Grounding (using a grounding rod) for two separate strings in a tranformerless inverter system (SMA)

midijeepmidijeep Registered Users Posts: 102 ✭✭✭
I have a SMA system (SunnyBoy with Sunny island) with two strings going into the SunnyBoys disconnect (provided with the inverter).  I want to put a surge protection device between the panels and the SunnyBoy disconnect and using a  grounding rod for the grounding. 
I am trying to figure out the best thing to use for the surge protection device i.e.  combiner box, another disconnect and being able to run the DC ground to a grounding rod.
The SMA system does not use a ground wire from the solar panels to the inverter (tranformerless).  I want to place something like the Midnite Solar surge suppressor MNSPD-300-DC in between the panels and the DC disconnect of the SunnyBoy.
Should I use a separate ground rod for both strings?
What could be a possible way to do this?


  • BB.BB. Super Moderators, Administrators Posts: 33,143 admin
    The path from the metal box (combiner/disconnects/etc.) to the ground should be as short and straight as possible. I suggest 6 AWG wire minimum (or get "real" lightning rated braided cable). No sharp bends (i.e., transition from wall to ground rod). Lightning is a "radio frequency" current and follows "different" rules vs DC and 50/60 Hz AC. Generally lightning will only flow a couple dozen feet down a 6 AWG cable before it finds a "different path" (or jump to a different path at a sharp bend).

    Lightning wise, just a single ground rod right at the base of each box/surge suppressor is fine.

    For "safety", I would highly suggest you run a minimum 6 AWG cable between each/all ground rods. That way if there is a short circuit between any of the Hot wires (whether DC or AC wiring) and metal racking, all DC / 50/60 Hz current flows between the short and the source (hopefully to trip a breaker).

    Some other links:

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • midijeepmidijeep Registered Users Posts: 102 ✭✭✭
    edited October 2018 #3
    Thanks Bill!.  Do you recommend a disconnect/combiner box from Solar-elecric  or any other source that would allow me to put a surge suppressor such as the mentioned Midnite Solar?
  • BB.BB. Super Moderators, Administrators Posts: 33,143 admin
    I don't know your wiring.... Typically, if you have three or more parallel strings of solar panels going to one DC input, then you need a breaker/fuse per parallel string (i.e., a combiner box). With less than that, you do not need the breakers and can use a simple junction box at the array (or where ever you make your array to DC input connection).

    If you have a DC disconnect, in may be on the outside wall of the house (a good place for lightning suppression)--Or the disconnect may be on the bottom of the inverter/DC charge controller)... If the integrated DC disconnect is mounted on the outside wall, or just inside an outside wall, that is a good place for lightning suppression. If the DC stuff takes place on an interior wall--You potentially are bringing lightning into the home--A bad thing all away around--And installing a J-Box on the outside wall to interior transition is usually a good idea (with lightning suppressors to ground).

    You will notice I am being very generic here... Much of what is required will be set by your local building codes/inspectors. There has been a lot of new code added to protect firemen (roof top disconnects, setbacks of arrays from roof edge for cutting ventilation holes and access, etc.).

    I guess you are around Salt Lake Utah? ... Perhaps somebody here can add some more details (I am not a code person).

    Also note that the NEC has not been very good about lightning grounds vs standard Green Wire Safety Grounds. NEC by itself (historically) has not be a good reference for lightning control

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