Combiner box for SolarEdge 10kW inverter

[Amy@altE]

I just started designing systems with the SolarEdge inverters. I'm trying to find out what most people are using for a combiner box for the bigger ones, 10kW and up. I'm using 4 strings, and including the Rapid Disconnect kit, so I don't need a fancy disconnecting combiner box. Are most people just combining the 4 strings into 1 string and using a regular combiner box like the Midnite PV12, and using 4 fuse holders for the + and 4 for the -? Or does it make sense to combine the 4 into 2 strings, running 2 sets of wires back to the inverter? The current is so low, it seems fine to run the 1 combined string back. What say you with experience?

[HX_Guy]

Why 4 strings? What's the setup? I'm almost willing to bet it could be done with 3.

The route I went with 3 strings was a Midnite Solar PV4 or whatever the model is that's prewired with MC4 connectors and added 2 more fuses so both the DC+ and DC- are fused. If you don't want to modify the combiner then the model you suggested would work.

BTW there's an SMA Sunny Boy combiner specifically for this application available as well.

[sensij]

The optimizer output is not a PV circuit. The current per string needs to be designed around 15 A (the rated output of the optimizer), with a 1.25 factor, as it would for any other DC power supply in a circuit. Combining four into one could require some heavy wire.

[Amy@altE]

Why 4 strings? What's the setup? I'm almost willing to bet it could be done with 3.

The route I went with 3 strings was a Midnite Solar PV4 or whatever the model is that's prewired with MC4 connectors and added 2 more fuses so both the DC+ and DC- are fused. If you don't want to modify the combiner then the model you suggested would work.

BTW there's an SMA Sunny Boy combiner specifically for this application available as well.

Love you, but not designing based on your system ; ). I was inspired by your PV6 + fuses to go with the PV12.

I looked at the SMA combiner, but it wants to be mounted vertically on a wall, I wanted something that can be angled on the roof.

[Amy@altE]

The optimizer output is not a PV circuit. The current per string needs to be designed around 15 A (the rated output of the optimizer), with a 1.25 factor, as it would for any other DC power supply in a circuit. Combining four into one could require some heavy wire.

Good point, now that you mentioned it, I remember that from HX_Guy's discussion. So do you have a suggestion of what to use? My first thought was a PV12 with 4 of the small finger bus bars to combine from 4 down to 2 strings. Seems like there's got to be something standard that everyone's using, awfully late in the game to be redesigning the wheel. Someone at SolarEdge suggested just using couplers to combine the 2 strings, I'm not crazy about that idea.

[Amy@altE]

Why 4 strings? What's the setup? I'm almost willing to bet it could be done with 3.

It could be done with 3 strings, it's 40 modules into a 10kW, but I'm old fashioned and like even strings. I know, embrace change. But I still run into the same question of how to combine them.

[foo1bar]

The optimizer output is not a PV circuit. The current per string needs to be designed around 15 A (the rated output of the optimizer), with a 1.25 factor, as it would for any other DC power supply in a circuit. Combining four into one could require some heavy wire.

I agree - I think it winds up somewhere like 2-4AWG.
So combining just pairs would be better, and I *think* would only need to be 6 or 8 AWG.
Just running the 10AWG down off the roof to a box next to the inverter might be better than doing a combiner box on the roof. (I'd guess something with enough DIN rail (8"?) would be what you'd need - weatherproof if mounted outdoors)
It'd be 8 wires in the conduit (or maybe 4 wires in each of a pair of conduits) - so the conduit would have to be big enough.

My calculations for my own setup - a 3/4" conduit (EMT or IMC) has sufficient space for four #10 PVwire + a #6 ground (THHN or THWN-2).
So a pair of 3/4" conduit would work to get it down to next to the inverter.

[HX_Guy]

It could be done with 3 strings, it's 40 modules into a 10kW, but I'm old fashioned and like even strings. I know, embrace change. But I still run into the same question of how to combine them.

Same exact panel count as me. Do 3 strings, it'll probably be better for the whole system anyway.

I'm guessing there are multiple orientations and/or shading issues? The SolarEdge system needs a minimum of 8 optimizers if I remember correctly for that string to be on. You're thinking of going 4x10 panels but what happenss if 3 or 4 of those panels are shaded?

That's exactly why I went from 4 strings as originally designed to 3 strings, so that I actually had more panels per string so there were panels to spare if some were shaded. Talked to SolarEdge tech support about it too and they agreeded with the logic.

Big added benefit is that then you can run 6AWG wire from the combiner box and you just need two wires coming down, instead of 4.

[sensij]

Same exact panel count as me. Do 3 strings, it'll probably be better for the whole system anyway.

I'm guessing there are multiple orientations and/or shading issues? The SolarEdge system needs a minimum of 8 optimizers if I remember correctly for that string to be on. You're thinking of going 4x10 panels but what happenss if 3 or 4 of those panels are shaded?

That's exactly why I went from 4 strings as originally designed to 3 strings, so that I actually had more panels per string so there were panels to spare if some were shaded. Talked to SolarEdge tech support about it too and they agreeded with the logic.

Big added benefit is that then you can run 6AWG wire from the combiner box and you just need two wires coming down, instead of 4.

Anytime this guy says "just do it the way I did", better think carefully about whatever it is you are doing.

[HX_Guy]

Anytime this guy says "just do it the way I did", better think carefully about whatever it is you are doing.

Really man? I have probably one of the better systems out there in terms of equipment used, from roof mounts to the optimizers. And it was researched probably 100x more than most systems that are just thrown up there.

[sensij]

Really man? I have probably one of the better systems out there in terms of equipment used, from roof mounts to the optimizers. And it was researched probably 100x more than most systems that are just thrown up there.

A functioning system does not necessarily mean it was designed well. I'm glad it is done and working, but the advice you are offering is not sound.

Sorry to derail the thread. I don't think there is a stock combiner that will go from four strings to two in a floating system, with + and - fused. Off the shelf, a pair of smaller combiners might be the way to go.

[HX_Guy]

What aspect do you assume was not designed well?

[sensij]

It could be done with 3 strings, it's 40 modules into a 10kW, but I'm old fashioned and like even strings. I know, embrace change. But I still run into the same question of how to combine them.

If you can keep the total panel wattage under 10500, you could just run two strings, no combiner needed at all.

Edit: If not, you could go with three strings, and consider combining just two of them in a fused combiner while running the 3rd through its own fuses and then all the way to the inverter. That would give you a 30 A and a 15 A circuit, probably less copper than a 45 A combined circuit.

[Amy@altE]

Oh! I forgot I can have up to 5250W per string! You are right, I can do 2 string of 20 Kyo255. That makes it much easier, no fuses even needed for a single string, as long as the wire is sized right. It still may be easier just to use a PV6 with 4 fuses, just don't combine them, use it as a pass thru.

Thanks so much for your help guys! What a team!

[DanS26]

Amy, if you are looking for something neat and tidy on the roof, you might want to check out the ACE combiner manufactured by Wiley Electronics.

It can be ordered in Pass-through or Combiner configurations. Non-metalic, small footprint, NEMA 3R and can be mounted in many different orientations. One limitation though...it will only accept wire size up #6.

I use the ACE-3C a three string combiner connected to a #6 home run. Also did manage to install a Midnite Solar SPD on the housing also.

[emartin00]

If you're using SolarEdge, you'll need the ACE-3C-DF. This version comes with 3 positive fuses and 3 negative fuses. Transformerless inverters require both positive and negative legs to be fused.