Splice USE-2 to THWN - best way?

Hi all,
I am installing racking and panels on the roof and will be starting the wiring process shortly. I have 2 strings of 14 panels and already have 3/4" EMT running from the Inverter to the roof. Now for the question - what would you recommend for a roof-mounted junction box and splicing method to go from the USE-2 wire to the THWN going down to the inverter?

I'm a noob installing my first system, thanks in advance for your help!

BTW - Maricopa County (AZ) is who will be inspecting the system.

Craig

Comments

  • solarix
    solarix Solar Expert Posts: 713 ✭✭
    Re: Splice USE-2 to THWN - best way?

    Water proof junction boxes (they are never really waterproof due to condensation - so make a way for it to drain) with basic wire nuts (code rated to 600V) is fine. You can go with sealed wire nuts like Polaris connectors) but really - wire nuts make a direct, gas-tight, contact between the actual conductors and will be reliable. Inspectors will be good with that, but they rarely go on roofs and don't open junction boxes either (inspectors rarely catch real problems - thats why you should have a licensed electrician). Junction box needs to be listed for exterior use. They have plastic ones as such, but in Phoenix, I'd stay away from any kind of plastic (even gray PVC goes bad in dry heat). If you want to look professional, spend a bit more for a steel NEMA-3R box that doesn't need a gasket for the cover (you gotta mount it on a vertical surface though).
    One thing we've started doing is to use unusually colored wire coming off the roof. Ya, code labeling requirements are coming to warn about the evil "photoelectric power source" in your DC circuit with labels on the EMT and the J-box, but if a future electrician opens up your DC circuit and sees a bunch of weird colors in there, some thinking will occur. Don't use black and red. If you have a grounded inverter, the grounded minus conductor has to be white (or gray), but the positive (hot) conductors can be anything else like yellow and orange. If your inverter is non-grounded (transformerless) make the minus conductors, brown and blue. As an installer, it is really nice to have a different color for all the conductors in the DC conduit - which can be 3 or 4 circuits on many arrays. Really eliminates wiring errors, but does mean an investment in #10 wire inventory.
    The array wiring is generally always black though - so tape the positive leads red. If you have a grounded type inverter, you can get white USE wire for the minus side, but it is a lot less available (can just tape white the ends of black wire in this case). I've also seen USE wire in red, but it doesn't stay red when exposed to sunlight! By the way, for inverters that are ungrounded (that is the dc conductors) generally referred to as transformerless, the array wiring ought to be the double-insulated type called PV wire. USE wire stands for Underground Service Entry and has great, tough insulation, and is rated for, but was never intended for exposed roofs (even the -2 version). PV wire is- although I don't like how fat it is and the outer insulation layer is no where near as tough as USE wire. Also - do search on how to minimize your array wire through "skip wiring".
  • CDN_VT
    CDN_VT Solar Expert Posts: 492 ✭✭✭
    Re: Splice USE-2 to THWN - best way?

    Great Info Solarix , I was thinking the same , as in colours . Best to explain "skip wiring" , since many slang terms take on different meanings . I can see the skip wiring on the DC negative , only .

    VT
  • solarix
    solarix Solar Expert Posts: 713 ✭✭
    Re: Splice USE-2 to THWN - best way?

    "skip wiring" is where you skip every other panel as you wire a string together. Take a row of 8 panels. Normally, you would wire them together one to the next, 1-2-3-4-5-6-7-8 and then run a home run wire to each end of the row. With skip wiring, you start at one end, skipping every other panel to the end and come back on the alternate panels, 1-3-5-7-8-6-4-2. You end up back at the same end and both home runs are at the same end. Saves wire, work, less excess panel leads flopping around, and if you are clever, all the strings on an array can end up very close to your j-box. We use way less PV wire now and that means less voltage drop too. Because leads are polarized with MC4 connectors, its easy to get the polarity right and not get confused by the skipping.
    A lot of panel manufacturers must be clueless about this as they often make the leads an inch or two too short to reach across a skipped panel.
  • peakbagger
    peakbagger Solar Expert Posts: 341 ✭✭✭
    Re: Splice USE-2 to THWN - best way?

    I picked up a Soladeck combiner for my system even though I didn't need one as I just have one string. It made a nice place to do the transition from the USE to THHN and included some fused terminal blocks.
  • Vic
    Vic Solar Expert Posts: 3,208 ✭✭✭✭
    Re: Splice USE-2 to THWN - best way?

    Hi solarix,

    Thanks! For the great suggestions!! 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.
  • Craigaz
    Craigaz Registered Users Posts: 4
    Re: Splice USE-2 to THWN - best way?

    Excellent feedback, guys. I went ahead and ordered a couple Polaris splices and I'll put them in a NEMA 3R or better enclosure with a weep hole for drainage. I'll also see if skip wiring is possible, I have Canadian Solar panels and have read on other posts that the cables may be too short. Thanks again for all the great input, I really appreciate it.