Grounding of residential subpanel

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pbunge

Member
Location
California
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
 

karl riley

Senior Member
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

Senior Member
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
 

dereckbc

Moderator
Staff member
Location
Plano, TX
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 ]
 

jxofaltrds

Senior Member
Location
Mike P. Columbus Ohio
Occupation
ESI
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.
 

jxofaltrds

Senior Member
Location
Mike P. Columbus Ohio
Occupation
ESI
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
 

hurk27

Senior Member
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.
 

karl riley

Senior Member
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.
 

pbunge

Member
Location
California
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.
 

dereckbc

Moderator
Staff member
Location
Plano, TX
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
 

pbunge

Member
Location
California
Re: Grounding of residential subpanel

Thanks, Derick
I was not aware of the effect of the magnetic field to reduce impedance. Is the affect of a magnetic field necessary to reduce the impedence for say 20+ amps fault current flowing through a #4 wire to to trip the breaker? I guess the concern is how much voltage is necessary to drive the current in order to trip the breaker. The higher the voltage the less safe the situation. A person grounding a faulty device would be subjected to whatever voltage is needed to drive the 20+ amps of fault current through the bonding wire to ground. Still seems that a #4 wire about 100' long would have very low impedance for 20 amps. Guess I need to do some math.
Peter
 

dereckbc

Moderator
Staff member
Location
Plano, TX
Re: Grounding of residential subpanel

Peter; sorry for getting your name wrong, please accept my apology. 20-amps of fault current is not going to trip a 20-amp breaker in any reasonable amount of time if ever at all. It would appear as load.

A person making the fault would not likely exhibit a low enough impedance to trip a 20-amp breaker. That is why we have GFCI breakers in certain locations where it is likely. By design it takes at least 6 times the rated device current to make the device operate instantaneously, for a 20-amp breaker that would be 120 amps. In order to drive 120 amps by ohm?s law the impedance at 120 volts/120 amps is 1 ohm. The 1-ohm impedance is not the impedance of the EGC, but the impedance of the whole circuit, from the main panel where the N-G bond is located, through the breaker(s), phase conductors, contact resistances, and the EGC in series. So the EGC impedance would have to be less than .5 ohms.

Not sure where the # 4 AWG comes from, I assume you are talking about the feeder EGC with a 70-amp breaker. That is a different story. The # 4 AWG would protect the feeder circuit, and the down stream branch circuits. For a 70-amp feeder you are required to have a # 8 minimum EGC, so a 4 AWG is permitted. 20-amp breakers require a minimum 12 AWG EGC.

I am not trying to say what you have would not work, but it does not meet code, and could be dangerous. If there were property damage or someone was hurt, guess who is liable.
 

pbunge

Member
Location
California
Re: Grounding of residential subpanel

Dereck, please accept my apology for mispelling your name! I appreciate your time to respond. What I'm needing is an answer to meet code and safety requirements preferably without replacing the existing H-H-N romex from the service with a H-H-N-G. Instead, adding a seperate #8 or larger ground wire in conduit to bond the subpanel with the service neutral and ground buss.

I am concerned about the issue Karl raised when he stated that the ground must run with the conductors because of the effect of the magnetic field. Would running a larger ground wire compensate for this effect of being routed independently?
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Re: Grounding of residential subpanel

Peter,
There is no exception to 300.3(B) for this application. The required equipment grounding conductor must be in the same raceway or cable as the feeder circuit.
Don
 

dereckbc

Moderator
Staff member
Location
Plano, TX
Re: Grounding of residential subpanel

Peter, I have been looking all morning trying to find an exception. The only thing I can find is for "nongrounding receptacle replacement" in 250.130(C). It does not apply to a feeder. 300.3(B) and 215.6 have got you between a rock and hard place. I concur with Don.

Good Luck
 

pbunge

Member
Location
California
Re: Grounding of residential subpanel

I guess I should say thank you, Dereck and Don, again I appreciate your digging, yet it's like being between a hunk of concrete and the building (rock & hard place). How to do this without tearing up the building or concrete, not to mention dealing with the association. I really would like to know WHY the ground must be alongside the feeder cables. Is it primarily for organizing to avoid confusion about what wire is with what feed, or as Karl stated, has to do with the interacting magnetic fields?
Peter
 

dereckbc

Moderator
Staff member
Location
Plano, TX
Re: Grounding of residential subpanel

It is to cancel out the magnetic fields, lower impedance, and reduce heating effects.
 

rb

Member
Location
Tennessee
Re: Grounding of residential subpanel

Pbunge,

I can give you a very elementary reason (its the only one I know). When a conductor returning the fault current is located in close proximity to a conductor sourcing the fault current, the magnetic flux created in the two conductors cancel each other out. When these conductors are not located close to each other the magnetic flux created during the fault causes inductive reactance in the circuit and increases the impedence of the fault return path. Once this occurs the fault current may seek alternative paths back to the source. Paths that are not designed to conduct electricity.

Im sure there are many fellows on this site that can explain it a lot better than I can.

Ron Bethea
 

pbunge

Member
Location
California
Re: Grounding of residential subpanel

Thanks Ron,
I appreciate the reasoning as it helps me to understand and accept the code better. I feel much more inspired to follow laws if I can understand the reasoning behind them. I did order Mike's book so I hope to be able to be less dependent on you guys, thanks for your input.

By the dialogue above it seems this is a very hazardous situation. Does this mean that the City should require all the condo's to have their grounding upgraded? I doubt that would go over well with the homeowners unless they could be convinced of impending danger; remember these have been around for 40+ years without burning down or killing people.
Peter
 
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