Question on MC cable wiring

VolcanoSolar
VolcanoSolar Solar Expert Posts: 56 ✭
in General Solar Power Topics #1
Hi everyone,

I just found this forum the other day. It's timely for me: I'm setting up my second off-grid system, this time to code, on a new house I'm building in Hawaii. Funny how much harder things get when you have to take code into consideration :^0

Question on wiring the panels, which are Kyocera 170-watt: How does one properly terminate the MC cable from the panels into an outdoor, roof-mounted junction box? Not the wire splice itself, where I assume a wire nut will do, but the shift from unenclosed MC cable (which strikes me as a bad idea, I'd rather see it in flex conduit at least) to enclosed system. Just run it into the open end of a piece of conduit? Pointed downhill to keep the rain out?

Puzzled...
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  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    #2
    Re: Question on MC cable wiring

    You've got the idea I'd use - run the cable UP into a conduit / knockout in the J-box. Not much water will go uphill.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
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  • Roderick
    Roderick Solar Expert Posts: 253 ✭✭
    #3
    Re: Question on MC cable wiring

    It rains a lot in Hawaii, and you might want to go with watertight strain reliefs, like the Heyco part here. It's UV-resistant, rated for wet, and about a dollar. It tightens a rubber bushing around the wire, sealing it.

    I got mine from Mouser electronics on the web, but you can find them all around.

    I'll try to attach a spec sheet.

    Depending on your environment, you might also drill a tiny hole at the lowest point in your junction box to drain water accumulation. Bad idea for a marine environment, or where there are insects that would like to nest in such a place. I can't vouch that doing such a thing would meet code - I didn't drill a hole, but rather, trusted all the gaskets to be watertight.
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  • Vic
    Vic Solar Expert Posts: 3,208 ✭✭✭✭
    #4
    Re: Question on MC cable wiring

    Hi VolcanoS...

    Personally, I use the water tite Nylon cable grip (strain releif). Available from the fine hosts of this site, and also from local electrical supply houses (and some home improvement stores). Usually place this into an aluminum waterproof outdoor box -- the smaller ones used for outdoor outlets and switches. I have also screwed the releif into an EMT fitting as well, but this has never been inspected by our local inspectors, who may have problems with this approach.

    This item looks to work with most XLP/USE2 wire common to MC cables :

    http://store.solar-electric.com/nystreliti1.html

    Good Luck with the inspections. 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.
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  • Roderick
    Roderick Solar Expert Posts: 253 ✭✭
    #5
    Re: Question on MC cable wiring

    Never mind my post, the part is right here on NAWS! Thanks, Vic.

    Here is a pic of my junction box. MC extender cables cut in half, coming in at top and bottom, from each PV string. Plastic conduit down to the DC disconnect on the left.
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  • Vic
    Vic Solar Expert Posts: 3,208 ✭✭✭✭
    #6
    Re: Question on MC cable wiring

    Hi Roderick,

    Your photo shows almost exactly what I've done before, right down to the snippets of insulation where the grounds pass thru the releif. Have been using alunimum boxes and EMT only for the shielding it can/does provide for EMI/RFI.

    Thanks for the photo. 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.
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  • System2
    System2 Posts: 6,290 admin
    #7
    Re: Question on MC cable wiring

    would using metal conduit and metal box's help with EMI?

    Also are the wire nuts kosher and full Code?
    I'll have 2 strings now and a third later to 6 guage wire.
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  • Vic
    Vic Solar Expert Posts: 3,208 ✭✭✭✭
    #8
    Re: Question on MC cable wiring

    Hi Sky...

    Well, in my case, use the box only as a shield. Run the leads from the PV panels into the box and out the other side into the conduit as home runs to the combiner box. SSooo do not use wire nuts. It might be better this way, as each string of panels has its own run and breaker. I dunno about wire nuts vs code on PV wiring. Good Question. Thanks, 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.
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  • Roderick
    Roderick Solar Expert Posts: 253 ✭✭
    #9
    Re: Question on MC cable wiring
    SKYWLKR wrote: »
    would using metal conduit and metal box's help with EMI?

    Also are the wire nuts kosher and full Code?
    I'll have 2 strings now and a third later to 6 guage wire.

    I see that professionals use metal for some reason. The only reason I used plastic was frankly, I'm cheap. I didn't want to have to buy a tool to bend EMT. I kept looking through the NEC at the library to see if there was something that required metal, but couldn't find anything. They used to allow using the metal conduit as ground I think, but no longer allow even that.

    When the inspector opened up the junction box and stared at it for 30 seconds, I wet my pants. I was all prepared to defend the wire nuts as being rated for this kind of application, but he finally shrugged and said, "okay." On the subject of plastic conduit, he had no problems at all. It's as good as metal, as long as it's rated for the exposure and temperature. If the temperature's going to be 200 C, you'd better be using metal, but other than that, plastic's fine.

    I would think, that when wires get fat enough, wire nuts don't work, anymore, and you need to use a terminal block. I don't know what point that is. I do know that you can get these Elephant Nuts that say "3 - #8" on the box. Don't know about #6.
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  • VolcanoSolar
    VolcanoSolar Solar Expert Posts: 56 ✭
    #10
    Re: Question on MC cable wiring

    Thanks everyone. Hey, that was too easy :^)
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  • VolcanoSolar
    VolcanoSolar Solar Expert Posts: 56 ✭
    #11
    Re: Question on MC cable wiring
    Vic wrote: »
    Hi VolcanoS...

    Personally, I use the water tite Nylon cable grip (strain releif). Available from the fine hosts of this site, and also from local electrical supply houses (and some home improvement stores). Usually place this into an aluminum waterproof outdoor box -- the smaller ones used for outdoor outlets and switches. I have also screwed the releif into an EMT fitting as well, but this has never been inspected by our local inspectors, who may have problems with this approach.

    This item looks to work with most XLP/USE2 wire common to MC cables :

    http://store.solar-electric.com/nystreliti1.html

    Good Luck with the inspections. Vic

    I bought some of these, installing them yesterday.

    The gripping/sealing action works fine, but I wouldn't recommend these for PVC junction boxes, because the threads aren't long enough to compensate for the thicker-walled PVC boxes. I would just be getting the nut tight when it would pop loose again. Even another 1/8" of thread would've helped, preferably 1/4". These were with "Scepter" brand 2-gang PVC junction boxes.

    (As for using PVC versus aluminum junction boxes: where I live, the volcanic fog turns aluminum to chalk dust in not too many years.)

    - Ted
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