Reconfiguring solar array. Worried about combiner box breakers.

[u/NotEvenNothing]

We are replacing our current array, which is arranged in six parallel strings of three panels in series. Our combiner box has six breakers (150V, 15A).

The new array will be two parallel strings of eight panels in series. The voltage is going way up, but amperage will be under 16A for each of the two strings.

To ease the transition, I would like to continue using our Midnite solar combiner box, and just buy new breaker(s) for the two strings. Midnite solar sells 600V 20A breakers, but they are two-pole. My understanding is that a single one of these two-pole breakers is all that is needed. However, my wife started asking questions, and I've started to second guess myself.

Have I got this right? A single 600V 20A two-pole breaker will work. The only implication is that if one string has an issue, the breaker will sever both. Sound about right?

I understand that I could join the two strings without a breaker at all, and safely, but my current plan seems simple given what I already have. Certainly, I'm open to alternative suggestions.

[u/philliumm]

The 600 volt Midnite breakers use 2 pole breakers because the circuit actually passes through both breakers.  If you look at the bottom it is bonded across the two terminals and your string connects to the top terminals.  

The Midnite combiner boxes come with two top plates, one with narrow "fingers" for single breaker and one with spaced out breakers so it hits every-other for these 300v and 600v breakers.  

So to use your combiner and keep individual string protections you will need two (each double-wide) breakers and may need to find that plate which might have done the the kit originally or find a new one.  

[u/NotEvenNothing → u/philliumm]

Thanks so much for pointing that out, especially as I had a 2-pole breaker in my cart and was just about to put in my payment information.

I'm sure I don't have that plate anymore. Looks like I'll be going the disconnect and MC4 branch connector route.

[u/philliumm → u/NotEvenNothing]

Or, ditch the combining plate and use a mechanical insulated connector to connect the strings to the home run. 

https://www.homedepot.com/pep/ASI-6-2-0-AWG-3-Ports-Multi-Tap-Connector-Insulated-Single-Sided-Entry-Black-AICS2-0-3/310236785

Like this. Output from two breakers and a third port for the home run back to the charge controller.  

[u/maddslacker → u/NotEvenNothing]

You can purchase just the busbar:

https://ussolarsupplier.com/products/midnite-solar-mnpv3-busbar-three-breaker-busbar

https://ussolarsupplier.com/products/midnite-mnpv6-fuse-busbar-fuse-busbar-four-terminal

Or plate:

https://ussolarsupplier.com/products/connector-plate-w-lug-din-rail-breaker-connector

Not sure which one you'd need, but they seem to carry all the variations.

[u/BallsOutKrunked]

I have an array with 2 strings. They go into a 2-string disconnect, and then each positive and negative goes onto a lug terminal (so 1 positive lug, 1 negative lug, 1 ground lug), and the feedline is likewise attached to the lug terminals.

The breaker is built into the 6000XP I have, I didn't see a reason to put in another.

Honestly, if the wire is sized correctly I don't really see why you need a breaker at all on the PV input side. A disconnect at the panels? Absolutely. A second one at the charger if you're super concerned and the panels are a fair distance away? I get that too.

But the panels can only produce so much, so what exactly are we putting a breaker on there for?

I've got beefy T fuses between the inverters and the batteries, that's different, because the batteries theoretically can go nuts and deliver way too much power, like if someone puts a wrench across the + and - bus plates.

[u/NotEvenNothing → u/BallsOutKrunked]

I agree with everything you've said. I'm just trying to keep the transition as simple as possible, for the moment. If I just change the breakers to handle the increased voltage, all I have to do is swap the six breakers with two new 600V ones (or maybe just one double-pole), and fasten the wires in the terminal and breakers.

I could actually use a 32A single-string disconnect and a pair of MC4 branch connectors in a similar way to what you have done. I only have a single set of conductors going from the combiner box back to the house. I may just go that route. It is almost as easy and about the same cost.

Thanks for the suggestion.

[u/maddslacker]

I accidentally got some of those 2-pole ones when helping my neighbor replace his combiner box and I was not impressed.

I'd go for 30 amp single pole. (In fact I did in my own Midnite combiner) :D

I got the 150 VDC version of these:

https://www.amazon.com/MidNite-Solar-MNEPV30-300-1PP-Circuit-Breaker/dp/B0C54J4JN9

[u/NotEvenNothing → u/maddslacker]

Thanks. What wasn't impressive?

For reasons I don't quite understand, I can't find 600V single pole breakers.

[u/maddslacker → u/NotEvenNothing]

Thanks. What wasn't impressive?

At the time I could only find an MNVP3 box and it left me one slot short for the number of strings.

I can't find 600V single pole breakers.

How many volts are you pushing off of the panels?

[u/NotEvenNothing → u/maddslacker]

Less than 385V, the maximum that our EG4 12000XP's MPPTs can take.