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Help With Choosing Breakers, Combiner Box, Wire Size, etc For My System

nick777

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Nov 26, 2021
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I need help with choosing wire size, combiner box, breakers, etc for my system. I have the bulk of the equipment which is:

1. 4 x 250w panels (photo with specs below)
2. Renogy Rover 40a MMPT charge controller.
3. 4 x 28a SLA/AGM batteries. I'll be upgrading these batteries in the near future.
4. 1000w 24v inverter.

Battery will be within 10' of the charge controller and the inverter can be set up right next the charge controller.

Many thanks in advance.
 

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What configuration are you going to use for the solar panels? 4s, 2s2p, 4p? Probably not 4s, but asking just in case.

8 gauge cable between the solar charge controller and the battery.

The same gauge cable could be used between the battery and inverter (1000 watt / 24 volts * 1.15 efficiency = 48 amps) as long as the distance of the cable is kept short.

To figure out cable size for yourself, I suggest you use this calculator. The distance is round trip, not one way.

 
This basically becomes a set of simple math problems. You have a 40A controller, with a 100Voc max limit. You can NOT wire all four of your panels in series. That would give you a 148.8Voc, far higher that what your controller can handle. You could wire them in parallel, which would give you 33amps at 30.1V. Four parallel strings though at relatively low voltage will allow for some voltage drop because you are pushing so many amps through wire at relatively low voltage. A second consideration is that with four parallel strings, fusing/breakers is mandatory!

The best configuration with your equipment would be 2S2P, that is two parallel strings of two panels in series. That would be delivering ~ 8.3A + 8.3A at 60.2V to the controller. The Voc for two in series would be only 74.4Voc, which is safely below the 100V limit. Even below freezing, that panel configuration will still be below 100V.

So, your numbers into the controller are ~16A at about 74V. Here's a chart documenting what wire can handle. So, standard 10 gauge copper wire will handle the 16A going into your controller.
1640364366566.png

Now, for the math for the power coming out of the controller.

You have two strings of two panels, producing a total of 1000W of power. The controller is going to take that raw solar power and transform it down to battery CHARGING voltage, creating extra amps with the extra volts. Assume the battery will charge at least 25V. So the math becomes 1000W/25V = 40A. Luckily, you have a 40A controller so you won't be exceeding the controllers amp limit. But, you'll need thicker wire coming out of the controller than what's going in. To safely handle at least 40A of current, looking at the chart, you need at least 8 gauge wire, though for a significant safety margin, I'd go with at least 6 gauge. I personally went with 4 gauge.

The same numbers would apply to the inverter, so I would go with 4 gauge coming out of the controller, and 4 gauge going from the battery to the inverter.

The last issue will be the fuses or breakers. The recommendation is protection 1.5X the amperage flowing through the circuit. According to code, if you only have two parallel strings of panels, you don't need protection, but I personally like having breakers, because it greatly simplifies troubleshooting when something goes wrong.

I'm partial to Midnight Solar products, so I'd recommend their three breaker combiner.

You'll want to use breakers 1.5X times your current, which if around 8A, you'll want a 12A breaker. Downstream of the controller with as much as 40A going into the batteries, you'll want a 60A breaker. You can go with the same rated fuses if you don't want breakers, but keep in mind you never want to disconnect a live DC circuit under load. Nasty arcs can be made.

The last thing I can comment on is that 28A is far too small a battery for this amount of current. For this sized system I'd go with at least 250Ah. A Trojan T-105 bank would be a better choice. Even Costco has a 210Ah golf-cart battery that would be far better than what you want in place.

Good luck to you!
 
I think you want to put the inverter by the battery. Usually with inverters, wire sizing is based off ampacity, not votlage loss. My inverter sets 2' away from my batteries but took 8' of cable to wire. If its 10' away, that could be enough to drive thicker wire because of voltage loss.

This is what I use to determine wire size based off ampacity:
1640365406572.png

This is the link I use to make sure that voltage loss stays under 3%:
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For solar panel placement, I've found just the opposite to be true. Voltage loss drives wire size selection and with staying below 3% loss, usually amapcity is not a factor.
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You also have a lot of panels for SLA batteries. THey have a lower maximum charge amps, and if this magic number is 1/3rd the charging amp hour battery rating, you could easily exceed that. I think that will be not just in lab conditions but between the hours of 10 and 2.
 
Thanks for everyone's feedback.

Yes I'll most likely go with a 2S2P panel setup.

Regarding the Midnite combiner box, can I use just a outdoor small breaker box like this from Lowes/Home Depot?
https://www.lowes.com/pd/Square-D-100-Amp-6-Spaces-12-Circuit-Main-Lug-Load-Center/3129229
What is the advantage of Midnite's combiner box vs these standard ones?

Also regarding the amps that my 12v batteries (in 2S2P) would be a 58a correct? So that means they can take about 19a (58 divided by 3) of charge power? And the solar panels wired in 2S2P would be about 16a (8.3 x 2), which is about the max charge amps the batteries can take?
 
To me, it’s more do you know what you’re doing. I could not take that box and turn it into a combiner box made like midnight does. Not sure how they fasten the positive and negative terminal blocks to the case and have it rated in excess of 100 volts.

If you do, go for it.

I do have a plastic box I made a combiner out of. Simply a 6 x 6 plastic box from home depot with cable glands drilled on the side. The 4 position ground terminal blocks are secured to their own piece of wood which is secured to the case. Not exactly UL listed and that is an outside fair weather box.

For your charge question, I’d have to refer you to your manual. I mentioned 1/3 rd, but may be lower. For the amperage part you did the math right for the solar panel output.
 
Thanks for everyone's feedback.

Yes I'll most likely go with a 2S2P panel setup.

Regarding the Midnite combiner box, can I use just a outdoor small breaker box like this from Lowes/Home Depot?
https://www.lowes.com/pd/Square-D-100-Amp-6-Spaces-12-Circuit-Main-Lug-Load-Center/3129229
What is the advantage of Midnite's combiner box vs these standard ones?

Also regarding the amps that my 12v batteries (in 2S2P) would be a 58a correct? So that means they can take about 19a (58 divided by 3) of charge power? And the solar panels wired in 2S2P would be about 16a (8.3 x 2), which is about the max charge amps the batteries can take?
The only issue with that system is that while the QO series breakers ARE rated for DC, they're only rated up to 48v, so unless you do a 4p setup with a breaker for each one, your strings will exceed the DC voltage rating of the breakers.

Sadly, AC and DC breakers are NOT interchangable.
 
Ok got it, thanks guys. I'll most likely go with the Midnite combiner box.
 
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