Where to combine the strings

gkerlin · July 2015

I'm at a decision point.
My system will be 4 strings 8 panels total, on the roof of the motorhome. 35.4vmp 7.91imp each string. My approx round trip run from the panels to the controller and back would be about 40' max

My ideal situation is to combine them down below. That allows me to install the box in an accessible place and be able to turn off each strings breaker as desired.

If I combine up on the roof then my breakers are not accessible - might as well use fuses.

If I combine down below it means bringing 8 #10 pv wires down a vent. Kind of cumbersome. I've brought 6 down before but 8 is getting to be a crowd.

I was wondering - would it be ok to combine the negative up on the roof, then use an appropriate size connector for the negative side to controller (I think #6 would give me room for longer conductors or room for more growth) and then I'd bring down 5 wires. (The 4 Pos PV wires and the combined #6 negative)?

I could use something like a Midnight Solar Baby enclosure to combine the 4 pos leads down below after each string passes thru the breakers. Then after the breakers heading to the controller I'd also use #6.

I haven't found any rooftop combiner boxes for the positive side and would likely have to make one and I'd rather have the positive combined below. Any issues with combining the negative up on the roof and the positive below?

Thanks as always.

Greg

vtmaps · July 2015

From an electrical circuitry point of view, I can't think of any reasons not to. But there may be practical reasons. You should not cut off the connectors on your PV wires. You need a weatherproof junction box to transition from the PV wires to the #6 cable. The #6 cable needs to be PV rated or at least be USE cable. Seems like a lot of trouble just to avoid bringing the PV cables into the rig.

I'm not too familiar with marine equipment, but I would think that there must be some marine combiners you could put up on the roof.

--vtMaps

gkerlin · July 2015

I'm going to use PV cable with MC4's at each end and cut it in half anyway. The MC4 connectors connected to leads coming from the strings. The other (cut) ends go down below or into the combiner box. Then the #6 wire would go right down thru the roof from the combiner and not see daylight.

I thinking it would be easier to pull 5 wires vs 8 down thru tight spaces to get them all below.

Vic · July 2015

gkerlin wrote: »

I'm at a decision point.
My system will be 4 strings 8 panels total, on the roof of the motorhome. 35.4vmp 7.91imp each string.
I haven't found any rooftop combiner boxes for the positive side and would likely have to make one and I'd rather have the positive combined below. Any issues with combining the negative up on the roof and the positive below?

Thanks as always.
Greg

MidNite Solar boxes have become the Industry Standard for Combiner boxes. Here is one that will accommodate 6 strings, outdoor, or indoor. It has a 3R outdoor Rating. Believe that it needs a minimum slope of 15 degrees from horizontal to meet 3R -- elevated 15 degrees from flat on the roof of your Motorhome.

http://www.solar-electric.com/installation-parts-and-equipment/midnite/pvarco/mnpv6.html

Then there is the SolaDeck Combiner:
http://www.solar-electric.com/installation-parts-and-equipment/midnite/pvarco/soarcoandac.html

The MidNite Baby Box is very, very small, tiny really. For INDOOR use, the MN Big Baby box allows more room (four breakers of fuses in a Baby box is a real stretch, with any room to run conductors, and, you probably would need room for the PV Negative busbar, so would suggest the MN Big Baby for indoor combining, if you choose that route.

The Big Baby would also do fine for CC In/Out breakers. If you need only space for two DIN Rail breakers, plus some cables, the Baby Box may well be fine.

Just a drive-by post. Good Luck, Vic

Where to combine the strings

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