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Thread: Wire Size Help Please

  1. #1

    Wire Size Help Please

    I have a single phase 220 volt air conditioner installed in my workshop (garage) and I need to run power to it. A sticker on the AC says to use a 20 A circuit breaker (got it). The run is up into the un-insulated area above the garage and then down through an uninsulated wall to the panel for a total run of about 50 feet. What wire do I need to run? Any other questions I need to ask and consider.

    Thanks in advance.

    Bill Clifton

  2. #2
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    12 gauge should be fine for this run.

    Bruce
    Epilog TT 35W, 2 LMI SE225CV's
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  3. #3
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    I guess you know you need a double 20 A breaker for 220v.

  4. #4
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    We are talking about running NM wiring, correct?
    In other words, you're not making a 50' long "extension cord".
    "The first thing you need to know, will likely be the last thing you learn." (Unknown)

  5. #5
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    How hot does your attic get in summer? you may have to derate the wire if it is too hot. Will it be NM in the attic or will it be in conduit? I hate to think how hot the attic gets when it is 120 in the shade.
    Bill
    Last edited by Bill Dufour; 12-05-2017 at 8:30 PM.

  6. #6
    12 gauge wire is good, 10 is better, especially if it does get hot as Bill says.

  7. #7
    I would use 12. 10 is OK but likely wasting money and is harder to work with. 20A=12 gauge. 220V makes no difference other than you need another hot conductor.

  8. #8
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    If using NM-B, it utilizes 90° C rated conductors. The need to compensate to #10 for high ambient temperature would only arise if the attic temperature ever got above 149° F.
    Last edited by Matt Marsh; 12-06-2017 at 4:17 PM.

  9. #9
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    Quote Originally Posted by Matt Marsh View Post
    If using NM-B, it utilizes 90° C rated conductors. The need to compensate to #10 for high ambient temperature would only arise if the attic temperature ever got above 149° F.
    90c is 194f, not 149. If his attic is hitting 194, he’s got bigger problems.
    10awg is a waste of money. 20amp=12awg.

  10. #10
    Not enough distance or potential heat to require #10. One 20A, 2 pole breaker and some 12-3 NM and you're good to go, as long as that works with your local code.
    “Travel is fatal to prejudice, bigotry, and narrow-mindedness..." - Mark Twain

  11. #11
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    He needs 12/2 nm not 12/3.

  12. #12
    Quote Originally Posted by Mike Heidrick View Post
    He needs 12/2 nm not 12/3.
    And with 12/2, make sure to wrap both ends of the white wire with black or red electrical tape to denote it is being used as a 'hot'. It may not matter to you, but will keep everybody happy - - code enforcement and/or a home inspector (when you sell).

  13. #13
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    Quote Originally Posted by Ben Grefe View Post
    90c is 194f, not 149. If his attic is hitting 194, he’s got bigger problems.
    10awg is a waste of money. 20amp=12awg.
    Ben, the wire itself is rated 90° C, but derating would necessitate increasing the size of the wire above an ambient temperature of 149° F. The derating factor for 141-149° F is .71. The derating factor for150-158° F is .63. The full rating of a 90° C rated #12 is 30 amps. 30 X .71 = 21.3 amps, still above the required maximum overcurrent device of 20 amps. But .63 X 30 = 18.9 amps, so at that point you would need to go to the next larger sized 90° C conductor.
    Last edited by Matt Marsh; 12-14-2017 at 11:20 AM.

  14. #14
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    Quote Originally Posted by Julie Moriarty View Post
    Not enough distance or potential heat to require #10. One 20A, 2 pole breaker and some 12-3 NM and you're good to go, as long as that works with your local code.
    I have another take on this, perhaps from my tendency to over-engineer things. If I'm going to the effort of running wires in the walls for and ceiling for a 240v circuit to a power tool and #12 is sufficient, I will usually run #10 instead. That gives me flexibility in the future to use that circuit for a larger load with a larger breaker. In fact, I generally just install the larger breaker at the time I run the wire or make a note to my future self in the panel.

    The extra cost is insignificant compared to the installation effort. #10 is harder to work with but after working with #1, 4, 6, and 8 everything else is easy.

    JKJ

  15. #15
    Quote Originally Posted by Mike Heidrick View Post
    He needs 12/2 nm not 12/3.
    I guess it's easy to tell I come from the land of conduit.

    Quote Originally Posted by John K Jordan View Post
    I have another take on this, perhaps from my tendency to over-engineer things. If I'm going to the effort of running wires in the walls for and ceiling for a 240v circuit to a power tool and #12 is sufficient, I will usually run #10 instead. That gives me flexibility in the future to use that circuit for a larger load with a larger breaker. In fact, I generally just install the larger breaker at the time I run the wire or make a note to my future self in the panel.

    The extra cost is insignificant compared to the installation effort. #10 is harder to work with but after working with #1, 4, 6, and 8 everything else is easy.

    JKJ
    John, he says it's for an air conditioner. Considering he said the unit requires a 20A feed, I'm taking that to mean it's a window unit or some portable floor unit. Practically every one of those type units sold today tops out at the 20A trip requirement. Plus, just think of the PITA wiring up a 20A receptacle to #10 wire in a 2x4 box.

    On the other hand, if that AC receptacle will one day become a feed for a big machine and he doesn't mind tackling wiring up a 20A receptacle to #10, then have at it.
    “Travel is fatal to prejudice, bigotry, and narrow-mindedness..." - Mark Twain

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