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Reverse PV Combiner Box?

AlaskanNoob

Solar Enthusiast
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Feb 20, 2021
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I see combiner boxes that will take strings of PV and combine them, in parallel, onto one wire. Is there a box on the other side that can split that back into several wires per string to plug into several MPPTs?
 
Nope.
MPPT each needs to be independent, can't handle someone else yanking their wires.
Exception is some equipment with multiple MPPT inputs in one unit, that allows paralleling.
You're going to need a separate PV run for each device.
 
So a combiner box is only used with a single MPPT? If a combiner box takes several strings, then puts them in parallel and spits them out on a single wire, then the end of that wire needs to feed into a single charge controller and can't be split back up?

Assuming 4 strings of PV, a combiner box that parallels all 4 PV strings into one wire, that wire can't be split back up into 4 wires to feed into the four trackers of a Victron 450/200 SmartSolar MPPT? Or is that kind of charge controller one of the exceptions you mentioned?
 
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Why use a combiner box at all? Why not directly run each string to its respective SCC/tracker?
 
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So a combiner box is only used with a single MPPT? If a combiner box takes several strings, then puts them in parallel and spits them out on a single wire, then the end of that wire needs to feed into a single charge controller and can't be split back up?

Yes, except some boxes have multiple isolated circuits. Often I see two.
You can make your own with DIN rail in a rainproof box and mount fuses, breakers, lugs that accept multiple wires, etc.
If no more than 2 PV strings going to one MPPT (not paralleled with additional MPPT in the SCC), then fuses not require. Could just splice wires. But a disconnect somewhere is handy. I shut off current at one end, unplug MC connectors at array.


Assuming 4 strings of PV, a combiner box that parallels all 4 PV strings into one wire, that wire can't be split back up into 4 wires to feed into the four trackers of a Victron 450/200 SmartSolar MPPT? Or is that kind of charge controller one of the exceptions you mentioned?

I would think so. RTFM, it should say whether or not they can be combined.

What I've found handy is separate runs from each string, because if change electronics I can swap around which string goes where. I have 12 awg runs to almost all, except one 8 awg which got two strings. I plan to make it 3 strings later, will add fuses at array end.
 
It has nothing to do with the manual. It has nothing to do with MPPTs.

I just want to know if I put string wires A, B, C, D, E, and F into a combiner box and it spits that out in one (or two) wires (paralleling them)....can I take that wire on the other end and uncombine it back into A, B, C, D, E, and F again? Regardless of what I plug it into or not.

If I put two 350V / 10 amp strings into a combiner box, and it sends out 350V / 20 amps on a wire, at the end of the wire can I split that back into two wires of 350 V / 10 amp each?
 
a PV Combiner box is just for feeding a single charge controller. If you are looking for redundancy I would use two smaller controllers and have two arrays feeding into a PV box for disconnecting each array via breaker or two single disconnect boxes.
 
It has nothing to do with the manual. It has nothing to do with MPPTs.

I just want to know if I put string wires A, B, C, D, E, and F into a combiner box and it spits that out in one (or two) wires (paralleling them)....can I take that wire on the other end and uncombine it back into A, B, C, D, E, and F again? Regardless of what I plug it into or not.

If I put two 350V / 10 amp strings into a combiner box, and it sends out 350V / 20 amps on a wire, at the end of the wire can I split that back into two wires of 350 V / 10 amp each?
You can but the box would not be combiner box. You can do this with combiner box if you remove all the wiring making all the Positive and negative connections together. Just buy a water proof box, add a din rail, install two breaker & or fuses. This would allow two seprate PV circuits to enter and exit the box.

Hope this helps
 
You can but the box would not be combiner box. You can do this with combiner box if you remove all the wiring making all the Positive and negative connections together. Just buy a water proof box, add a din rail, install two breaker & or fuses. This would allow two seprate PV circuits to enter and exit the box.

Hope this helps
Thanks.

I'm essentially trying to figure out if I can run my PV into a combiner box, reduce the wires down to one (or maybe two) and run that the 500 feet to my cabin and then split it back up to feed multiple trackers in multiple MPPTs, OR do I have to have 7 smaller wires running the 500 foot distance to plug into the MPPT.

I'm not sure which would be better, I'm just asking about what can be done. Thanks for that info.
 
It has nothing to do with the manual. It has nothing to do with MPPTs.

I just want to know if I put string wires A, B, C, D, E, and F into a combiner box and it spits that out in one (or two) wires (paralleling them)....can I take that wire on the other end and uncombine it back into A, B, C, D, E, and F again? Regardless of what I plug it into or not.

If I put two 350V / 10 amp strings into a combiner box, and it sends out 350V / 20 amps on a wire, at the end of the wire can I split that back into two wires of 350 V / 10 amp each?

Once two strings are combined into a single pair of wires, you generally can't split into two 10A circuits. If two identical loads were connected (e.g. light bulbs), they would pull equal amounts.

I have some inverters with a single MPPT input and 4 fuses. If I brought a pair of fat wires from an array with multiple strings combined and branched it into four fuses, no guarantee the current would divide evenly enough. Would probably blow one fuse that carried more than its share. There is a 5th input unfused if I want to use a single fat wire.

I do have splitting like that for AC, from grid through relays of inverters connected in parallel and combined again at my loads panel. They carried current imbalanced 3:1 due to resistance of some circuit breakers. Changing to a different model fixed that.

Feeding MPPT, you can't do that because each pulls its array to different voltages implementing its algorithm. Unless they are designed to work as a team. I know of some inverters with 2 MPPT which can do that. I don't know about the Victron.

Thanks.

I'm essentially trying to figure out if I can run my PV into a combiner box, reduce the wires down to one (or maybe two) and run that the 500 feet to my cabin and then split it back up to feed multiple trackers in multiple MPPTs, OR do I have to have 7 smaller wires running the 500 foot distance to plug into the MPPT.

I'm not sure which would be better, I'm just asking about what can be done. Thanks for that info.

You need at least a pair of wires per SCC. You may or may not need separate pair of wires per MPPT input (depending on what manual of SCC says.)
 
Edit: I see Hedges just posted something similar to the following:

So the real question is if you were to combine the 4 strings, run the 2 wires (pos and neg) 500', then split the 2 wires back into 8, with 2 per tracker, how would the 4 trackers deal with sharing the combined voltage and amperage? Would each get exactly 1/4 of the wattage or would they fight each other trying to take as much as it could handle? Presumably the combined wattage would be too much for any one tracker. Is that a risk?

As a side thought, consider the cost difference of the two possibilities:
For four strings It's either 8 wires combined into 2 wires then split into 8 or just the original 8 wires. Let's assume the amps and voltage state that the 8 500' wires would need to be 6AWG and the 2 500' wires would need 1/0AWG. What's cheaper? 4000' of 6AWG or 1000' of 1/0AWG?
 
Thank you both for that.

I was under the impression that the MPPT simply got whatever the PV could provide, and then did its voltage magic in order to send that power on to the batteries based on battery conditions. I wasn't thinking the MPPT had any effect on the PV, but perhaps if the MPPT "pulls its array to different voltages" then I'm misinformed. I didn't think the MPPT was changing the voltage of the PV feeding it.

I was envisioning the current being split evenly from the fiction I referred to as a "reverse combiner box" into the output wires going into the MPPT trackers. But the idea of an MPPT fighting for more of its share of power in my imaginary scenario makes sense as to why this thing doesn't exist.

Thanks for the info. Now instead of planning to trench one fat wire with room for adding more PV later, we'll trench 7 individual small wires, and 7 more unused small wires just so we have them in place in case we expand our PV & SCCs and such in the future.
 
This is where "AC Coupling" has an advantage.
You could put PV arrays with grid-tie inverters all over the countryside. They share one set of AC wires.
For off-grid system, at one location you have a battery inverter which forms the grid (sets frequency and voltage).

Advantage of running wires in DC includes lower IR losses if higher voltage (about 400VDC), and voltage drop in wire has no impact on system other than efficiency loss.

Thanks for the info. Now instead of planning to trench one fat wire with room for adding more PV later, we'll trench 7 individual small wires, and 7 more unused small wires just so we have them in place in case we expand our PV & SCCs and such.

PVC conduit, and you can pull more things later. Maybe a few conduits, thinking you might want AC and data connections out there as well.
Don't assume you can fit as many wires as allowed in a conduit. Gets difficult when cramped.
 
Curious why you keep saying 7 wires. Why not 8? Four strings means 8 wires (2 each - 1 positive and 1 negative).
 
I was under the impression that the MPPT simply got whatever the PV could provide, and then did its voltage magic in order to send that power on to the batteries based on battery conditions. I wasn't thinking the MPPT had any effect on the PV, but perhaps if the MPPT "pulls its array to different voltages" then I'm misinformed. I didn't think the MPPT was changing the voltage of the PV feeding it.

I was envisioning the current being split evenly from the fiction I referred to as a "reverse combiner box" into the output wires going into the MPPT trackers. But the idea of an MPPT fighting for more of its share of power in my imaginary scenario makes sense as to why this thing doesn't exist.

Plot of current vs. voltage and power vs. voltage for a PV array:


Plot showing local maxima and global maxima that can occur with some shading conditions. SCC MPPT may or may not find the higher one.
Ideally you avoid having such shading on an array (separate MPPT per string can help, if shade falls on one string.)

 
Curious why you keep saying 7 wires. Why not 8? Four strings means 8 wires (2 each - 1 positive and 1 negative).
Yeah, we will run 16 wires total I think. 7 of them will be connected to PV and will plug into 7 of the 8 trackers on our two MPPTs. The rest of the wires will just be in there in case we want to add PV in the future.
 
Yeah, we will run 16 wires total I think. 7 of them will be connected to PV and will plug into 7 of the 8 trackers on our two MPPTs. The rest of the wires will just be in there in case we want to add PV in the future.

All MPPT and GT inverters I'm familiar with want dedicated positive and negative wires to their on private PV array. No wires shared except ground wire (which you should have for safety.)

If 7 MPPT trackers in two SCC, I think you need 15 wires. 7 positive, 7 negative, 1 ground. (then we can consider just how heavy the ground needs to be.)
 
Yeah, we will run 16 wires total I think. 7 of them will be connected to PV and will plug into 7 of the 8 trackers on our two MPPTs. The rest of the wires will just be in there in case we want to add PV in the future.
16 wires for 8 trackers. 14 of the wires will be connected to 7 of the 8 trackers leaving 2 wires left for the 8th tracker if you add one more string of PV in the future.
 
Ah, yeah, 32 wires total to future proof it. I may have to dig the trench twenty feet deep just so the stack of wires stays under ground.
 
Ah, yeah, 32 wires total to future proof it. I may have to dig the trench twenty feet deep just so the stack of wires stays under ground.
So you want up to 4 of these big SCC with 4 trackers each. Yes, 32 wires. Maybe 36 wires with 1 ground wire per SCC.

Also note that with a large bundle of 32 wires running that far, you will likely need even larger gauge wire to deal with the extra heat, not just the amperage and voltage drop.
 
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