Odd Grounding Question

I've run out of room for solar panels on the main structure where the panels, inverters and other equipment are housed. I'd like to add more panels on an outbuilding that is located about 96 feet from the main structure. The outbuilding has AC power fed from the main structure. Unfortunately, when the outbuilding was constructed, they didn't put in a ground rod, but it does have system ground (#4 wire) running back to the ground buss in the main structure, which is where the ground rods are located. Accessing the sub-panel wiring to add a ground rod at the outbuilding would be a big project (I'd have to cut through some expensive concrete work) so I've never done it. The outbuilding is a small guest house.

Considering the above, if I want to add solar panels on the roof of this outbuilding, would It be best to:

1. Run only the solar panel ground wire (from panels/combiner box on the outbuilding) directly to the ground buss in the main structure, or...
2. Should I run a wire from the combiner box to a ground rod that I could install at the outbuilding, and then run another ground wire from the combiner box to the ground buss in the main structure? or...
3. Other suggestions?

Thanks....

Comments

  • jaggedbenjaggedben Solar Expert Posts: 230 ✭✭
    Re: Odd Grounding Question

    I'm a little confused whether the subpanel at the outbuilding is part of your new installation plans in any way, or not.

    From your equipment list in your signature, I would gather that you are putting in a new DC wire run from the outbuilding to your existing inverter setup. In that case, either 1 or 2 is fine, IMO.

    In case you are using the subpanel at the outbuilding in any way, I'd say the existing #4 ground is going to be adequate for your needs, although it might be good to add bonding bushings to it if they aren't there.

    I'm also a little skeptical that it would be so difficult to add a ground rod to the subpanel. I think it probably shouldn't require anything more special than a long 1/4" masonry bit to get a #6 wire into the panel from behind it or something. But I'm also gathering that since your in Mexico(?), that strict adherence to the NEC isn't required. In that case it doesn't matter much.
  • nielniel Solar Expert Posts: 10,311 ✭✭✭✭
    Re: Odd Grounding Question

    i too agree it could be any of those choices. if too difficult to put a rod in then use the connection with the running #4 ground wire. at a distance of 96ft there shouldn't be much chance for ground loops should you go with a rod at the outbuilding.
  • BigwoooBigwooo Solar Expert Posts: 60 ✭✭
    Re: Odd Grounding Question

    The subpanel in the outbuilding is old construction and it is not part of the new solar panel installation. They will be separate systems. The new panels will be a new DC run from the outbuilding to the existing inverter setup. I'd like to avoid cutting into the subpanel at the outbuilding and keep the systems independent of each other.

    Retrofitting the old subpanel with a ground rod by cutting into the subpanel from the back would normally not be an issue. Unfortunately I only have one small section of the building with no concrete surrounding it, and it happens to be on the opposite side of the building from the subpanel. I'll use that area to get the new DC wiring underground, but getting wire to a ground rod from the existing outbuilding sub panel would require damaging a lot of rock work on the patio and/or exterior of the building.

    I tried cutting into the conduit that runs from the outbuilding to the main structure, but I can't get the wire for a ground rod to run back into the subpanel. Being Mexico, they used 90's instead of sweeps. Impossible to run wire through after the fact. I think that unless I want to re-do a lot of intricate rock work, I'm stuck without a ground rod at the outbuilding. Yes we're in Mexico...No NEC
  • nielniel Solar Expert Posts: 10,311 ✭✭✭✭
    Re: Odd Grounding Question

    the ground really won't be separate as it will go back to the main building and tie into the common ground rod there. systems can be separate, but ground should be common to all.
  • BigwoooBigwooo Solar Expert Posts: 60 ✭✭
    Re: Odd Grounding Question

    Thanks Neil,

    From what I understand then, I'm OK to run a separate (independent) ground wire from the array back to the ground buss in the main building? In that case, I will have one ground wire running back to the main building ground buss from the Solar array, and a second independent ground wire running from the outbuilding AC subpanel back to the main ground buss. No ground rod at the outbuilding.
  • nielniel Solar Expert Posts: 10,311 ✭✭✭✭
    Re: Odd Grounding Question

    only one ground lead. 2 will set up ground loops.
  • ggunnggunn Solar Expert Posts: 1,973 ✭✭✭
    Re: Odd Grounding Question
    Bigwooo wrote: »
    The subpanel in the outbuilding is old construction and it is not part of the new solar panel installation. They will be separate systems. The new panels will be a new DC run from the outbuilding to the existing inverter setup. I'd like to avoid cutting into the subpanel at the outbuilding and keep the systems independent of each other.

    Retrofitting the old subpanel with a ground rod by cutting into the subpanel from the back would normally not be an issue. Unfortunately I only have one small section of the building with no concrete surrounding it, and it happens to be on the opposite side of the building from the subpanel. I'll use that area to get the new DC wiring underground, but getting wire to a ground rod from the existing outbuilding sub panel would require damaging a lot of rock work on the patio and/or exterior of the building.

    I tried cutting into the conduit that runs from the outbuilding to the main structure, but I can't get the wire for a ground rod to run back into the subpanel. Being Mexico, they used 90's instead of sweeps. Impossible to run wire through after the fact. I think that unless I want to re-do a lot of intricate rock work, I'm stuck without a ground rod at the outbuilding. Yes we're in Mexico...No NEC

    If there are only to be DC modules on the roof of the outbuilding, then why is the (AC) subpanel even in the equation?
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Odd Grounding Question
    ggunn wrote: »
    If there are only to be DC modules on the roof of the outbuilding, then why is the (AC) subpanel even in the equation?

    I believe on of the choices was to use the #4 ground on the sub panel for connecting the DC ground to.
  • BigwoooBigwooo Solar Expert Posts: 60 ✭✭
    Re: Odd Grounding Question
    ggunn wrote: »
    If there are only to be DC modules on the roof of the outbuilding, then why is the (AC) subpanel even in the equation?

    This is an outbuilding, that prior to installing the panels, had no DC wiring. The AC sub panel is where the system ground block is, and is the connection point where the ground wire runs back to the main power room.

    After several days of cutting through concrete, I was able to run the system ground for the solar panels into the sub panel. Now the combiner box and panel frames are grounded to the system ground block at the sub panel, and there is only one ground wire running back to the main building.

    BTW, I bought Midnight Solar DC lightning arrestors for the combiner box and new charge controller. If you plan to install these, leave plenty of room, they are MUCH larger than the Delta arrestors.
  • BigwoooBigwooo Solar Expert Posts: 60 ✭✭
    Re: Odd Grounding Question

    Another note, I had an installer tell me that it's better to leave the panels un-grounded (off grid) in lightning prone areas, but I'm not sure why. I remember reading a thread on this site where that theory was discussed. I can't find the thread, anyone remember this, and have a link to the thread?
  • nielniel Solar Expert Posts: 10,311 ✭✭✭✭
    Re: Odd Grounding Question

    solar guppy likened to that theory, but i disagree with it. with a ground static charges can be bled off and avert dangerous build up. also, in the event of lightning you will control the direction to some degree of where that lightning is to travel making equipment and you less likely to be hurt.
  • BB.BB. Super Moderators, Administrators Posts: 29,636 admin
    Re: Odd Grounding Question

    Here are some lightning threads:
    BB. wrote: »
    A couple threads about Lightning:

    Off Grid Grounding Technique?
    Another Question, this time about Lightning

    Note, the above are discussions, not a do A, B, and C--and you will be "safe". There probably is no such thing with lightning. Several different techniques are discussed--and a few of those posters even have experience with lightning. :cool:

    And our host's consolidated FAQ page:

    www.windsun.com
    Lightning Protection for PV Systems

    From other past posts here, Windsun (admin/owner of NAWS), he said that most of lighting induced failures he saw were in the Inverters' AC output section.

    Towards the end of this thread is a very nice discussion of proper generator grounding.

    -Bill

    The second link includes Solar Guppy's method.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • ggunnggunn Solar Expert Posts: 1,973 ✭✭✭
    Re: Odd Grounding Question
    Bigwooo wrote: »
    Another note, I had an installer tell me that it's better to leave the panels un-grounded (off grid) in lightning prone areas...
    Why would whether the system is on or off grid make a difference to the EGC?
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