Help me be solar ready


Hello, I have a question that I was wondering if I can get some much needed information. I have an out building which I would eventually like to put grid tie solar on that I am about to run power to. My questions is, in order to have the electric “solar ready”, what is needed in between the out building and main panel , as far as disconnects between the two?  Do I need a seperate wire for the solar to back feed the main panel or can I utilize the subpanel that is supplying power to the building it self? Any information is greatly appreciated.

 Distance between the Main(200amp) and Sub (125amp is 20 ft.  The distance between the Sub (125amp) and Outbuilding (100amp)is 200ft.

Main (200amp) Panel -------20ft-----------> Sub Panel (125amp w 100amp main) -------200ft------------> Subpanel (80amp Main)




  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭

    You will have to calculate how much larger than normal, you have to up-size your wire. You might be ok feeding loads with 8 ga cable, but to backfeed the Grid, and prevent inverter overvoltage shutdowns, you may need 2 ga cable. And don't forget the run from your meter to where ever the grid transformer is, that's also going to add resistance your inverter will see and can trip offline from.

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  • Dave Angelini
    Dave Angelini Solar Expert Posts: 6,780 ✭✭✭✭✭✭

    How much solar in KW?

    "we go where power lines don't" Sierra Nevada mountain area

  • Clinton_FL
    Clinton_FL Registered Users Posts: 3

    Yea, I was going to feed the outbuilding with 0/2 wire. Is it allowed to backfeed over that wire or do I need to run a seperate wire?

    For the sake or argument 5.5 KW (max 20 panels)


  • BB.
    BB. Super Moderators, Administrators Posts: 33,476 admin

    To add detail to Mike's post...

    Grid Tied inverters look for an acceptable range of input voltage (and frequency too) of something like (in USA/North America) 212-264 volts (+/- ~4 volts or so).

    And lets say that your system is sending 40 amps (40 amps * 240 VAC = 9,600 Watts).

    First, you "up size" the wiring to support 1.25x maximum sustained current or (1.25*40a=) 50 Amp branch circuit breaker+wire minimum rating. (breakers and wiring are really rated to run "fine" at 80% of rated current. If you run at 100% of rated current of breaker and wiring, the breaker may/will eventually trip, and the wiring can overheat when run for hours at rated capacity).

    Next, you need to look at your maximum AC Mains voltage you see at your home (people at the "end of the lines" in remote areas can see wildly varying voltages). Say your maximum is 250 VAC. Simplifying (you may have runs/sub panels rated >50 amps and using heavier cable and less voltage drop) lets say you have 225 foot run of 50 Amp rated cable (one way run, some voltage drop calculations use "round trip" length). Looking at a simplified NEC wire rating chart for 50 amp rated cable (use real NEC to figure out wire gauge in conduit, ambient temperatures, etc.):

    8-6 AWG cable could be used. Using worst case of 8 AWG, 40 amp current, 225 run, we get a voltage drop of:

    8 AWG, 40 amps, 225 feet:

    Voltage drop: 11.31

    Voltage drop percentage: 4.71%

    Voltage at the end: 228.69

    And in your case, this is a voltage "rise" (increase in voltage because you are "back feeding" your circuit. The inverter is set to shutdown (typically) around 260 to 264 VAC, and your base voltage is 250 VAC (example):

    • 250 VAC + 11.31 VAC "rise" = 261.31 VAC estimated voltage at GT inverter

    So, in this case, you could have your GT inverter shut down if your AC mains hit 250 VAC during the middle of a sunny day.

    Heavier cabling may be required to get you out of that GT Inverter Trip zone (6 AWG is 7.11 volt rise, gives you some headroom--And some inverters have adjustable over voltage set points, so the installer may be able to address higher line voltages--However, remember that any loads sharing the cable run would be >260 VAC line voltage and could be damaged).

    Another issue is that your 200 amp panel is usually allowed 1.2x rated capacity for solar... So, you could have a 200 Amp breaker and (1.2 x 200 amps=) 240 Amps of total breaker capacity (200 amps of loads + 40 amps of solar). If your subpanel out to the shed is 60 amps (run some other loads, etc.), then you need to derate the main panel breaker by 240 amps - 60 amps solar = 180 Amp maximum main breaker.

    Also, there can be issues with mixing solar+loads on main+subpanels with code. You need to check with your inspector on what is acceptable to them (i.e., if you have a 60 amp sub panel for your GT Solar, they may not allow any other load connections to share that panel--GT solar only--and require you to run a separate sub panel cable from the main panel.

    Other issues to look at... New NEC regulations are much more complex/restrictive on solar panel wiring (safety disconnects) and roof space (paths for firefighters to access array/roof and cut smoke vents).

    If you use ground mounts, watch for rocks thrown by rotary mowers, kids playing ball, etc. Solar panels are "single weight" tempered glass and pretty easy to shatter with thrown objects (large hail) and/or scratched by hard objects.

    Check how your utilty will bill for solar. Some require separate AC meters for GT solar (especially on larger installations) and you may not be able to us a single main panel (and issues rated to that). Also, some smaller utilities will not allow GT Solar, and other utilities may have billing plans that do not pay you very much for your solar power (high monthly connection fees, low payback for power generated, separate Home and GT solar meters, etc.). Over time, utilities are removing subsidies for GT Solar installations (real value to utilities can push solar rates down to $0.05 or less per kWH, or >$48 minimum monthly connection fee).

    Obviously, the solar array should be pointing mostly south (some areas with array facing slightly east will harvest more for areas that afternoon thunderstorms, for example). Array should be in clear sun (no shade) from 9am to 3pm (at least). Shaded arrays do not generate power. And look at winter sun angle--Trees (or trees growing over time), mountain valleys, etc. can reduce your overall harvest.

    And you know that GT Solar inverters shutdown during utility power failures (no backup solar power if main power fails). There are a few systems that can provided a limited amount of backup solar power during the middle of sunny days--But that may or may not be of use for you (as always, the details matter).

    Are you planning on paying for the system directly? Or will this be some sort of Lease system? (Leased systems--read contracts closely, they can place a lot of restrictions on you, the owner, and be difficult to remove from your property without high fees/lease buyouts).

    Note trying to scare you off of GT solar--Just go into this with both eyes open.

    Good luck,


    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Clinton_FL
    Clinton_FL Registered Users Posts: 3

    I would be paying for the system directly, and with micro invertor systems, the plan would be to "plug and play" as funds allow. Of course this would mean the infractructure is in place. I am aware of the grid tie systems not being "backup power" . I am ok with that as energy storage is costly and something that I do not want to maintain. Main things about my question is I do not want to dig a 250ft trench twice. The main panel has the capacity... I am mainly concerned about backfeeding through two panels to get to the main. Also having a disconnect at the meter for the solar away for emergancy works to kill all power coming to the main panel.

    I read on here that some places do not want your solar subpanel/combiner box not to have any feeds to outlets from it. Which is why I want to ensure I have the proper items in place while I have the ground open. Thanks for the information.

  • BB.
    BB. Super Moderators, Administrators Posts: 33,476 admin

    On occasion, for buried runs--I use oversize pipe and/or add one or two parallel pipe runs (even ABS sewer pipe) in the trench, so I can pull other stuff later (trenching is more work than the cost for the "extra piping"--At least for me).


    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset