Question re: PV grounding to combiner in attic.

MJSullivan56MJSullivan56 Registered Users Posts: 42 ✭✭
I plan on having two strings of six panels each on the roof of my house. Both strings will share the same racking. I plan on running two nylon JBOXes through the sloped roof, one for each string. The strings will be combined together downstream using a combiner in the attic. This combiner will naturally be grounded back to the inverter.

My question is straightforward: given that the rack will have a shared ground, do I run a single grounding wire through just one of the two JBOXes, or do I run dual grounding wires (one grounding wire per JBOX)? 

My concern: Won't running two grounds present a possible "ground loop"?


  • BB.BB. Super Moderators, Administrators Posts: 33,140 admin
    As always, grounding is an "it depends/its is complicated) answer...

    You have the + and - lines from the panels/Combiner back to the Solar charge controller / GT Inverter panel inputs.

    In general, you want the +/- lines to run from the array/combiner box directly to the Panel Input. Different controller mfg. have different implementations, and running +/- wires "unconnected to anything else" is always the "right answer.

    For Schneider charge controllers, they need +/= wires ungrounded because the Charge controller measures current in the negative lead (at least the older models of MPPT controller) if you connect to green wire or battery ground directly to the panels, the controller will not MPPT correctly (can't measure array current correctly).

    Others like the older (and current?) Outback controllers... the Vpanel - is directly connected to Battery -.

    For some GT inverters--The array Vpanel input may be floating, or even "directly" connected the 240 VAC utility power and will short circuit the GT inverter....

    Then you have Green Wire grounding. This is to keep the panel frames/racking from becoming "electrically" hot from a +/- short circuit to ground. This can be through a greenwire pulled from the array with the +/- all the way to your home electrical/grounding point...

    And there is Lightning Grounding: OR, especially if you are in an area that has chances for direct (or indirect lightning) strikes, you need to run a ground (suggest 6 AWG minimum from panels to racking (via approved ground clips) and metal combiner box (if used) from the roof to the side of the building and down to a ground rod driven outside the exterior wall/foundation. You do not want to "bring" lightning energy into your home... The way lighting works (like a low frequency radio wave) and if you bring it into the building, it "naturally" wants to migrate (i.e., arc) to the outside edge of the building.

    The ground rod at the base of the wall/array should also have a 6 AWG wire that runs from that "local array ground rod" back to the home's ground rod/cold water connection (and eventually to the AC Neutral+Ground bus bar). This second wire between the two ground rods/points is to provide an AC current path to "trip" any breakers/fuses if there is a "hot to ground" short circuit in the system.

    If you have two ground rods that are not connected together, they will work fine for Lightning (a few million volts "don't care about some ground resistance). However, shorting DC or or AC to "metal" will not generally trip a circuit breaker or fuse (a typical short to "ground rods" may only allow 4-8 amps of fault current--Not enough current to even trip a 15 Amp breaker). The 6 AWG cable between the two (or more) ground rods/grounding system will allow the >>15 amps to flow and trip any protective breakers/fuses.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • MJSullivan56MJSullivan56 Registered Users Posts: 42 ✭✭
    Problem might be how our AHJ interprets the NEC. It seems to suggest that there must be (green) rack grounding accompanying the +- wires once they leave the PV area. Personally I would rather run the (green) rack grounding down the exterior to a 2nd grounding rod then bond the two grounding rods with a connector cable. This would leave the PV power “ungrounded” until it gets to the combiner. Making matters worse, all the example diagrams provided by the manufacturers of the UL gear I intend on using show rack grounding accompanying the PV +- 
  • MJSullivan56MJSullivan56 Registered Users Posts: 42 ✭✭
    edited April 2022 #4
    Another option I suppose would be to provide independent “real” lighting protection along the peak of the roof and let the rack grounding “go inside” to the combiner as per the manufacturers’ public documentation.
  • BB.BB. Super Moderators, Administrators Posts: 33,140 admin
    You can bring the solar cabling down the roof and side of the home... Then bring the cable in a few feet off the ground into the building. And run a 6 AWG ground to the rod at the base of the wall. Is not pretty. (no sharp corners in lightning ground wire--Soft large diameter bends to lessen the chances that the lightning will leave the 6 AWG wire at a right angle bend--Higher Impedance because of sharp bend).

    And you can put surge suppressors at the box as the solar power enters the building...

    If your home is not in a high probability of lightning area--Then the lightning grounding is much less important.

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