Solar combiner box questions

[johnm1]

I'm looking for a solar panel combiner box for the RV's roof.

I'm going to start with 2 panels with the option to add 2 more if needed. I've seen some that are stupid expensive and some that aren't even remotely weather resistant (and still really pricey). Some people say to use waterproof junction boxes (like from the home stores) and some say they don't last very long.

Also, do I need to add fuses/circuit breakers in the combiner box of do I add them down nearer the charge controller? Seems in the combiner box is "better"?

What are my options here?

[2oldman]

time2roll wrote:
Midnight 150 converts the 75vdc to 12vdc for charging. This is how they are designed to operate..

That's a monster controller that would handle this with ease.

[red31]

Sunmodule Plus SW 290 - 300 MONO specs

[time2roll]

SunWorld data sheet

So so easy to just snap these together in series. MC4 #10 extension cut in half. MC4s to the panels, cut end to the controller. No box, no fuses, no extra connections to give trouble, minimal power loss. Midnight 150 converts the 75vdc to 12vdc for charging. This is how they are designed to operate.

The whole combiner box, #4 feed to the PWM controller revolves around 12 volt panels and just complicates these installs.

These large panels have bypass diodes splitting the panel in three sections. With two panels you basically have six sections producing power and any shade just affects that section. The bypass diodes allowing the remaining sections to push power through to the controller. Again designed for home systems with 10 to 20 panels in series where shade issues on a few panels are not going to shut the whole thing down.

[azrving]

johnm1 wrote:
I stand corrected. The panels (SolarWorld Sunmodule Plus SW 290 - 300 MONO) are 36ish volts.

But it's a 24 volt panel, isn't it? 12 volt panel will often do 20 to 21 volts oc so 24 volt would be about 40 oc. Maybe I'm reading it wrong?

[red31]

with an Imp of 9.25A, 2 in parallel would be 18.5A in full sun and cool, Vmp 32.3v

To limit voltage drop to 2% of Vmp or .02 x 32.3v = 0.65v and assuming 20' ya go to a voltage drop calculator like
http://www.genuinedealz.com/voltage-drop-calculator
20', 18.5A, 8 AWG yields v. d. of .478V which is less than .65v
20', 18.5A, 10AWG yields v. d. of .76 v

In series 9.25A, 64.6v, 20' 10awg vd of 0.38v
0.38v is less than 1% of 64.6v

[johnm1]

I stand corrected. The panels (SolarWorld Sunmodule Plus SW 290 - 300 MONO) are 36ish volts.

[red31]

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.

[johnm1]

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.

[brulaz]

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.

[red31]

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 ...

[TechWriter]

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:

[westend]

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?

[2oldman]

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

Yes, these are not likely 12v panels.

[2oldman]

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.

[johnm1]

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.