size up is 20A. Minimum circuit ampacity is a formula that is (roughly) the sum of the full load amps of the motor(s) inside the piece of equipment times 1.25 to determine the conductor size required to power the equipment.

So the condensing unit likely draws ~9.5-10A max and the air handler around ~2.4A, and both will have variable speed motors that would probably only need about half of that to remove 12k BTU of heat, so ~5-6A or thereabouts should do it, which is around 1/3rd of the 16A server, or a COP of 3.

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Dylan16807 7 days ago | parent [-]

Well I don't know why that unit wants so many amps. The first 12k BTU window unit I looked at on amazon uses 12A at 115V.

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quickthrowman 5 days ago | parent [-]

That is probably just bad data entry at Amazon. I don’t ever trust the specification data on Amazon, I look for the manufacturer’s spec sheet/cutsheet.

In this case, 12A is the maximum continuous load allowed on a 15A breaker. The unit itself probably uses between 900-1000w (7.5A to 8.3A), the spec sheet might say 12A to encourage a dedicated circuit for the A/C unit which then gets added to Amazon’s specs on their website.

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Dylan16807 5 days ago | parent [-]

I think I finally found an actual product page: https://bdachelp.zendesk.com/hc/en-us/articles/2319602600002...

The amazon page specifically said 1354 watts, but I think that's actually for the 14300BTU model. 12000BTU is 9.72 amps.

Anyway, doesn't this make my actual argument stronger? These units fit even better into a normal circuit than I thought, and make the mini-split look even worse in comparison.

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quickthrowman 4 days ago | parent [-]

4.5-5A at 240V = 9.72A at 120V

It’s the same level of power consumption. I’m not even sure what you’re asking at this point, to be honest.

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Dylan16807 4 days ago | parent [-]

You were talking about needing a second 240V 20A circuit, and you later backed that up by citing the spec sheet of 230V mini-split with a minimum circuit rating of 15A.

My argument was that you do not need such a circuit.

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quickthrowman 4 days ago | parent [-]

Technically you’re correct, a 12000 BTU minisplit only uses around 1000 watts while running which is just over 4A.

The breaker size being 20A 2P is a consequence of the NEC requiring you to size the wire based off the equipment nameplate rating of 15A, which is based off the full load amps of the motors inside the equipment.

Full load amps is the max amount of current a motor can draw at a specific voltage and is used for sizing wire and overcurrent protection for a piece of equipment. It doesn’t always match up the current a motor draws while it’s running normally. You take full load amps times 1.25 to get minimum circuit ampacity, which you use to size the conductors.

So while you are correct that a 240V 12000 BTU minisplit wont draw anywhere near 20A, the specific minisplit I looked at required a 20A breaker due to the minimum circuit ampacity being 15A. If the MCA was 12A, you could use a 15A breaker; an MCA of 8A would allow using a 10A breaker, and so on.

If you use fuses, you can size the overcurrent protection at 100%, breakers require 125% of the load for a continuous load. So you could use a 30A fusible disconnect switch fused at 15A for a unit with an MCA of 15A.

	▲	Dylan16807 4 days ago \| parent [-]
		That's not the angle I'm taking. I'm not saying anything about what the mini-split actually uses. Give it the circuit that the nameplate asks for. Instead I'm saying that particular minisplit is a lazy design and we can get a 12000 or higher BTU unit with a much smaller nameplate rating. Not only will it only need a single-pole breaker, the required circuit probably already exists.