Any issues with adding fuses to my DC Negative inside the combiner box?

[sensij]

Integdog: So is there a chance he wired it wrong? Or since it's working it's wired properly?

Is there anything specifically that I can check for in the wiring to make sure it was wired correctly? I assume the only points he could have done wrong wiring is in the combiner box or in the DC disconnect?

Based on the pictures you've shared, the wiring is ok. The only major thing he could have done wrong (besides using white wire) would have been to connect Dc- to ground. He probably thought he was doing that when he ran the bare ground from the inverter to the electrode, which is why I asked about it earlier. However, all he did was create a somewhat lower impedance path to ground for the EGC. The fact you've generated some power proves this. The solaredge inverter has gfci protection that would have tripped if either dc line was actually grounded.

[HX_Guy]

Integdog: So is there a chance he wired it wrong? Or since it's working it's wired properly?

Is there anything specifically that I can check for in the wiring to make sure it was wired correctly? I assume the only points he could have done wrong wiring is in the combiner box or in the DC disconnect?

[inetdog]

I haven't tried yet but I'm thinking you'd undo the top plugs first and the you can access the bottom row.



Good question, I have no idea really, doing some reading on it now. I don't even understand how the wiring differs between the two...what does it mean neither has a bond to ground? I'm hoping my installer hooked up everything correctly as he seems a bit confused, I don't think he has any experience with transformerless inverters.

The transformerless inverter basically uses the PV array to pump up an AC output voltage in the + direction relative to the neutral, with the - of the array temporarily referenced to ground internally and then uses that same panel with the + now referenced to ground instead to pump up a negative voltage to drive the AC line negative with respect to neutral. (For an inverter with only a 240V AC output it is a little more complicated than that, but the principle is the same.

if the installer actually bonds the negative to the ground at any point in his wiring, then the inverter will not work, and will actually blow a fuse or trip an internal breaker to indicate the fault.
If the installer does not bond the negative to ground (or to either the AC EGC or AC neutral) then during the operation of the inverter the + and - leads will both change voltage with respect to ground, although of course the difference between them will stay constant at the panel Vmp voltage.
To deliver AC out a transformerless inverter which has a buck regulator design must get a DC voltage from the panel which is greater than the peak to peak AC output voltage (for a 240V output). For 240V AC that comes to about 350V DC.

It sounds like your installer is clinging to his rote learned rules of the past, for grounded arrays, and is not even reading the instructions for the SMA TL model inverter that he is installing. I am not sure at this point how you can best prevent him from doing more damage in a misguided quest to follow his "plan".

[HX_Guy]

I have a couple questions. First, if you plug into the MC4s in the back row of that box, how
will you ever get in there with a tool to release them?

I haven't tried yet but I'm thinking you'd undo the top plugs first and the you can access the bottom row.

And then, just what is a "transformerless inverter"? My impression is NOT using a big iron
core running at 60 HZ, but maybe that's not it at all. Bruce Roe


"In a transformerless inverter, neither the positive nor negative conductor has a bond to
ground. The result is that you no longer have a grounded current-carrying conductor. This
requires both conductors in each circuit to be protected with OCPDs and have disconnecting
means as required in Art. 690, Part III."

Good question, I have no idea really, doing some reading on it now. I don't even understand how the wiring differs between the two...what does it mean neither has a bond to ground? I'm hoping my installer hooked up everything correctly as he seems a bit confused, I don't think he has any experience with transformerless inverters.

Here is a good article explaining the differences:

http://www.homepower.com/articles/solar-electricity/design-installation/ungrounded-pv-systems

So in a transformer based system, the PV- would be attached to the negative bus bar to ground it? The same bus bar as the equipment ground?
I'm hoping my installer didn't do this, and I don't think he did as I don't think the system would be operating.

[bcroe]

I have a couple questions. First, if you plug into the MC4s in the back row of that box, how
will you ever get in there with a tool to release them?

And then, just what is a "transformerless inverter"? My impression is NOT using a big iron
core running at 60 HZ, but maybe that's not it at all. Bruce Roe


"In a transformerless inverter, neither the positive nor negative conductor has a bond to
ground. The result is that you no longer have a grounded current-carrying conductor. This
requires both conductors in each circuit to be protected with OCPDs and have disconnecting
means as required in Art. 690, Part III."

[HX_Guy]

Nice. I was going to suggest heat shrink as a clean looking option, but didn't think you would have it available. I would definitely use that instead of a sharpie.

I told my installer that I was going to put the heat shrink wrap on the white wire to make it black and he said...

"It is grounded system. Both ground and equipment grounded.. please don't put anything on it...it is meant to b white. If you put anything on it will have to phase tape it white."

I think he's a bit confused that this is a grounded inverter.

[Amy@altE]

I was wondering if there was enough play in that wire to bring it all the way up. If you are seeing enough voltage to jump the space, you have more problems than we can cover here ; )

http://www.midnitesolar.com/pdfs/MNPV2-3-4-6_manual.pdf page 4 shows cutting the finger bus bar in 2 to make 2 circuits, that's why they make the lug removable on the bus bar, and have a spot on both ends. They do caution on 1000V systems to make sure the gap is big enough, I think they just added that warning recently, so they are used to it being used up to 600V.

[HX_Guy]

I guess I was imagining this massive arc happening between them.



I may still need to go the route I'm thinking only because the wire running from the combiner to the inverter (white wire with red arrow pointing to it) may already be pulled tight, so it wouldn't reach the bus bar. It would obviously be a cleaner looking install with the bus bar, but as long as it's not a safety issue with running wires like I am thinking, I don't mind...it'll all be covered up anyway.

[Amy@altE]

Oh right, I forgot you said that. That should be fine, but it is common with the PV6 to cut the bus bar in 2 and have the 3 strings each go to a separate circuit. With everything tightened down , there won't be any movement causing them to short.

[HX_Guy]

Don't forget to also order another finger bus bar to combine the 3 negatives. http://www.midnitesolar.com/productP...Order=13&act=p

Amy, I was actually going to run wire from each fuse holder to the Negative bus bar, like in my photo/diagram earlier (white lines).
Do you foresee any issue with doing it that way?

Only reason is because I feel having the PV+ and PV- finger bus bars so close to each other wouldn't be that good of an idea, would it?
I mean they would literally be millimeters apart with all the fuse holders on the DIN rail.

[Amy@altE]

Amy, I actually only need 6 total fuses, not 8. I have 3 strings in the system...right now the 4th fuse holder (far right) isn't being utilized.
The box is a MNPV4-MC4, but it's really a PV6 that's pre-wired with MC4 connectors for 4 strings (same box though).

Don't forget to also order another finger bus bar to combine the 3 negatives. http://www.midnitesolar.com/productP...Order=13&act=p

[sensij]

Nice. I was going to suggest heat shrink as a clean looking option, but didn't think you would have it available. I would definitely use that instead of a sharpie.

[HX_Guy]

Let me change that to the best suggestion I have. Sensij's suggestion is better.

What do you guys think of this stuff?



It's heat shrink tubing that's rated for 600V and 125ºC flammability. 10mm that shrinks down to 5mm (wire diameter is 6mm).

[HX_Guy]

Was able to talk to tech support at Midnite Solar and they said that while the box hasn't been tested for what I'm proposing and it would lose it's UL listing if modified in this way, he didn't see why it wouldn't work. He said there should be just enough same on the DIN rail to fit 6 fuse holders and actually even gave me the suggestion that I could cut out the opening in the black dead front wider so that it would fit around all 6 fuses and also keep then together so they don't fall off the rail.





Also here is another picture I found online that shows the available space on the DIN rail, definitely looks like 2 more fuses would fit on there.

[HX_Guy]

Amy, I actually only need 6 total fuses, not 8. I have 3 strings in the system...right now the 4th fuse holder (far right) isn't being utilized.
The box is a MNPV4-MC4, but it's really a PV6 that's pre-wired with MC4 connectors for 4 strings (same box though).

My install does not require it to be disconnect able at the array. I have looked at the MNPV12, which has enough fuse holders, and it would definitely work...it's basically what I'm trying to build.



The other option is the SMA SBCBTL6 combiner, which is specifically for this purpose. Again basically does the same thing, but like with the above Midnite Solar, it's pretty oversized for my application since I only need 6 total fuses for 3 strings.