DYI researched and needed advice on a 17.4 k system

**Ampster** · 03-20-2021, 01:52 PM

You may not need Rapid Shut Down for ground mount. Check your local codes.

**khanh dam** · 03-20-2021, 08:15 PM

1. at the ground mount do i need to run a combiner box or can i just run each string in conduit to the two inverters i seen video's and i see a box used , do i need to have a breaker at the round mount and if i do need one what box is good to get ?

Midnite Solar MNPV12 Solar Array Combiner

Midnite Solar MNPV12 Solar Array Combiner | Northern Arizona Wind & Sun (solar-electric.com)
Midnite Solar MNPV12 Solar Array Combiner | Northern Arizona Wind & Sun (solar-electric.com) is what I used. wayyy too biggggg, but allows you to keep the 2 inverters seperated.

2. the room i want to install the system into is a 20 x 12 foot room thats behind the house . it is 28 feet from the utility poll and main panel
can i install a subpanel and connect it to the main panel .also were can i place the disconnect ? can i place the disconnect before the main panel or between the panel and subpanel ?

if the inspector/AHJ thinks the room could be used as a place for people to gather they rapid shut down will probabaly be required. I'd fight hard as heck to avoid RSD devices which will add $1500 or more to total price. 2017 technically allows a building that is used ONLY for housing solar equipment to be excempt from RSD. ask your local inspector. mine requires that disconnect be at the main meter/panel because that makes it easier for fireman to find

3. what other items required beside the backed feed breaker and the disconnect for the system ?

4. any other advice ?

make sure your busbars can handle that much back fed power, you might need a line side tap.

**morf33** · 03-21-2021, 10:01 AM

can the inverter be installed into a subpanel then the subpanel to the main and have the disconnect between the subpanel and main ?
if have two set as seen in the picture do i need to run two seperate paths or can i run one to the other then into the building
if i have 2021-03-21_16-38-32.png

**khanh dam** · 03-21-2021, 10:11 AM

save your self a lot of headache and $1500 by not needing RSD modules under each array by just putting inverter underneath each array. use sunmodo gasket between panels to keep it mostly water tight so rain does not get on inverters. Trench the ac wires and be done with it.
does your building have at least 250a main breaker. I think you need at least that big to handle the back feed. otherwise you need line side tap and fuseable disconnect.

**Ampster** · 03-21-2021, 01:27 PM

Originally posted by khanh dam

..........
does your building have at least 250a main breaker. I think you need at least that big to handle the back feed. otherwise you need line side tap and fuseable disconnect.

The 125% rule applies to the bus bar rating not the service panel breaker on that bus bar. A 7.7 kW inverter would require a 40 Amp breaker so the calculation would be based on 80 Amps backfeed. Depending on configuration of the panel and meter the options vary.

**oregon_phil** · 03-21-2021, 02:29 PM

4. any other advice ?

A) Did you know that SMA has a string design tool at SMA America, inverters, sunny boy inverters "Download our quick design reference tool"

For arguments sake, lets assume you like symmetry and each of your strings is 10 panels long to keep string voltage under 600VDC.

Without getting too much in the weeds, the SMA 7.7 inverter has 3 MPPT inputs. Only inputs A & B can be paralleled together. In other words, you cannot combined 30 panels together in a combiner box, run two DC wires to the SMA 7.7 and uncombine them into the three MPPT inputs. Download the manual and look at the electrical connections chapter. I personally would keep as many unique MPPT inputs as possible (not parallel them together).

B) I have seen quite a bit of discussion in recent years about the pros and cons of having the inverter close to the ground mount (short DC run, longer AC run) or having the inverter farther away from the ground mount (longer DC run, shorter AC run). At the very least, I would measure the AC voltage on each leg at the main panel to get a baseline. Maybe others can chime in to fill in the gaps.

**foo1bar** · 03-21-2021, 07:46 PM

Originally posted by khanh dam

save your self a lot of headache and $1500 by not needing RSD modules under each array by just putting inverter underneath each array. use sunmodo gasket between panels to keep it mostly water tight so rain does not get on inverters.

Don't waste money on a gasket. Inverters are built to be installed outdoors where they will get rained on.

Whether to put the inverters under the array or near the main panel is something you'll have to decide. I would look at resistive losses at the different voltages (AC vs. DC) and cost of wires to make my decision.

if have two set as seen in the picture do i need to run two seperate paths or can i run one to the other then into the building

I probably would do 1 trench to make my life easier, rather than 2 trenches.
I don't know if you would need two separate conduits in the trench for the two arrays. I don't think so, but don't know. May depend on size of conduit you use.
I'd probably run another conduit or a direct burial wire that provided for GFCI outlets near the arrays. (Easy to run the wire/conduit when you already have the trench there)
Make sure you follow the minimum cover depth rules for buried wires/conduit.

You will probably want to change wires in a box at the array - switching from PV wire (to connect to the modules) to THWN.

can the inverter be installed into a subpanel then the subpanel to the main and have the disconnect between the subpanel and main ?

I think so - I think you're describing:
pair of 40A breakers for inverters in 100A subpanel. Subpanel's wires then go to disconnect. wires from disconnect then go to solar breaker in main panel. And in the main panel it's appropriately done for solar backfeed. (ex. 175A main breaker with 225A bus bars and a 80A backfeed breaker, using the 120% rule. Or 200A main breaker and total of the non-main breakers is 200A or less including the backfeed. Or one of the other methods like a dedicated solar breaker like some main panels now have)
I'm not an electrician though - and whatever you do you'll want it documented in your plans so that the planning dept / building inspector signs off on it.

FWIW, I wouldn't install inverters in an area where people are - I'd install them on an outside wall (preferably on a north wall - out of the sun). Some inverters have a hum that you may find annoying..

**oregon_phil** · 03-22-2021, 04:54 PM

FWIW, I wouldn't install inverters in an area where people are - I'd install them on an outside wall (preferably on a north wall - out of the sun). Some inverters have a hum that you may find annoying..

The SMA 7.7 has a cooling fan to add to the noise factor if that is a concern. I don't mind it because mine is in an outbuilding.

**morf33** · 03-23-2021, 01:58 AM

i have a 20 foot container office that i will use for the inverters and main wiring so i got plenty of space and isolation away from the main house .
noise is not a factor and my servers will be ten times more noisy that the inverter itself

DYI researched and needed advice on a 17.4 k system

DYI researched and needed advice on a 17.4 k system

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