Grounding diagram is attached with questions

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Kgelles
Kgelles Solar Expert Posts: 25
I'm attaching a few questions I have about grounding my solar setup. I'm trying to figure out the best and most economical way to ground my off-grid equipment. In the basement I have a ground busbar that ties all of my equipment, battery bank and ac load center....I'm not sure where to ultimately terminate the ground wire. I have included the questions right in the diagram...

Attachment not found.

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  • BB.
    BB. Super Moderators, Administrators Posts: 33,479 admin
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    Re: Grounding diagram is attached with questions

    Will the off grid AC power be connected to Utility Grid power (via transfer switch)?

    Also, do you have a risk of lightning strikes at your home?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
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    Re: Grounding diagram is attached with questions

    am i reading you right that they are using your steel water pipe as the ground? i didn't think utilities still did this kind of stuff.

    i would say running from the pv array to the utility ground would be optional being it is quite a distance. now where in the house does the utility connect to ground? if it's at the top where you list option 1 and it has to travel through to the other end of the house to have it contact the ground towards the water utility feed then i would disconnect the water pipe ground and buy a proper 8ft ground rod driven into the soil where either the electric utility enters the building or you may need it opposite the service breaker box, but outside, if the wires travel through the house some. a hole would then be drilled to the outside to feed the bare #6 to the ground rod. minimize bends as much as possible and any necessary bends should go somewhat gradually.
  • SolaRevolution
    SolaRevolution Solar Expert Posts: 410 ✭✭
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    Re: Grounding diagram is attached with questions

    Here is how I would interpret your diagram as applied to the NEC:

    If your "Off Grid Setup" is connected to the same AC distribution system as the utility by way of a transfer switch it is not a "Stand Alone System".
    It is not an "Interactive System" unless it can "sell" to the grid.

    Your house is grid tied. Your utility AC distribution system is grounded. Unless the "Off Grid Setup" is isolated from your utility's "Electrical Production and Distribution Network" your "Solar Photovoltaic System" is grid connected and the solar AC grounding should be common to the utility AC grounding system with a single AC neutral bonding point. If the utility and the solar AC system outputs are completely isolated, each one will require it's own Neutral to Ground Bond.

    Whether they are isolated or not, NEC 250.58 requires both systems to be connected to the same grounding electrode(s). "They shall be considered as a single grounding electrode system in this sense."
    They must have a minimum size conductor based on NEC 250.66.
    NEC 250.53(D)(2) requires a "Supplemental Electrode" for under ground water pipe.
    NEC 250.104 (B) requires bonding "Other Metal Piping" to the grounding electrode system.

    The "can of worms" opens when you look at the battery system's DC ground connection requirements (NEC 250.166) and neutral bond. The large battery cables can require a large "grounding electrode conductor". NEC 250.166(C) specifies "...that portion of the grounding electrode conductor that is the sole connection to the grounding electrode shall not be required to be larger than #6 copper wire..." :confused: (This is the murkey part. It is vague and some AHJs will interpret this very differently. I'm not looking at the newest version of the NEC so this may have been recently cleared up.)

    NEC 690.47(C) requires that both AC and DC systems be bonded together either:
    (1) with seperate grounding electrodes which are then bonded by a conductor sized for the larger of AC or DC grounding conductors.
    or
    (2) connected to the same grounding electrode with seperate conductors sized for the AC or DC system which they serve.



    In short, you should probably run a "grounding electrode conductor" sized per NEC Table 250.66 from your utility service entrance to a new "grounding electrode" (ground rod) placed at, and connected to, the DC side (however you or the AHJ chooses to interpret the DC bonding requirements) of your "Off Grid Setup".


    Alex Aragon
  • Kgelles
    Kgelles Solar Expert Posts: 25
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    Re: Grounding diagram is attached with questions
    BB. wrote: »
    Will the off grid AC power be connected to Utility Grid power (via transfer switch)?

    Also, do you have a risk of lightning strikes at your home?

    -Bill

    The off grid AC power won't be connected to utility grid power via transfer switch.
    We have a moderate risk for lightning as compared to other parts of the country.
  • Kgelles
    Kgelles Solar Expert Posts: 25
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    Re: Grounding diagram is attached with questions
    niel wrote: »
    am i reading you right that they are using your steel water pipe as the ground? i didn't think utilities still did this kind of stuff.

    i would say running from the pv array to the utility ground would be optional being it is quite a distance. now where in the house does the utility connect to ground? if it's at the top where you list option 1 and it has to travel through to the other end of the house to have it contact the ground towards the water utility feed then i would disconnect the water pipe ground and buy a proper 8ft ground rod driven into the soil where either the electric utility enters the building or you may need it opposite the service breaker box, but outside, if the wires travel through the house some. a hole would then be drilled to the outside to feed the bare #6 to the ground rod. minimize bends as much as possible and any necessary bends should go somewhat gradually.

    I looked and the steel water pipe ground appears to be a telecommunications ground wire.
    The utility connects to a ground rod at the service entrance.
    I'll create another sketch to see if some changes I will make will work ok as a ground.
  • Kgelles
    Kgelles Solar Expert Posts: 25
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    Re: Grounding diagram is attached with questions

    Here is a new diagram that is revised based on everyone's excellent feedback. Will this be safe?
    (please note: I decided to locate the panels to the roof of the main house to avoid a small shading issue.)

    Attachment not found.
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
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    Re: Grounding diagram is attached with questions

    ok that's a little clearer. i would still add the rod at the solar equipment area and bury at least 1 ft down in the outside dirt a bare #6 copper wire to tie the solar ground rod to the utility ground rod with the proper rod connections. although the pipe is convenient i don't recommend using it due to lightning aspects as this aspect you want to keep outside of your home in the ground where it belongs.

    now this i didn't make myself clear on as the underground tie from the utility ground rod to the pv ground rod is what i said would be optional, but do understand that the nec requires that ground lead go along with the pv leads and into the home. if you were to tie the pv ground rod to the utility ground rod that 2 paths would be created to ground and that isn't recommended by me. the one with the pvs is required, but not hearing this from me i'd disconnect it at both the pv and solar equipment and just use the underground rod to rod tie later after any inspectors have already approved the system. remember that you can only tie the pvs one way or the other and underground with rod to rod is best, but the code requires a connection that travels with the pv wires.

    i do recommend the burial of ground wires to be at least 1 ft down, but soil type and conditions may interfere with such a requirement. this is to minimize any possible damage to the wire from digging or natural occurrences. there aren't any guarantees damage to the ground wire will not happen in the ground, but the efforts you make there can minimize any chances of damage. an example might be to put small rocks or large gravel just above the wire and then continue to throw dirt onto the rocks to allow grass or other small vegetation to grow. if growing larger vegetation like bushes would mean the roots could surround and possibly push through the wire thus breaking the wire. one could encase the wire in pvc or something similar, but the wire itself aids the contact with the ground and increases the effectiveness of the ground. one possibility would be to increase the thickness of the wire to lessen the possibility of breakage, but copper wire is soft so no matter how thick it is breakage is still possible.
  • Kgelles
    Kgelles Solar Expert Posts: 25
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    Re: Grounding diagram is attached with questions

    Thank you Niel.

    This is what I plan to do:

    1. As recommended I'll add a ground rod at the solar equipment area and run a 6awg bare copper wire from my solar equipment ground bus to that rod.

    2. I'll tie a #6 copper wire underground between the new solar equipment ground rod and the utility ground rod.

    3. I'll tie a #6 grounding conductor from the PV frames to the new solar equipment ground rod which is tied underground to the utility ground rod.

    QUESTION: So, it might be safer if I keep the PV frame grounding conductor outside the home and run it to the new solar ground rod? (keeping it separate from the PV leads)
  • BB.
    BB. Super Moderators, Administrators Posts: 33,479 admin
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    Re: Grounding diagram is attached with questions

    Yes, keep the lightning ground on the outside of the home/walls (no matter what NEC/Code guys say).

    Lighting is not DC current, it is (very roughly) a 7kHz Radio Frequency current and does not want to follow wiring blindly. Keep very soft angles (like 18" diameter or larger?) and remember that lightning wants to flow away from the center of wiring/homes/etc (skin effect).

    Some more reading material:
    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
    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Kgelles
    Kgelles Solar Expert Posts: 25
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    Re: Grounding diagram is attached with questions

    Excellent. Thanks for the quick response. And I'll read the links. Thank you for providing them.
  • Kgelles
    Kgelles Solar Expert Posts: 25
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    Re: Grounding diagram is attached with questions

    One more question about the ground rods:

    If my solar equipment is completely isolated from any of the home's grid power and the solar equipment and PV array has its own ground point...is the 25 feet of underground wire linking to the utility ground optional or required? See attached diagram.

    Attachment not found.
  • SolaRevolution
    SolaRevolution Solar Expert Posts: 410 ✭✭
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    Re: Grounding diagram is attached with questions
    Kgelles wrote: »
    If my solar equipment is completely isolated from any of the home's grid power and the solar equipment and PV array has its own ground point...is the 25 feet of underground wire linking to the utility ground optional or required?

    NEC 250.58 Common Grounding Electrode
    "...Where seperate services, feeders, or branch circuits supply a building and are required to be connected to a grounding electrode(s), the same grounding electrode(s) shall be used. Two or more grounding electrodes that are effectively bonded together shall be considered as a single grounding electrode system in this sense."
  • Kgelles
    Kgelles Solar Expert Posts: 25
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    Re: Grounding diagram is attached with questions

    OK. I see. Thank you sir.