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Thread: where to buy THWN-2 cable?

  1. #1
    JoeT Guest

    Default where to buy THWN-2 cable?

    just got back from home depot...

    Solid or stranded, preferably in #10 AWG

    Home depot only has THHN in that gauge, or #8 is THWN-2 (and #6)

    my application is a wet-rated (in conduit) run of 6 pairs of cable to a PV combiner box with a total 1-way run length of 25', each pair to carry 5 amps @ 24v nominal.

    thanks, I'm striking out.

    one option is to run a #6 pair through the conduit and have the PV combiner box on the roof I suppose, but I'd still need #10 or #12 in THHN-2 or THWN-2 and I can't find that either for inter-connecting the arrays to the combiner box.

    Also: what metallic conduit would you recommend that I buy? (metallic conduit is required right? or at least recommended?) will be a roof install, and secured to the roof directly (stand-off not required for the conduit right?)


  2. #2

    Default Re: where to buy THWN-2 cable?

    How about something like this?

    Jim / crewzer
    120618: System off-line for a while...

  3. #3

    Default Re: where to buy THWN-2 cable?

    I'm assuming that you want an economical solution, and that there aren't other reasons for #10 THWN-2.

    Plain THWN should serve for your application, unless you expect ambient temperatures in your location to exceed 70C. (For example, I hear that in Death Valley, ground temperatures can exceed 100 C, and presumably the air temp just above a hot roof is similar. Even THWN-2 is no good in that heat.) In another posting, I mentioned that for my system, #10 THWN in conduit, when derated for temperature, safety margin, and extra-bright sun per the 2005 NEC, was good to about 11.7 amps. Look for the John Wiles document link in other postings, and run through his step-by-step calculations for sizing wire.

    I noticed the same thing, that #10 generally only comes in THWN for common building wire. If you're dead-set on #10, you can get THWN-2 for that guage from electric distributors, but it seems to be a special order, and that would defeat the cost advantage. USE-2 is fairly available on the internet, and you can get it in #10, and it is superior to THWN-2 in some ways. But again, cost premium. Since it seems you need only 150 feet of wire, if you really want THWN-2, consider getting the #8, anyway. That's perhaps $90 of wire, compared to maybe $60 for #10 THWN.

    RNC (Rigid Nonmetallic Conduit) is equivalent to EMT (Electrical Metallic Tubing) in most cases. If it will be outdoors, then you want the UV-resistant gray PVC conduit, and they stock that at most hardware stores. I went with 3/4", because of considerations mating with the disconnect switches. Cheap, and you don't need a conduit bender, just glue the pre-made fittings. It passed inspection for my jurisdiction; your mileage may vary.

    If it's a location where no one is likely to snag the cables, you may be exempt from running conduit for the first run from the panels to your combiner box - again, check with your local inspectors.

  4. #4
    JoeT Guest

    Default Re: where to buy THWN-2 cable?

    thank you very much!

    USE-2 sounds like a good plan, THWN-2 simply because...

    1. roof install requires conduit (?)
    2. if conduit is used on a roof it is considered 'wet'
    3. if wire is exposed to 'wet' it has to be rated 90*C

    those wires that meet 90*C are only THWN-2 or USE-2 (or some others like that)

    I thought metallic conduit was a requirement per grounding needs? Anyways that's great if it's not.

  5. #5

    Default Re: where to buy THWN-2 cable?

    The part about exposed to wet needs 90 C is not strictly true, only if you need to carry a certain amount of current in the wire. Carrying current heats the wire, and there are tables in the NEC at the library that tell you how much you need to derate the insulation based on ambient temperature. Again, if you run through the steps of the calculations in the John Wiles document, you'll most likely find that THWN will serve for you. I realize now that you live near me, and the temperature above a hot roof here is about 65 C max. You might even be able to get away with #12 THWN for meeting code. However, you'd probably want heavier wire to minimize voltage drop in your 24 volt system. The John Wiles document explains this calculation, too.

    It used to be acceptable to use metal conduit itself as a ground, saving one wire, but recent code no longer allows this in the general case. That doesn't mean you can't use metal conduit, just that you need an explicit ground wire inside, just like you would with PVC.

  6. #6

    Default Re: where to buy THWN-2 cable?

    Reviewing this topic today prompted me to check my wire scrap bucket. It turns out that the 10 AWG wire I get at my local Home Depot strores is 600 V UL-listed type "MTW or THWN-2 or THHN or Gasoline and Oil Resistant II or AWM".

    Jim / crewzer
    120618: System off-line for a while...

  7. #7

    Default Re: where to buy THWN-2 cable?

    If it does not say specifically "UV resistant" stamped on the cable it is not approved.

    That is why we sell the tray cable.

    Per NEC 90°C, wet-rated conductors are necessary
    Northern Arizona Wind & Sun Forum & Website Administrator

  8. #8

    Default Re: where to buy THWN-2 cable?

    Hmmm... What about when the wire is run in a conduit, which is what JoeT originally had in mind?

    Jim / crewzer
    120618: System off-line for a while...

  9. #9

    Default Re: where to buy THWN-2 cable?

    I guess it would be prudent to use 90 C wire, but if you follow the John Wiles "Photovoltaic Power Systems and the 2005 National Electrical Code," and go through the calculations step-by-step, you may find that even 75 C wire suits your purpose. In some parts of the country, I'm sure its a given that roof temperatures will exceed 70 C, and in such places, there is no hope for 75 C wire, but not everywhere. If the current is low enough, and the deratings are applied properly, you might be ok. Especially with only 5A running through the wire, I'm pretty sure you could meet code with #10 THWN in San Jose, Joe (I live here, too). If uncertain, you could always go to City Hall and request a review of your plans by an inspector.

    I originally had TC cable in conduit, and failed. Not all TC cable is rated for 90 C, so you will want to check the specification carefully if you ever plan to put more current through it. With a lower current, I would have passed. Or if the wire was out of conduit, I could have passed. The inspector was fine with non-UV resistant wire entirely enclosed in conduit, which is what I used as a replacement. The conduit itlsef was UV-resistant, of course. I think you only need UV-resistant insulation if you plan to run naked wire (no conduit) outside.

    In summary, my recommendation is to go with #8 THWN-2 in conduit. And have the John Wiles document on hand, the local inspectors revere him, and most of them had a course from him last month when he visited. I was able to reverse one of the inspector's objections by showing an article by Mr. Wiles where he recommended exactly the practice I had employed.

  10. #10

    Default Re: where to buy THWN-2 cable?

    Not all TC cable is rated for 90 C
    All the TC cable that we sell is 90C wet rated. But this may all become moot anyway soon - see notes below

    Just as a note, for the NEC 2007 or 2008 (forgot which) there will be a new wire type designation specifically for PV systems. Excerpt from the article:

    Type USE cable was removed from the list of acceptable cables because it does not have the
    necessary 90°C, wet-rated insulation required in PV module wiring. Type UF cable was
    removed from the list because it is not readily available as a single conductor, and even when
    available, is restricted in the Code (340. and in the UL White Book (General Equipment
    Directory-2005) to a 60°C temperature rating. Type SE was removed from the list because it is
    commonly available only as a multi-conductor cable and may have only a 75°C insulation. A
    listed and labeled Photovoltaic (PV) Wire, complying with a new UL Subject 4703 for such
    cable, was added and is available for these installations. This cable has a 90°C, wet-rated
    insulation that is more durable than SE and USE cable insulation and it has passed the longduration
    720-hour accelerated sunlight/UV exposure tests. Passing such a test will allow the new
    PV wire to be marked “Sunlight Resistant.” This PV Wire will also meet the requirements for
    PV cables on the ungrounded PV systems allowed by 690.35.
    Northern Arizona Wind & Sun Forum & Website Administrator

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