Grounding 2 buildings from 1service

[justin7]

Does it seem dangerous to anyone to run an equip.gr.cond. and not bond to the neutral at the structure ? No one would know if the E.G.C.was lost. Would the breaker trip?

 

[bphgravity]

Re: Grounding 2 buildings from 1service

Well, the code gives options. Meet the requirements of 250.32(B)(2) and you will eliminate this issue.

But to answer your question, I guess this is were proper wiring methods and good workmanship comes in. If you size you EGC properly, make the proper terminations, and follow all requirments as far as buiral depths and proper protection for the conductors, there should never really be a problem.

Here's a good time to put in an excellent statement that can be found in NFPA 70E, "It can be debated that all of the requirements of the NEC, when traced through a chain of events, may relate to an electrical hazard."

 

[roger]

Re: Grounding 2 buildings from 1service

Well, by bonding them at both locations it is putting two conductors (the Grounded and the Equipment Grounding Conductors) in parallel and most likely violating 250.6 and 310.4.

Roger

[ May 15, 2004, 10:02 AM: Message edited by: roger ]

 

[don_resqcapt19]

Re: Grounding 2 buildings from 1service

This is a part of the code where the CMP seems to think that the electrons know if they are on the line or load side of the service disconnect. Multiple bonding of the grounded conductor is required where a common transformer serves multiple buildings that are also served by a common metal underground water piping system. How is this really any different then a single service feeding multiple buildings? Why is multiple bonding a bad thing on the load side of the service and a good thing on the line side of the service?
Don

 

[bennie]

Re: Grounding 2 buildings from 1service

Don: I like your style

[ May 15, 2004, 11:40 AM: Message edited by: bennie ]

 

[renosteinke]

Re: Grounding 2 buildings from 1service

This was the subject of an article in the Nov/Dec 2003 issue of IAEI News. See it at IAEI.org.

Code language is somewhat confusing on this point. Probably the best model is to think of the second panel (building) as a sub-panel, then add a ground rod.
Remember, the purpose of the ground rod is for lightning protection- NOT fault clearance. At the 25 ohms resistance allowed by the code, the GEC won't trip a breaker anyway.

Also, don't forget to (I repeat myself here) run a ground wire between the panels, and have the ground bond with the neutral ONLY at the main panel.

 

[justin7]

Re: Grounding 2 buildings from 1service

My point is that not bonding at the remote structure is dangerous. Especially with directly buried cables. That E.G.C. is about the only thing that makes that circuit breaker work if you dont bond at the remote structure.

 

[justin7]

Re: Grounding 2 buildings from 1service

It seems like you would know something was wrong if you somehow lost your feeder nuetral. If you lost your E.G.C. you would not know there was a problem.

 

[bphgravity]

Re: Grounding 2 buildings from 1service

Trust me when I say that we all understand your point. The fact remains that there many circumstances that could indeed create a hazard. In most cases, there are other options or opportunities to ensure the problem does not arise.

Check out Article 250.58. Suppose you lose the grounded conductor one of the two services. By having the GEC's bonded together, normal operating current will use the grounding electrode system and whatever conductive materials on the way there to get back to the ssource. You would never know it was going on.

It just points out the reality of the final statement in my first post above.

 

[hassaf]

Re: Grounding 2 buildings from 1service

Having two separate bars (for neutral and Equipment grounding), the current flow is controlled, and there is only one path for the neutral current to flow back to source on.

But, when the neutral conductor is bonded to ground in panels other than the service equipment, many paths are created, and the flow of neutral current (or electrons) will take each path offered back to the source, since The current (or the unbalnced neutral current) needs to go back to the source to complete the circuit, or no electricity will flow.

By bonding the neutral to ground, and under normal circumstances (no fualt), a path for a circulating ground currents (the return current of the neutral) are established on the metal enclosure at the sub-panel of the structure, and two parrallel paths for the neutral current flow to source are established; one through the neutral conductor, and the other through earth ground. Also, if you have a metal water pipe between the two buildings, then the neutral current will also flow back to source through yet another path.

 

[don_resqcapt19]

Re: Grounding 2 buildings from 1service

How do the electrons know the difference between the multiple bonds and paths that are code required on the line side of the service disconnect, but are code prohibited on the load side???? If multiple bonds and paths are a hazard, they are a hazard on both sides of the service disconnect.
Don

 

[justin7]

Re: Grounding 2 buildings from 1service

For my situation I like 250.32(B)(2) (only three wires) better than (B)(1). Only because I think its safer.

 

[hassaf]

Re: Grounding 2 buildings from 1service

Don, can you please rephrase your question regarding the electrons current flow? I don't quite understand your question.

But to explain further what i mean, the electrons will take every possibly path that they are offered, if you bond the neutral at the sub-panel, then the electrons will flow back on the neutral from the load, say a receptacle load, and when they reach the bond at the sub-panel, they will split into two or three different path in an attempt to go back to the source, one path would be through the electrode grounding conductor for the building, one through the nuetral and one could be through the metal water pipe (which you should have bonded already in the building).

So, when that happen, the water pipe for example, would have a potential or a voltage (becuase of the current going through it, small but enough to cause harm to a person touching it.

 

[don_resqcapt19]

Re: Grounding 2 buildings from 1service

hassaf,
All I am saying is the same type multiple bonding that is prohibited by the code and considered to be a safety hazard on the load side of the service disconnect is required on the line side of the service disconnect. How can it be safe and required on the line side and be unsafe and prohibited on the load side?
Don

 

[hassaf]

Re: Grounding 2 buildings from 1service

Thanks for the clarification Don.

I'm assuming the line side would be the main service equimpents downstream of utility transformer, right?

I'll try to clarify with a drawing of a circuit showing the current flow, but i'm not sure how to insert within my response. bare with me.

 

[don_resqcapt19]

Re: Grounding 2 buildings from 1service

hassaf,
I'm well aware of the parallel paths for current flow that result from multiple points of bonding. These same paths also exist on the line side of the service disconnect. In many cases the metal underground water piping system will carry 20% or more of the service grounded conductor current. Even the shield of the TV cable system is required by the NEC to be connected in parallel with the service grounded conductor.

So, when that happen, the water pipe for example, would have a potential or a voltage (becuase of the current going through it, small but enough to cause harm to a person touching it.

This same thing happens on the line side of the service. Why is it only a hazard on the load side and not on the line side. Do the electrons change their behavior when they pass through the servive disconnect? By the way, in my opinion, if the grounded conductor and its connections are in tact, that there would be no shock hazard as a result of the current flow in the water pipe. It will, however, create an electromagnetic field that many consider to be a health hazard.

Don

 

[brentp]

Re: Grounding 2 buildings from 1service

I would like to see a diagram of the parallel neut current on the line side of the service disconnect(s). (That's if someone has a diagram 'handy')

Brent

 

[roger]

Re: Grounding 2 buildings from 1service

Brent, Ed will be able to provide some excellent graphics when (if) he sees this.

Roger

 

[iwire]

Re: Grounding 2 buildings from 1service

Brent, Roger is correct but this is also easy to picture in your mind.

Think of a neighborhood of houses with all metal water piping, often one power company transformer will feed many houses.

At every house the neutral is bonded to the metal water piping, this puts all the metal water piping in parallel with the neutral.

This is only one example, there are others.

 

[brentp]

Re: Grounding 2 buildings from 1service

Roger & iwire(Bob),

Thanks.

I don't mean to be lazy in my research, but a diagram from the experts on this site would help me with my jobsite discussions.

In other words, a little backup would be helpful.

Brent

[ May 17, 2004, 07:31 PM: Message edited by: brentp ]