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I am running power for a Mitsubishi mini split MSZ-GL12NA

https://hvacdirect.com/hvac/pdf/Mitsubishi-MSZ-GL12NA-U1-Installation-Manual.pdf

It says max 15amp min 14AWG. I will be running the romex wire about 90 feet from power panel in attic to the unit in Arizona heat. Do I run a double pole 15 amp breaker or double 20? Do I use 14awg wire or 12awg wire? Should I run 3 conductors or 4? Hot Hot Ground Neutral, or just Hot Hot Ground? Thank you in advance for any help. Thank you, Pete

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  • BTW: Is your panel in the attic proper, and is your attic vented/outside-the-insulation, or conditioned/inside-the-insulation? – ThreePhaseEel Apr 17 '20 at 22:53
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The problem is your attic temperature

Phoenix makes outside-the-insulation-envelope attics just stupidly hot, and so you run into trouble at NEC 310.15(B)(2)(a) - the thermal derate.

Suppose we use 12 AWG wire that is allowed 90C for thermal derate purposes (NM-B or UF-B). The 90C column in 310.15(B)(16) gives you 30A "to play with". When you derate that to 50%, you find yourself at 15 amps. Well, look at that. That's enough. And that 50% derate will cover you all the way to 167F, according to 310.15(B)(2)(a). Which hopefully will be enough.

If we need 176F we'll need 10 AWG wire; if we need 185F we'll need 8 AWG wire.

You may want to discuss with your inspector what they expect.

Now, the circuit breaker must be 15A, because the UL-approved instructions say so.

Now it looks like this unit will pull less than 7.5 amps normally. That's less than half of circuit capacity, so you have the option to run a neutral wire and install a service receptacle as ThreePhaseEel describes, and for those reasons.

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  • Thank you for the information! I appreciate it! – Pete Apr 17 '20 at 13:44
  • BTW: if his panel is in the attic, he may need to rerate his breaker, too, due to that same stupid heat you describe. :) – ThreePhaseEel Apr 17 '20 at 22:53
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Use a 15A breaker with 14AWG wire (/3 W/G unless you don't need a service outlet at the disconnect)

The rule for an air conditioner is that you need to use wire that's fat enough to handle the unit's maximum operating current (as given by the Minimum Circuit Ampacity), and you cannot use a breaker larger than the Maximum Overcurrent Protection Device (Max. Fuse on the specsheet you have). This is because the air conditioner has built-in overload protection, so the breaker just has to protect the wiring if it gets shorted.

Also, 240V air conditioners, as a rule, never need a neutral wire by themselves, just hots and a ground. (This is true for everything from large window/packaged-terminal units, through minisplits, up to 5+ton monsters used in light commercial work.) However, running the neutral does let you have a service receptacle at your air conditioner disconnect, something that is required (by NEC 210.63) if there isn't an outdoor receptacle within 25' of the condensing unit already. So, you might as well run the neutral to the unit disconnect box and have it terminate there.

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  • IF, however, you want to run a "service outlet" receptacle to be next to it on the outside wall, so that you have a place to plug in a drop light or power tool, then you might want to run a 14/3 cable, even if you don't use the neutral now. Better to have it and not need it than to need it and not have it. But, it's dealer's choice, the unit doesn't need it.And there would be nothing wrong with running #12 instead of #14, even though you would still use the 15A breaker. Slightly more $$ but less chance of a voltage drop under load (not that it's a high chance anyway). – JRaef Apr 17 '20 at 4:00
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    @JRaef -- fair point re: the neutral! 90' isn't enough for me to get concerned about voltage drop, though, especially on a 208-230V appliance. – ThreePhaseEel Apr 17 '20 at 4:08
  • Thank you for the information! I appreciate it! – Pete Apr 17 '20 at 13:44

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