I have until June 30th to get my system installed and running.

[Palmtree]

After June 30th, 2019 the rebate from the utility is cut in half. I have decided on the basics. It's a ground mounted system without shading issues. I have been approved for a 13.8 kw grid tied system. I plan to use 2 string inverters mounted in my garage with an AC disconnect outside next to the meter (required by utility). I think I have it fairly well squared away but I do have a few questions on DC disconnects. I need to run 4 strings (2 for each inverter) 200 feet to my house. I feel like it would be nice to have a DC disconnect outside by the panels. The SMA inverters I plan to use have a DC disconnect built in to them. Is that sufficient for my situation? Is a DC disconnect outside by the panels considered overkill? What is my best option? Thanks

[bcroe]

Are you combining a pair of strings at the array to be disconnected there, for each inverter? Experience
here is that some subtle issues can be avoided by breaking both leads with the switch; a DC switch of
sufficient voltage rating (and fairly large/expensive) will be required for each set. I picked up a deal on
some used outdoor ones on Evil Bay. Bruce Roe

[Palmtree]

Yes. A 600 volt DC disconnect is really expensive and it looks like I would need 4 of them. Interestingly a 4 string combiner box with a disconnect is much less expensive. What's up with that . However I don't want to combine strings. I need to connect then to my inverters separately unless I'm confused by the SMA instructions .

[Palmtree]

A few more details may help. I have planned a system with 46 300 watt panels and two 6 kw string inverters. If I run it as only two strings then they will be approximately 920 volts for each string at about 10 amps. According to SMA that is too high to connect a single 920 volt string to to each inverter so they say I should connect one string of 14 (560 volts) and another string of 9 (360 volts) to each inverter. That means running 4 strings and would require 4 DC disconnects at the array which would be very expensive. So expensive in fact that I'm not sure I would save any money over micro inverters. How have others handled a situation like this?

[ButchDeal]

A few more details may help. I have planned a system with 46 300 watt panels and two 6 kw string inverters. If I run it as only two strings then they will be approximately 920 volts for each string at about 10 amps. According to SMA that is too high to connect a single 920 volt string to to each inverter so they say I should connect one string of 14 (560 volts) and another string of 9 (360 volts) to each inverter. That means running 4 strings and would require 4 DC disconnects at the array which would be very expensive. So expensive in fact that I'm not sure I would save any money over micro inverters. How have others handled a situation like this?

or you could put 22 on one inverter and 24 on the other. Then you could have two strings of 12 on one inverter combined at the array, and on the other inverter two strings of 11 combined at the array.
now you just need two DC disconnects. Well you don't really need them as there should be DC disconnects on the inverters...

[Palmtree]

OK. So you are saying I could connect two 12 module, 480 volt, 10 amp strings in parallel into a single 480 volt, 20 amp string at the disconnect, and then do the same with the other two 11 module strings? Previously I was merely regurgitating what the SMA design tool told me to do in this situation. If I do what you say then I would need to separate the two strings again before conncecting them to the inverter. Is that correct? It's true that the Sunny boy inverters have a DC disconnect on the inverter. I'm not sure why it is that I feel I should have a disconnect at the array. Is a disconnect at the array completely unnecessary?

[ButchDeal]

OK. So you are saying I could connect two 12 module, 480 volt, 10 amp strings in parallel into a single 480 volt, 20 amp string at the disconnect, and then do the same with the other two 11 module strings? Previously I was merely regurgitating what the SMA design tool told me to do in this situation. If I do what you say then I would need to separate the two strings again before conncecting them to the inverter. Is that correct?

Why would you need to separate them at the inverter?
no you wouldn't need to do that.

It's true that the Sunny boy inverters have a DC disconnect on the inverter. I'm not sure why it is that I feel I should have a disconnect at the array. Is a disconnect at the array completely unnecessary?

Depends on your local code but not usually. You could also use the combiner box for the two strings as a disconnect.
You generally don't need fuses when combining just two strings but you could used a fused combiner which would give you a means for disconnecting if you really wanted one.

[Ampster]

Rapid Shutdown Devices are required for rooftops but I am not sure if needed for ground mount. The hardware is expensive and that may be what you were referring to.
In any event my standard advice when digging a trench is to throw in an extra conduit to give you some flexibility especially if you are under a time constraint. That deadline is probably for final inspection and certification by Power Company. At least that is how it worked in California a while back. The advice above is good from the guys with more experience on issues of disconnects and o am just fguring out what my own local code requires.

[Palmtree]

Code is not an issue. There is no code in my rural county. I do need to meet certain requirements to tie in to the utility grid. The only disconnect the utility requires is an AC disconnect accessible by the utility employees. Maybe the DC disconnects built in to the inverters are all I need.
I do wonder if there is a problem with dividing the 13.8 kw in to 4 strings and then combining them in parallel to 2 strings. Run the two strings (approx 460 volts and 20 amps each) to the inverters. Then can I land them each on a single input on each inverter as 20 amps at 460 volts. Or would it be better (voltage drop wise) to run the higher voltage at 10 amps and forget about having a DC disconnect at the panels?

[Palmtree]

Why would you need to separate them at the inverter?
no you wouldn't need to do that.

Then I would have 1 string of 24 modules at 960 volts to first inverter and the other string of 22 modules at 880 volts. The inverters have 3 input connections with a max input of 600 volts and 30 amps. So I figured I would separate them and connect them in parallel to the inverters. Then the 960 volt 10 amp string would be 480 and 20 amps (x 2) and the 880 volt string would be 440 volts at 20 amps (x 2).
Also the voltage drop calculator says that I can run 960 volts at 10 amps over 200 feet with 12 gauge copper and only a 0.76% drop. Maybe I'm confused about something.

[ButchDeal]

Then I would have 1 string of 24 modules at 960 volts to first inverter and the other string of 22 modules at 880 volts. The inverters have 3 input connections with a max input of 600 volts and 30 amps. So I figured I would separate them and connect them in parallel to the inverters. Then the 960 volt 10 amp string would be 480 and 20 amps (x 2) and the 880 volt string would be 440 volts at 20 amps (x 2).
Also the voltage drop calculator says that I can run 960 volts at 10 amps over 200 feet with 12 gauge copper and only a 0.76% drop. Maybe I'm confused about something.

no when you combine strings you combine them in parallel which does NOT increase the voltage.
Yes the inverter has 3 inputs but you would only use 1

you would have two strings of 12 modules or 2P12S at 480V into one inverter and 2 strings of 11 for 2P11S at 440V into the other.

[Palmtree]

no when you combine strings you combine them in parallel which does NOT increase the voltage.
Yes the inverter has 3 inputs but you would only use 1

you would have two strings of 12 modules or 2P12S at 480V into one inverter and 2 strings of 11 for 2P11S at 440V into the other.

I guess I wasn't clear. I understand that combining in series multiplies voltage and not current, and the opposite is true for parallel connections. I guess what I need to know is if there is any reason I shouldn't make the two 200 foot runs at 920 volts and 10 amps each. Then wire each 6 kw inverter in parallel at 460 volts and 20 amps on each input (SMA says 600 volt max per input). Is there a reason that I couldn't use 2 inputs on each inverter. That's what the SMA solar design program says I should do. The difference is that the design program assumed that I was running a separate cable from the array to each input at 460 volts and 20 amps. The main reason I ask is because line voltage loss calculators say that it would only require 12 gauge wire to achieve 0.76% loss. I would likely bump that up to 10 gauge and would still save greatly on copper wire and keep the losses really low. I guess if I did that then I would need to install some sort of distribution panel before the inverters -- or could I just wire a combiner box backwards?

[bcroe]

Many inverters have a ground connection to inputs, which would prevent tying them in series. Generally 600V
is the limit for consumer grade equipment. Bruce Roe

[ButchDeal]

I guess what I need to know is if there is any reason I shouldn't make the two 200 foot runs at 920 volts and 10 amps each. Then wire each 6 kw inverter in parallel at 460 volts and 20 amps on each input (SMA says 600 volt max per input). Is there a reason that I couldn't use 2 inputs on each inverter. That's what the SMA solar design program says I should do.

No the design program is not telling you to crank up the voltage to 920V. It will not work the way you are describing.

The difference is that the design program assumed that I was running a separate cable from the array to each input at 460 volts and 20 amps. The main reason I ask is because line voltage loss calculators say that it would only require 12 gauge wire to achieve 0.76% loss. I would likely bump that up to 10 gauge and would still save greatly on copper wire and keep the losses really low. I guess if I did that then I would need to install some sort of distribution panel before the inverters -- or could I just wire a combiner box backwards?

there is pretty much no way to run the voltage that high and it would not be to code anyway.

[Palmtree]

I guess I'm not making my question clear. Sorry for that. Here's another attempt.
Is there a difference (to the inverter) if I make the 200 ft run from the array as a single string of 23 modules (920 volts and 10 amps) and then just before the inverter I divide that into 2 lines (460 volts and 20 amps each) and then connect them on 2 of the 3 available inputs? As opposed to making the 200 foot run with a string of 12 (480 volts, 20 amps) and a string of 11 (440 volts, 20 amps) and then connect each to on of the 3 available inputs on the inverter.