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My mini split system call for a 12/3 wire on a 30amp circuit. I thought i had to use 10/3 wire on a 30amp circuit. Is it safe to use the 12/3 wire?

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  • Make/model of system? Dec 26, 2022 at 16:39
  • Usually manufacturers installation instructions are what you must use, but no harm checking if there is a mistake.
    – crip659
    Dec 26, 2022 at 17:34
  • Details matter such as make/model, run length, etc. Update your question with more details. Dec 27, 2022 at 5:58

2 Answers 2

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Motors are weird

240.4(D) lays down a hard limit of 20A for #12 wire. However it exempts a list of exceptions.

Two of them are motors, for which you follow the rules in NEC article 430 instead... and Heat pumps, where you follow Article 440 instead (which turns around and pulls in Article 430 again).

What's going on? Breakers are there to provide three protections: overload (provided by their delayed/thermal trip mode), short-circuit, and bolted hot-ground fault (both provided by their magnetic instant-trip mode). Many motors have overload protection built right into the motor, which works well because it can sense motor temperature.

Thus, the breaker is not needed for overload protection, and the breaker size can go up a bit to reduce nuisance trips from normal motor startup. That's what the Article 430 rules are saying. However, the "nuisance trip" issue should be a non-issue on an inverter drive motor.

The motor or equipment will state the minimum and maximum breaker size. Use any breaker in that range.

Note that as of Jan 1 2023, NEC 2020 states will need GFCI breakers on their hardwired outdoor mini-splits (because they're outdoors) unless your state deleted that #$?§& requirement. Which many did.

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The NEC allows 30 A through an AWG12 copper wire if the cable is rated for 90°C.

Note that this is for 30°C ambient; if you live in a very hot climate you may actually exceed this if you have an outdoor run to your mini-split on a hot day (when, presumably, you'd be running the mini-split a lot). I'd personally run 10/3 anyway, even if it's not strictly necessary.

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    I suspect there is something else going on here. Cable - and "/3" typically is shorthand for a cable, not (though allowed) for "3 individual wires" - is normally at 60 C or possibly 75 C, not 90 C rated. To blindly say "12/3" would encourage people to use cable that really is not valid for 30A under normal usage. My hunch is that it has to do with motor ratings, which are different. Possibly the mini-split really only needs a 25A breaker but they say 30A anyway (my electrician actually put in a 25A breaker for the A/C to replace 30A fuse when replacing my panel) as 30A is more common. Dec 26, 2022 at 17:13
  • That would at least fit in terms of 75 C rating rather than 90 C rating. But still wouldn't work with standard cables. Dec 26, 2022 at 17:13
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    I was thinking of just using 10/3 anyway, but i will need to look at the terminal block to make sure it will handle 10 gauge wire. Thank you!!! Dec 26, 2022 at 17:16
  • From an engineering perspective I agree the breaker spec is most likely 30A to handle inrush current. From a code perspective though a 30A breaker must be connected to a 30A-rated cable. This unit likely pulls about 50% or less while in operation and will never be stressing even a 12/3 90°C cable near its limits. HOWEVER, when this unit fails, or something else goes on this circuit? It's much easier to run 10/3 for every 30A circuit at install time, rather than replace it later.
    – Matt S
    Dec 26, 2022 at 17:17
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    240.4(D) limits 12 AWG to 20A, but for a list of exceptions covered elsewhere in Code. The 90C thermal value is in practical terms, useless except for computing thermal derates. Dec 26, 2022 at 20:00

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