Separate Ground Wire for Each String?

mjp24coho
mjp24coho Solar Expert Posts: 104 ✭✭✭
in Off Grid Solar & Battery Systems #1
Sorry for another grounding question, but I haven't been able to see this specifically addressed in any of the other threads. I'm mounting 8 new panels this weekend - 4 on two rails, and the other 4 on two other rails. Both strings will be parallel to each other, and will be wired into a midnite combiner. I'm using WEEB grounding lugs at the end of each rail. My question - can I run one ground wire for both strings - run it along the bottom of each rail, connected to each rail with the WEEB grounding lugs, and then run the one ground wire into the combiner, and then one wire from the combiner to the ground mount? All the diagrams I've seen have a separate ground wire from each string into the ground bus bar in the combiner, and then one ground wire from the ground bus bar to the ground rod. I can't see the reason behind each string having a separate ground wire, and I'd like to cut down on the number of wires if possible.

I have two existing rails (next to the new ones I'll be mounting), and on these rails I have 3 separate panels for my grundfos pump. These panels are wired into a separate combiner box. If I can wire the other strings with one ground wire, can I also include these rails on the same ground wire?

Also, I'm planning on using 6 AWG bare wire - no conduit, so it will be exposed. Is that OK, or should I use green-jacketed THHN?

Thanks!
· Share on Twitter

Comments

  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #2
    Re: Separate Ground Wire for Each String?

    Grounding is for two major reasons... The first is if there is a short circuit somewhere in the wiring system. So, a heavy wire, properly connected and protected against damage/weather is "good enough". Tie the wire to the main system ground rod/water pipe/etc...

    For lightning, you want the ground cable to take as short and direct as path possible from the panel frames/mounts, down the roof and outside of the wall to the nearby main ground rod/connection. Or to a local ground rod next to the exterior of the foundation/wall. you don't want to bring the ground wire into the home, because it could jump from the wire to other places inside the home.

    And if the main ground is not close (10' maximum?), drive a new ground rod near the foundation and run the ground wire there. And run a second cable from the local lightning ground back to the main ground rod/water pipe. You still want a solid electrical path such that a short to the lightning ground can get back to the AC neutral/DC negative bonding points (to trip a breaker or fuse).

    Lightning ground cables should have "soft curves" (something like an 18" radius). Lightning is a Radio Frequency current and you need to follow some of the basic rules for running the cables (soft curves, run on outside of building, if there are multiple lightning ground cables, run them at opposite sides of the building--Far apart).

    Metal siding/roofing/drain pipe are not good lightning grounds.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
    · Share on Twitter
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #3
    Re: Separate Ground Wire for Each String?

    I think he's asking about connecting the panel frames/mounts grounds together at the array site and running one wire to the ground rod from there, as opposed to certain diagrams which show a separate ground wire from each string of the array joining at the combiner box.

    Technically there is no reason an array of two (or more) rows in the same location can't be considered one and be ground connected at the array with a single wire leading to the Earth ground.

    AHJ's may not see it this way, though.
    · Share on Twitter
  • mjp24coho
    mjp24coho Solar Expert Posts: 104 ✭✭✭
    #4
    Re: Separate Ground Wire for Each String?

    Thanks - that was exactly what I was getting at. I couldn't see any reason why I couldn't connect the panel frames with one ground wire between them, run that wire to the combiner box ground bus bar, and then run one wire to the ground rod from there.
    · Share on Twitter
  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #5
    Re: Separate Ground Wire for Each String?

    For lightning--You want to keep the runs short and as direct to earth ground rod a possible.

    If the two arrays are within (say for example) 10' from each other, then a wire from one array to the other, then to ground, should work.

    However if you are looking at arrays on two roof surfaces, more than 20' apart, etc... Then, from a lightning point of view, you are (probably) better off with a wire run from each array to a local ground rod. And for safety grounding (to trip a circuit breaker/fuse, run a ground wire between the two arrays or between the two ground rods (and the home's master ground rod/water pipe--and the AC neutral/bond and DC Return/bond).

    There are a lot of issues with lightning, and here are some links/discussions we have had:
    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
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
    · Share on Twitter
  • SkiDoo55
    SkiDoo55 Solar Expert Posts: 414 ✭✭✭
    #6
    Re: Separate Ground Wire for Each String?

    Are you using WEEB's on the panels to frame's? Are ther WEEB jumpers on all rail splices if you have them? Are they acceptable for local codes?
    #6 bare copper. Jacket on THNN won't hold up in direct sun.
    GT3.8 w/4600W Trina 230W, TX5000 w/5000W ET-250W, XW4024 w/1500W ET-250W, 4 L16, 5500W Gen. (never had to use) Yet!!
    · Share on Twitter
  • mjp24coho
    mjp24coho Solar Expert Posts: 104 ✭✭✭
    #7
    Re: Separate Ground Wire for Each String?

    The panels strings are all on the same roof - within 2' of each other, so they're pretty close. I'm using WEEB mounts for the panels to the rails, and there are no jumpers (no rail splits). This is a remote mountain cabin - no local code to follow or have inspected. I just want to make sure I'm being safe as possible with the wiring setup.
    · Share on Twitter
  • SkiDoo55
    SkiDoo55 Solar Expert Posts: 414 ✭✭✭
    #8
    Re: Separate Ground Wire for Each String?

    You are going in the right direction. All metal frames and rails bonded together and taken to earth ground.
    GT3.8 w/4600W Trina 230W, TX5000 w/5000W ET-250W, XW4024 w/1500W ET-250W, 4 L16, 5500W Gen. (never had to use) Yet!!
    · Share on Twitter

Categories