Solar wiring question

[Muddydogs]

Currently I have 2 100 watt panels on top of the trailer (VOC 21.6 volts, ISC 6.34 amps, VPM 17.4 volts, IMP 5.75 amps) with a 30 foot run of 12ga wire to the charge controller.

Last fall my system stopped charging and upon inspection one or more of the MC4 connectors are not making a good connection. My plan is to remove the MC4 connectors and make a water proof bus bar box on the roof to connect the panels to the main line.

I was thinking when I was up there that I should install a third panel. From what I can tell if I'm running the numbers right the 12ga wire will safety handle the max output from 3 panels but my line lose is high. If I switch the 30 foot of main line to the controller to 10 ga am I really gaining a lot with line loss? Will I be getting anything extra by adding a 3rd panel?

Looking at a wire table it appears that even going to 4ga wire won't get me down to a 2% loss for my run length.

[KD4UPL]

I've installed over a megawatt of PV panels and not seen any with aluminum wire. I've run several miles of PV wire and not seen any #10 or #12 that is aluminum.
You aren't supposed to hook more than 2 panels (or strings of panels) in parallel without over current protection to each string. So, just connecting 3 panels in parallel to bus bars or thru Y connectors isn't recommended.
Technically #12 isn't big enough for 3 panels in parallel. Solar arrays are considered "continuous" current by the NEC's definition so you have to multiply the Isc by 125%. You also have to multiply by another 125% to account for the fact that solar panels can put out more than their rated current under certain conditions. This means you really end up multiplying the Isc by 156%. This means your main wire down from the roof, with three panels, needs to be able to handle 29.67 amps. That means even #10 is barely large enough.
I would either replace your charge controller with an MPPT unit that allows you to run all 3 panels in series or upgrade your wire to at least #8 and put a fused combiner box on the roof.

[red31]

If you get 2 more double branch connectors, unplug one panel, plug in new double branch connector, connect old and new panel.

[BFL13]

I use a Solar30 PWM with my three 100s in parallel. The Windy Nations P30L is the same. Good controller for the job. You must wire the 100s in parallel with the PWM.

If the two 100s worked for you, a third might not be necessary but could be better.

AFAIK there are some different sizes of MC4 and singles vs double. Also the panels might have junction boxes you can connect wire to or else pigtails with MC4. I can get #8 in the panel junction boxes so then I just need a combiner box up there and a single pair of #8 down to the controller.

If the panels have pigtails, likely they are aluminum #12 with MC4 so you need to wire from the pigtails to the combiner box too. One way is to cut the pigtails short, and join the pigtail al #12 to your copper #8 (however you like--but need to tape it up to keep it dry) and then go with #8s to the combiner.

If the pigtails are long enough to reach the combiner then it is easier--if you are that lucky!

I am suspicious, based on the pigtails, that when you buy say 25 ft of wire with MC4s it might be aluminum wire, which acts like a higher number gauge from copper even though they call it #12. That can matter for calculating line loss.

[Lwiddis]

I wouldn’t change the wire either but then again I wouldn’t use 12 gauge. 10 from each panel to the combiner box, 8 from the box to the controller, 6 from the controller to the batteries.

[bpounds]

Muddydogs wrote:
Currently I have 2 100 watt panels on top of the trailer (VOC 21.6 volts, ISC 6.34 amps, VPM 17.4 volts, IMP 5.75 amps) with a 30 foot run of 12ga wire to the charge controller.

Last fall my system stopped charging and upon inspection one or more of the MC4 connectors are not making a good connection. My plan is to remove the MC4 connectors and make a water proof bus bar box on the roof to connect the panels to the main line.

I was thinking when I was up there that I should install a third panel. From what I can tell if I'm running the numbers right the 12ga wire will safety handle the max output from 3 panels but my line lose is high. If I switch the 30 foot of main line to the controller to 10 ga am I really gaining a lot with line loss? Will I be getting anything extra by adding a 3rd panel?

Looking at a wire table it appears that even going to 4ga wire won't get me down to a 2% loss for my run length.

What I would do:

It is definitely worth adding a 3rd panel if you want one. I would not change the 12g wire. Your voltage loss won't be enough to worry about, and that 3rd panel will still be well worth it.

I would not replace the MC4 connectors until I had tried to save them. Probably just the act of disconnecting and reconnecting has cleaned up whatever corrosion you had. Work on cleaning the pins a little more, use some DE grease, and reassemble. If you can't get them cleaned up well enough to suit yourself, just replace them with new connectors (don't eliminate them).

If you feel corrosion would be an ongoing problem (an indication would be that you had more than one connector failed), you could use heat shrink tubing with an adhesive lining and shrink it over the entire connector. This stuff is available in large diameters and is used for battery cables and the like. Amazon has it. Just be aware that it is good stuff, and if you ever need to remove it, it is a job to get off.

[pauldub]

Two good sources of large wire are genuinedealz.com for boat wire or Temco on eBay for welding wire. The boat wire is tinned so it's more expensive.

[pauldub]

10 gauge wire is a little small for such a long run. I used 8 gauge for about a 10' run on mine from the combiner box down to the controller. If they are truly MC4 connectors, you should be able to interconnect between brands.

[Muddydogs]

Another question.
I see I can get a 3 way MC4 connector, are all MC4 connectors the same? If I purchase a 3 way will my existing connectors of another brand hook up right? Or will I have fit issues between brands?

My current MC4 connectors are from Windy Nation, they don't have a 3 way and to hook up 3 panels using there system looks like just to many connections to fail.

Just wondering if I decide to take the easy way and just replace MC4 connectors instead of doing a combiner box. I have a bag of extra MC4's around someplace.

[Muddydogs]

I had to go look at what PWM and MPPT where again. 4 years ago when I first did this I studied up an knew a thing or two but have since forgot much of the info. The system worked and kept my battery's charged very well so I didn't have to think about it.

I am using a PWN 30 amp controller. The Windy Nation P30L model.

[BFL13]

I have three 100s in parallel with PWM, and can get about 18.6 amps to the battery with a high sun on the array. I use #8. Can't say if #10 would be as good.

The main thing is that with PWM, your line loss is not very important. You will still get your 18 amps.

BUT if you have MPPT then it is important, because the amps to the battery you get works differently from with PWM. MPPT amps is all about watts from the controller divided by battery voltage. Line loss reduces wattage.