Why can't you transition from PV Wire to THWN-2 via MC4 connectors?

HX_Guy
HX_Guy Solar Expert Posts: 296 ✭✭✭✭✭✭
in Solar Beginners Corner #1
Or can you? I'm guessing it's not to code, but having a hard time understanding why.

For example if you use PV Wire to link all the panels together in series but then need to run through conduit to the combiner box, the usual way seems to feed the PV Wire into a junction box and splice it with THWN-2 in the box. If MC4 connectors are ok for interlinking modules, why is it not ok as a splice?
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  • Photowhit
    Photowhit Solar Expert Posts: 6,006 ✭✭✭✭✭
    #2
    Re: Why can't you transition from PV Wire to THWN-2 via MC4 connectors?

    MC connectors are made to be water tight, any time you need to run through conduit, you must have wires exit into a weather proof environment. You could use a MC connector in the weather tight box to connect to THWN, or more typically have a bare end spliced to the THWN wire. MC connectors aren't cheap and the correct crimper isn't cheap. UL listed butt connectors run 35 cents, MC4 connectors run $2+ and you should have the correct crimping tool ($200+)

    In normal installations have an MC connector that connects to the string of panels and then the other end sans connector goes through a water proof wire restraint and is combined or spliced for the run to the combiner or the inverter, Newer combiner boxes often have external MC connectors rather than wire restraints.
    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.
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  • HX_Guy
    HX_Guy Solar Expert Posts: 296 ✭✭✭✭✭✭
    #3
    Re: Why can't you transition from PV Wire to THWN-2 via MC4 connectors?

    So a conduit must always terminate in a junction box? I've seen installs where it doesn't, for example if you have a small group of panels on one portion of the roof, and the next group of panels is say 20' away...I've seen where the module wires enter into conduit and the back out 20' later to connect to the next panel. Is that not proper?

    Something like this:

    185515-roof-wiring-conduit.jpg
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  • Photowhit
    Photowhit Solar Expert Posts: 6,006 ✭✭✭✭✭
    #4
    Re: Why can't you transition from PV Wire to THWN-2 via MC4 connectors?

    Never having to deal with code, my guess is they just were trying to keep things clean, wires off roof...

    I have 15 foot runs with wire clipped to the panel frames... conduit serves little use if you are running use2 wire through it, other than keeping things neat...
    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.
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  • HX_Guy
    HX_Guy Solar Expert Posts: 296 ✭✭✭✭✭✭
    #5
    Re: Why can't you transition from PV Wire to THWN-2 via MC4 connectors?

    I have a 40' run where I need bring the wires from a group of panels over to the combiner box, so just ruling loose wire on the roof probably isn't the best solution, need to feed it through conduit. And since it'll be in conduit, it doesn't need to be UV resistant so THWN-2 wire should be good. Just wondering if I can go from the panel's PV Wire/MC4 connector to a THWN-2 wire that has an MC4 connector on it, then feed it through conduit and come out the other side where it would go into the combiner box.
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  • solarix
    solarix Solar Expert Posts: 713 ✭✭
    #6
    Re: Why can't you transition from PV Wire to THWN-2 via MC4 connectors?

    You can't run THHN wire exposed at all. To transition from PV to THHN using MC4 connectors is OK as long as the THHN is fully enclosed. That means the splice needs to be in a j-box, but then a simpler splice is all that is needed not the MC4. On a 40' run, its a tossup as to whether its cheaper to just run the PV wire through conduit or to add a j-box and transition to THHN. (assuming its just one string for that group of panels)
    That's a really nice conduit job in the pic btw. Except should really be up off the roofing more to prevent it from collecting leaves and debris.
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  • HX_Guy
    HX_Guy Solar Expert Posts: 296 ✭✭✭✭✭✭
    #7
    Re: Why can't you transition from PV Wire to THWN-2 via MC4 connectors?

    I've got 6 wires plus the bare ground that need to travel that 40' so I think going with THWN would be more cost effective.

    Another possibility would be I guess to go from PV Wire to USE-2 through the conduit? The only exposed part of the USE-2 would be where it comes out of the conduit which would be under the solar panels.
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  • solarix
    solarix Solar Expert Posts: 713 ✭✭
    #8
    Re: Why can't you transition from PV Wire to THWN-2 via MC4 connectors?

    USE-2 wire pretty much is PV wire and costs about the same. Before the PV wire type became available, standard practice was to use USE-2 (Underground Service Entry and the 2 means sunlight resistant) as it is rated for exposed locations like a rooftop. But USE is not rated for smoke so is not allowed within a building. Also, THWN is insulation rated for wet locations (underground) as opposed to THHN which is for dry locations. Actually, most wire sold today at your average big-box store is rated both THHN/THWN - so there you go. With 7 conductors, just put a simple 4x4 waterproof j-box at the far array and transition to THHN there. You can use the nice "polarus" type sealed splices or actually, regular wire nuts are rated for 600V so are code and can be used. Don't forget to ground the J-box and both ends of the conduit (if the ground wire is in the conduit - just one end if ground is outside) If you use the pot metal j-boxes with the threaded holes for conduit, none of the troublesome conduit ground bushings will be needed.
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  • HX_Guy
    HX_Guy Solar Expert Posts: 296 ✭✭✭✭✭✭
    #9
    Re: Why can't you transition from PV Wire to THWN-2 via MC4 connectors?

    Thanks Solarix for the help.

    What's complicating things is that the combiner box I bought is pre-wired for MC4 connectors, so I have to come out of the conduit with a wire that I can put an MC4 connector on and then plug into the combiner box.

    I was thinking of using USE cable because it's a little thinner than PV Wire, so should be able to fit it in the conduit easier.

    Midnight-MNPV4-MC4-2.jpg
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