Solar combiner box questions

Why combine on the roof? Because MC4 wire is not cheap, nor is a Midnite "Big Baby" box --$35 without three breakers, wire entrance ferrules, and the terminal strip. If he DIY's a combiner box, he'll probably have $20 into the whole thing and has access to connect more panels without pulling wire into the RV.

For circuit protection (or the more practical interruption of current to the controller), install one of these Manual reset breakers between combiner wiring and solar controller.

Won't the loss be too big if I run the MC4 cables (10 gauge wire) all the way to a combiner box sitting next to the charge controller?

I also need to do the math on the cost of MC4 extension wire vs 2 or 4 gauge wire and lugs and heat shrink, etc.

johnm1 wrote:
I didn't say anything about doubling the solar output. I said I want to start with 2 x 295W panels and might want to add more ... as in more solar, not necessarily more of the 295W units

Ok.

By the way, I haven't seen a mention of what brand these panels are, but that much wattage usually comes in 24 or 36v. Is that what you're looking at?

The only 295w panel I can find is nominal 36v. The good news is that you're dealing with much less amperage, bad news is you need a controller to handle it, and it's going to make it tough if you want to add smaller 12v panels to your array.

time2roll wrote:
These panels should be closer to 9 Imp each.

Yes, these are not likely 12v panels.

johnm1 wrote:
Won't the loss be too big if I run the MC4 cables (10 gauge wire) all the way to a combiner box sitting next to the charge controller?

I also need to do the math on the cost of MC4 extension wire vs 2 or 4 gauge wire and lugs and heat shrink, etc.

You shouldn't need 2 AWG from the combiner box. I'd suggest to buy a length of 4 awg welding cable, red and black heatshrink, a crimper and the necessary lugs to connect up this project. If you have some 4 AWG left over, it may be incorporated in a different project.

A lot of the solar users on this and other Forums are absolutely terrified of using a wire that is bigger than barely adequate. Most guys are also squeamish about pulling wire through the roof. A marine bulkhead fitting removes that worry. MO is that wire is still cheap and I want to get ALL of the solar harvest through the controller and out to the batteries. I don't want a 3 percent voltage drop. YMMV.

If you do extend MC4 down through to the controller, at least you'll know how it is easily pulled. When you add a module or two, you'll either be repeating that or adding in more "Y" connectors.

Also suggest to use an online calculator for different wire sizes, length of run, and current involved. It's a lot easier when you know how all this works and are not having a load on your shoulders about missing a piece of the puzzle. Here's a calculator

It sounds like you are developing a good plan. What controller do you have in mind?

westend wrote:
Most guys are also squeamish about pulling wire through the roof. A marine bulkhead fitting removes that worry.

OP, save your money. Use one (or in your case 2) of these cable entry flat panel:

johnm1 wrote:
Won't the loss be too big if I run the MC4 cables (10 gauge wire) all the way to a combiner box sitting next to the charge controller?

without the length who knows, maybe ask the one that highly suggested ...

johnm1 wrote:
Won't the loss be too big if I run the MC4 cables (10 gauge wire) all the way to a combiner box sitting next to the charge controller?

I also need to do the math on the cost of MC4 extension wire vs 2 or 4 gauge wire and lugs and heat shrink, etc.

There's 8ga MC4 as well.

Wholly cow people ... take a deep breath please.

I don't know what voltage the panels are but I "think" they are 12. I will check when at my pc tomorrow.

About panel brand ... is this like the "my Honda generator is better than your Champion generator? Does it matter?

The cc is midnight classic 150 or something like that.

I have assumed (and read) that I need/should run the panels into a combiner box and then though the roof to the charge controller and on from there.

Does it really matter what volts or amps I work with ... as long as it's wired correctly? I mean, we all have 30 or 50 amps runing through our extension cords!

Lastly, to the people who have given constructive feedback, thank you! It is very useful. I have NEVER EVER done anything like this and that is why I'm asking for guidance and help.

johnm1 wrote:

Does it really matter what volts or amps I work with ... as long as it's wired correctly?

how do ya wire it correctly without knowing.

1v drop is 6% for 12v (Vmp 17v)
1v drop is 3% for 24v (Vmp 32v)

18A, 28' 10 AWG yields 1v drop.