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Thread: Grounding of residential subpanel

  1. #1
    Join Date
    Feb 2003
    Location
    California
    Posts
    8

    Grounding of residential subpanel

    An existing subpanel, probably over 40 yrs old, has a local ground #8 ground wire bonded to a copper water pipe and the neutral and grounds share the same buss bar. No ground wire in the #2-3 romex back to the service, about 100' away. Service has one 70 amp breaker dedicated to this condo. I believe the bank of about 10 service panels for the condo complex is grounded. No easy or esthetic way to run a ground to the service.

    When replacing the subpanel is it permissible to install a ground rod under the subpanel, and bond a #6 copper wire to the ground rod and to a nearby 3/4" copper cold water pipe that enters the ground? Access openings to bonding clamps would be provided and the neutral buss would be isolated from the ground buss bar.

    I have not been able to find code to support this and the bulding official says this is not acceptable but cannot give a reason. He says grounding is for lighting, a moot point for this area in So. California. I thought grounding was for safety, a path for current to flow should a short to a device occur, thus preventing the user of a device from being shocked.

    Thank you. Should you want to reach me by phone:
    (949) 433-8891.
    Peter Bunge

  2. #2

    Re: Grounding of residential subpanel

    Peter, You have a dangerous situation here. At the subpanel, the path for a ground fault as you describe it would be to the copper water pipe to the assumed water pipe clamp at the service grounding point and then back to the transformer through the service neutral. This relies on the electrical integrity of the water pipe, which is not under the electrician's control.

    You are under the impression that connecting to earth is for electrical safety (aside from lightning). This is a myth. Just think of how much current will flow from 120V through a grounding electrode of a nominal 25 ohms. 120/25 = 4.8A. Will 4.8 amps trip a breaker? Where is the safety?

    I would suggest you go into the mikeholt website and order a book and/or video on grounding and bonding. It will clear up any misconceptions. (I don't work for Mike Holt!)

    The neutral bus in the subpanel must be insulated and all equipment grounds must have their own bus. There must be a grounding conductor with the feed conductors (it could be a conduit) and it must run with the phase conductors, not in some other more convenient path.

    Now, aside from basic safety, the situation you have now sets up a huge magnetic field loop from the normal use of power to the subpanel. The neutral return current for the subpanel is now splitting and returning on both the neutral conductor in the feed and the copper water pipe. Both paths will have a large magnetic field, depending on how much current. If there have been any malfunctions of computers, etc., you would be solving that situation as you solve the safety Code violation.

    Karl Riley

    I think you will haver some other comments by the end of the day.
    Karl Riley
    Moderator

  3. #3

    Re: Grounding of residential subpanel

    A quick correction of my post. As it is now, a fault current will return on both the water pipe and the feed neutral. The breaker will trip, but the parallel path of the current is not allowed.
    Karl Riley
    Karl Riley
    Moderator

  4. #4
    Join Date
    Feb 2003
    Location
    Plano, TX
    Posts
    3,773

    Re: Grounding of residential subpanel

    Peter, I agree with Karl, you have code violations. I am not real clear about the setup. Is the sub-panel in a detached building? It does not sound like it, but there a couple of options if it is.

    I will assume it is not attached. Sounds like your neutral is multi-grounded at the sub-panel. That would cause load current to flow on the water pipe and violate 250.6 and 250.24(A)(5). From the main panel you will need to run phase, neutral, and an EGC to the sub-panel. You can bond the ground again to an electrode, but not the neutral.

    If it is detached let us know, 250.32 offers a couple of options.

    [ February 24, 2003, 03:12 PM: Message edited by: dereckbc ]

  5. #5
    Join Date
    Feb 2003
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    Mike P. Columbus Ohio
    Posts
    3,568

    Re: Grounding of residential subpanel

    Peter

    As stated above this is dangerous.

    You may NOT drive a ground rod. You are in the same structure.

    I see multiple problems here.

    Stop do not do anything.

    I am sure that the waterlines are "interconnected" between the condos.

    You might have found a can of worms!

    Please give a detailed explaination of what water, gas, electric is present.

    Mike P.
    Inspector Mike
    ESI

  6. #6
    Join Date
    Feb 2003
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    Mike P. Columbus Ohio
    Posts
    3,568

    Re: Grounding of residential subpanel

    I do not work for MH or KR.

    Buy the books that these two "PROFESSIONALS" have. They will help you understand the dangers that others have made by their installations.

    There are many "PROFESSIONALS" here one this site, I do not profess to be one of those. They will give you great advise. Whether by a "PRO" or a want-to-be like me, when someone says danger, please research this further.

    Mike P. IMHO
    Inspector Mike
    ESI

  7. #7
    Join Date
    Feb 2003
    Location
    Portage, Indiana NEC: 2008
    Posts
    9,892

    Re: Grounding of residential subpanel

    fault current will return on both the water pipe and the feed neutral
    From What I'm getting is this subpanel is fed with a 3 conductor H-H-N and the neutral is bonded at the subpanel to the case and the neutral bar. and the egc's are also connected to this bar. now you have a #8 ran to a water pipe from this subpanel as a water ground.It might not meet todays codes but this sounds like that it was code compliant for the "1960"'s yes you will have parallel fault currents on both the water pipes and the feeder neutral because of multible connections in the building. but where in the nec of the 60's is this not allowed? as we have neutral currents on our water pipes that have a commoned connection with a house down the road. I would install a ground rod at the main service bank and size the gec to the service. the problem is that to bring
    this up to todays code would take running new SE 2/2/2/4 cable from the subpanel to the main service. and seperateing the grounds and the neutrals at the subpanel. and running the water ground back to the main service panel and bonding it there. and in no way do you ever depend on just the earth to take a fault to trip a breaker as the fault has to return back the path it came back to the neutral at the main service and earth will not do this. as was said before.
    Wayne A. From: N.W.Indiana
    Be Fair, Be Safe
    Just don't be fairly safe

  8. #8

    Re: Grounding of residential subpanel

    Hurk27: Are you assuming that the prohibition of connecting neutral to ground on the load side of the main disconnhect was allowed in the 60s?
    I doubt it, but I don't know the history of the Code on this point. Perhaps someone does know when this came in.
    Karl Riley
    Moderator

  9. #9
    Join Date
    Feb 2003
    Location
    California
    Posts
    8

    Re: Grounding of residential subpanel

    Interesting how these condos, which are connected and share water pipes have been around for 40+ years. Of course that doesn't mean there isn't a risk involved in the existing state, but I wonder if alarm is warranted. Do the existing panels need to be changed for safety concerns?

    I want to emphisize that the new panel neutral buss bar would be insulated from the panel thus there would not be a parallel flow of current back to the service unless it was a fault current. I do realize my mistake of assuming nill resistance posed by the ground rod/water pipe. I see the point of having the ground bond back to the neutral at the service in order to return current to the transformer and minimize resistance to fault current. I do not see why the path of the ground between the panel and service would make any difference, remembering that the only current flowing in the ground would be fault current. Thanks for the suggestion of the book on bonding and grounding, I'll order it.

  10. #10
    Join Date
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    Plano, TX
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    Re: Grounding of residential subpanel

    Paul, if your work is going to be inspected, it is going to be tagged if you do not run an EGC with the feeder 215.6. To keep the impedance at a minimum, it is necessary to run the EGC within the same raceway or cable as the phase and neutral conductors. This allows the magnetic field developed by the circuit conductors to cancel, reducing thier impedance. The water pipe is not part of the path and may not have the low impedance neseccary to operate the breaakers.

    Good Luck

    Dereck

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