NEC code and wiring

Hello,

I have read the NEC pdf that is for PV systems that is floating around the internet.

One thing I do not understand is that in his example systems, he is showing that expensive
sunlight resistant solar cable inside conduit to the inverter.

It seems to me that if the wire is in conduit, we should be able to use 600V THHN wire of the
appropriate diameter.

???

GB

Comments

  • Photowhit
    Photowhit Solar Expert Posts: 6,002 ✭✭✭✭✭
    Re: NEC code and wiring

    I know ground mounted systems must be in conuit or have some means of limiting access such as a fence, perhaps thats where your seeing this.

    You might provide a link to this PDF so we're all talking about apples.
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
  • solarix
    solarix Solar Expert Posts: 713 ✭✭
    Re: NEC code and wiring

    The problem with using standard wire like THHN is that somewhere you have to transition to PV wire when connection to the modules. Most of the time it is easier to just run the PV wire all the way to the inverter rather than add junction boxes etc. Actually, here in the sunny SW, I usually run the PV wire in conduit even though it is rated for exposure as I just don't trust any kind of insulation in direct sunlight for 25+ years. I only expose the wire (going into the array) where it is shaded. In the old days, the PV modules had regular junction boxes that could be hard plumbed with conduit all the way, but that was way slower than MC4 plug and play. Now with high voltage strings of modules it is so much easier clicking the wiring together. PV wire is now readily available and not so expensive in smaller quantities (250') that using it exclusively is not so bad. The special crimp tool is way expensive though. If I was a DIY'er, I would just use a universal crimp tool and solder them to be sure.
  • Photowhit
    Photowhit Solar Expert Posts: 6,002 ✭✭✭✭✭
    Re: NEC code and wiring

    I'd buy a non certified MC4 crimping tool (about $30) before I soldered anything that would flex often.
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
  • System2
    System2 Posts: 6,290 admin
    Re: NEC code and wiring

    I think that this article might relate to your question:
    USE-2 conductors undergo a 350-hour accelerated
    UV exposure test. This length of test time is not sufficient
    to allow the USE-2 conductors to be marked “Sunlight
    Resistant” because that marking requires a conductor
    or product so marked to be tested for 720 hours.
  • Photowhit
    Photowhit Solar Expert Posts: 6,002 ✭✭✭✭✭
    Re: NEC code and wiring

    I don't know if the original poster has gotten his answer, or indeed, if we understand his question...

    I read the PDF the previous poster provided and it just touches on the point I was making. There is a previous thread here but I can't search. Im at home on my kindle.

    The thread has to do with NEC requiring any wiring with DC voltage above 30 volts to be run in conduit or to be in a limited access area.

    I'll search when I'm on a proper computer.
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
  • Vic
    Vic Solar Expert Posts: 3,208 ✭✭✭✭
    Re: NEC code and wiring

    Regarding running USE-2/RHH/RHW/XLP etc PV wire in conduit:

    THese conductors have insulation that is thicker than the Compact Conductors like THHN, and as such, would require larger diameter conduit for the runs. So there are trade-offs.

    I have been running the PV conductors to a junction box or Conduit Body for the transition to THHN conductors. This works for me. Furthermore, most of the available wiring, like XLP are 7-stranded conductors, vs THHN's common 19-conductors. This finer stranding allows easier pulls in conduit .. JMHO, am not a real por. Vic
    Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH X2@48V, 11.1 KW STC PV, 4X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  25 KVA Polyphase Kubota diesel,  Honda Eu6500isa,  Eu3000is-es, Eu2000,  Eu1000 gensets.  Thanks Wind-Sun for this great Forum.
  • SolaRevolution
    SolaRevolution Solar Expert Posts: 410 ✭✭
    Re: NEC code and wiring

    "It seems to me that if the wire is in conduit, we should be able to use 600V THHN wire of the
    appropriate diameter." - GB



    Yes. THHN, THW is fine for PV output wiring inside conduit. If your PV modules have junction boxes which can accomodate conduit you have no need for the USE-2, RHH wire.

    If you do not have J-boxes then you must have the USE-2 wire. It would usually be run to a string combiner or a j-box at (or very near) the array location and terminate to busbars or breakers. The output from the combiner box can then be THHN in conduit to the PV disconnect which feeds to the charge controller or inverter. Depending on the specifics of the site you may not need a combiner or j-box in which case it may be more trouble than it's worth to to change from USE-2 to THHN.



    I do a lot of service work for older PV systems. I have seen USE-2, RHH which has been exposed for 20+ years. The places where it is exposed to direct sunlight tend to get bleached and dusty dry on the outside but the insulation remains effective and does not seem in danger of cracking or peeling off. THHN has no business being outside of conduit!

    The main problem I see (where everything is up to code) is where the connections are made. Damp enclosures often have white or greenish corrosion in the connections. I recommend using electrical joint compound in all DC connections. Rather than trying to completely seal an exposed j-box, expect that it will get wet and be sure that water can get out through weep holes in the bottom. Also, be sure your PV output wiring has drip loops so water does not just run down the wire and into the j-box.

    Zip-ties should not be in direct sunlight. Even the UV resistant ones don't last long when directly exposed. Try to keep them shaded.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: NEC code and wiring

    One person (electrician?) had good luck with "Lead-plate" anti-seize lubricant for "wet" electrical connections over the years.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Photowhit
    Photowhit Solar Expert Posts: 6,002 ✭✭✭✭✭
    Re: NEC code and wiring

    I think the original poster went south (or at least lives south in GA...LOL) but I found the former thread I had started about "IS it True? Code requires conduit for panels runs less than 30 volts?"
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
  • gfbankston
    gfbankston Registered Users Posts: 12
    Re: NEC code and wiring

    Thanks for the answer, had family problems, went out-of-town...

    Panels have just arrived and should start mounting them today. My mentor showed me his installation and he used weather heads to run the USE-2 wire into. Then, in his junction box he has his circuit breakers, and runs THHN from there to the inverters, maybe 75 feet away. He also uses split bolts to make junctions, then makes those connections weatherproof (outside the conduit), and I saw this is acceptable via the code.