Correct type of wire to connect to A/C outside unit.

Discussion in 'Electrical Forum discussion & Blog' started by pexhouse, Mar 2, 2014.

  1. pexhouse

    pexhouse New Member

    Messages:
    23
    Location:
    united states
    Replacing several runs of wire, including both 240s for the dryer and outside A/C unit. All is good except I'm not sure about the wiring for the outside unit.

    What is there now is in poor shape and is smashed in the walls between the 2x4s and the brick wall. I don't think there's a linear foot that is done to code and looks like it's been there nearly since the home was built in 1958 (judging by the paint colors that are splattered on it). Also it's 2 different sizes with half being 10ga and half 8ga (transition takes place in a metal ungrounded box with no cover and no wire nuts). Anyway, long story short was going to replace the whole run. First problem I think I solved. First the specs:

    Trane 3-ton unit manufactured 1994

    Phase 1
    Minimum Circuit Ampacity: 23.0 amps
    Recommended fuse/breaker (hacr): 40amps
    Max fuse/breaker (hacr): 40amps
    approximate length of run from breaker box to outside unit: 80' (5' covered by insulation, 10' in hot attic, the rest through a wall and a cool, dry crawl space)

    I already have a new Square D QO 40amp breaker on it. I understand before the NEC 2011 that it could run on 12ga wire since that allowed for up to 25 amps on an a/c circuit but now it's lowered to 20 amps, thus mine now would require 10ga wire. However, before I understood this I already purchased 125' of 8/2 (with 10ga ground), and with the price of the 10/2 being roughly the same price as the 8/2 (100' roll of 10/2 vs 125' roll of 8/2), and when the A/C gets replaced I want to put a larger unit in that will hopefully cool better, I'd just assume stick with and install the 8/2. That was the first problem but like I said I think it's figured out that there's nothing wrong with putting larger than required in there...???

    Second problem: What TYPE of wire? I'm attaching a picture of how it looks there with the disconnect box cover removed (just for a FYI). I've been reading that NM CANNOT go in that flex pvc tubing? However, that's what's currently in there and been fine for 40+ years. I'm not sure what else I could do and there's no way I'm running 3 single THWN wires through tubing up the wall, through the attic and down into the crawl space. I can think of easier ways to commit suicide.... Not to mention I'm not drilling or chiseling out any more where all of that comes through the wall since it's scrunched in there with the refrigerant lines...

    Some on the forums I was reading just said they install NM through to the disconnect, whether it's a DC on the side of the house or a pull-out DC like mine is, and then run the THWN from the disconnect to the unit. It isn't strict to the code but it is what it is I guess.


    Anyone have any ideas or input to throw me???

    TIA

    fr_302_size1024.jpg
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,948
    Location:
    New England
    As I understand it, you can always use larger wire IF the breaker or terminals are designed to accept it. It will have less voltage drop over longer distances. As to the rest of your questions, can't answer.

    If the issue with the a/c is that it doesn't dehumidify the house, it may be that it is too big already. One too big will not run long enough to dehumidify the air. Maybe the better thing to do is to add as much insulation and plug holes as you can, which may minimize the size of the a/c you need and make the house more comfortable, and will help with dehumidification just by closing the holes. Also, seal all of the ducts so they don't leak.
  3. pexhouse

    pexhouse New Member

    Messages:
    23
    Location:
    united states
    Thanks for your reply. :)

    No there's no problem with humidity--it's just a strictly cooling problem. My neighbor across the street has an A/C unit that really cools it down much faster (600 sq ft larger house, but correspondingly larger A/C system...only thing I know about its size is it's on 50 amp breakers) and he doesn't have any humidity problems or problems with it freezing. Compare that with my house and it takes much longer to cool the main part down while the back den (approx 300 sq ft) it essentially doesn't cool it at all and it stays warm all summer. I was looking at getting a 12,000 (1 ton?) BTU window unit for the back den. That's fairly large but with the doors open it would help give some extra cooling for the rest of the house. However, instead I think the main house A/C could easily take an extra 1/2 ton and possibly 1 more ton if a scoop is put in the duct work to aim more air at the back den because that would then make the rest of the house warmer.

    This house is poorly insulated. I've sealed up the airgaps best I can. One by one trying to get things in the house upgraded/fixed one step at a time...
  4. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,948
    Location:
    New England
    An oversized a/c unit, once the house is down to your set temperature, will not need to run very often. Having it bigger than desired, the only thing it's good for is that initial cool-down. After that, it won't run as much as one sized properly, and it can only dehumidify while it is running, so while the house may end up your set temp, it is uncomfortable because it is not as dry as a smaller one.

    Sounds like as long as it is working, the money would be much better spent on adding some insulation. Then, what you have can do its job better. Now, if you live in the desert, humidity isn't an issue, but still, the house is more comfortable when the thing can run longer, rather than shorter on/off cycling.
  5. jwelectric

    jwelectric Electrical Contractor/Instructor

    Messages:
    2,532
    Location:
    North Carolina
    NM cable is not allowed outside even in a raceway. You will need a disconnect at the unit unless you can see the breaker and it is less than 50 feet away. The insulation on the copper line set is a problem.
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