you have a transformer inside the blding and you create a separately derived system(480-208 step down ) you run the ground wire from the primary and secondary to the chassi and the bonding elecrode from the xo to the chassi and on to blding steel. do you bond the neutral to the can at the first means of disconnect. if so, doesnt this create 2 paths to ground?

The code allows a few options here.

Bond at the Transformer
or
Bond at the first panel/disconnect


Part of 250.30(A)(1)

or

Bond at both places if this will not create a parallel path.

Part of Exception 1 to 250.30(A)(1)

From your description you will have a parallel path (the grounding conductor and maybe the raceways) if you bond at both.

I bond at the transformer and never again, it is a policy of the company I work for.

Bob

[This message has been edited by iwire (edited 03-19-2004).]

even if i use a plastic raceway dont i still have a parallel path ?

Exactly, that's why I bond only at the Tformer, and have an insulated neutral bus at the 208 volt panel.

I can not think of any normal installation that the exception would apply to.

Like I said I do my bonding in the transformer only, I was just trying to give you all the allowable options.

Rizer, only if the first means of disconnect is mounted on a conductive material common to the Transformer or Grounding Electrode.

Roger

Roger Even with plastic raceways I still see the EGC and GEC as parallel paths to the grounded conductor.

How would you avoid this?

Bob

i agree that you are creating a parallel path to ground. this was explained to me many years ago by an inspector who stated you can bond it at the xfmr or the first means of disc but not both. none the less my boss insists he has " done it that way all (his) life". he then tried to find it in one of his electrical books and after 5 minutes said " it won't make any difference so do it my way".

Bob and Rizer, maybe I'm not understanding the discription of what's being done here.

What I'm talking about would be the diagram on the left below.



even bonded at both locations there would only be one path for the load to return to the source.

Roger

maybe ive been told incorrectly but i was told that the ground wire from the service is connected to the chassie not the xo. and that the bonding electode should be grounded to the chassie on its way to the xo. i would think a ground fault could travel back to your bonding electode via the neutral or the ground wire thereby having 2 paths to ground.

roger, the diagram to the left looks fine. but, if you were to add a bond. jumper to the neutral bar in the right hand diagram. this would be a clearer picture of what im being told to do.

Roger while I agree that with a plastic raceway and eliminating the EGC between the transformer and the first OCPD there will be no direct parallel path between the transformer and the panel.

But I still think and I may be wrong that you would have a path for for neutral current through the grounding electrode system.

If we bond at the transformer and the panel we can eliminate the EGC between them.

We still will have to bring a EGC in with the primary's to provide a fault path back to that system.

Considering that EGC will be connected to the buildings grounding electrode system, along with the transformers GEC I see a potential for current flow on these conductors.

Bob, the grounded conductor is all that is needed for the fault clearing path from the "Main" back to the "Source"

This would be the same for a "Service", just put a meter between the Transformer and the Service Disconnect and install the GEC at the meter or the panel.



Roger

Still having a problem with this.

I will stick with what I am familiar with.

480 delta primary 208 Wye secondary.

There is no grounded conductor on the primary side, we will need the EGC brought to this transformer on the primary side so we can open the 480 breaker, not relying on the building steel connection to do so.

This would be the same as having to bring the grounded conductor to a service disconnect even if we do not intend to use it for loads in the building.

Bob, looking at a residential service for simplicity.

In essence what you are saying is that you feel it necessary to install a Grounded Condutor and an Equipment Grounding Conductor from a pad mounted residential transformer and the Main Disconnect?


Maybe I'm misunderstanding what's being said.

Roger

We could just be misunderstanding each other.

Sticking with the 480 delta primary 208/120 Wye secondary transformer.

If the primary windings fault to the case what is the fault clearing path back to the 480 supply?

A house service includes a grounded conductor, the delta 480 supply has no grounded conductor.

If you do not provide a primary side EGC the only path back to the 480 supply neutral will be through the transformer GEC and most likely building steel back to the service main bonding jumper.

It also could be that I am misunderstanding the function of the bonding jumper at the transformer.

I have to go to bed now, I will talk to you later.

Bob

Bob, me too, good night Bob.

Roger

roger and bob, thank you for the insight into this problem.

just one more question roger, what would the diagram look like if the raceway were changed to steel? would you run a ground wire and use bonding bushings or a bonding bushing with a jumper on one end of the pipe

[This message has been edited by rizer (edited 03-21-2004).]

[This message has been edited by rizer (edited 03-21-2004).]

Roger,

In your diagram of March 19 7:03 PM on the left, I believe that you create exactly the parallel path that Rizer is asking about.

Not shown in the diagram is the ground connection between the disconnect enclosure and building steel or the grounding electrode system. The disconnect enclosure is not simply bonded to neutral and then isolated from all other ground contact, and there will certainly be electrical conductivity between the disconnect enclosure and building steel. This means that the neutral-panel bond in the disconnect is either explicitly or implicitly a bond between the neutral and building steel. In the transformer you also bond X0 to building steel. Net result: normal neutral current flow can follow two paths, either along the neutral wire, or through the multiple ground bonds and building steel or grounding electrode system.

If the PVC conduit were replaced with steel conduit, this would be an explicit and clear parallel path, with neutral to enclosure bonds at both ends of a metallic pathway. IMHO this would actually be safer than the PVC conduit case, because you would now have a situation much like the parallel path found when a residence meter pan is bonded to neutral at the same time that the service disconnect enclosure also has a neutral bond. In this case, there is an explicit parallel path, but it can be very carefully bonded. A low resistance parallel path is often not a problem (unless you get inductive heating), but an overall low resistance parallel path with a high resistance (per unit length) joint _is_ a problem. In homes with neutral problems, the neutral current flowing on the water pipes is not much of a problem until a plumber has to cut the pipe

I think that the policy expressed here (bond X0 only, treat all panels as sub-panels) is the most sensible way to go.

-Jon

Hi Jon,

As far as metallic conduits between meter sockets or CT cabinets and service panels, I will never do that any more.

I have learned much from hanging around these forums and I have come to the conclusion that for my own piece of mind I will not deliberately make a path for the unbalanced neutral current other than the Neutral conductor itself. OK the water serice may be another path, but I do not get a choice on that one

I know that there are literally tens of thousands of installations like you describe that are working fine and will continue to do so.

That said I find no compelling reason to continue to make a parallel neutral path, I know I have seen the results of this current eating away lock nuts and enclosures when the connection is not all it should be and I am sure most electricians have seen that too.

I have been and will continue to use PVC or SE between bonded metering enclosures and bonded service enclosures.

Just my opinion and certainly not a code requirement at this time.

Bob

PS Roger is a pretty sharp guy, he is aware of SDS procedures.

[This message has been edited by iwire (edited 03-21-2004).]

Morning all, thanks for the vote of confidence Bob.

First of all I think I see some of the misunderstanding.

If we go back to my first post where I said (I actually drifted from Rizers direct question here) my intent here and in my other posts have been that, baring this, (and of course other outside paths) there would be no common conductive path be it intentional or not, think wooden structure, and Bob's reference to Exception 1 to 250.30(A)(1)


With that said, I wholeheartedly agree that if we do identify a parallel path in any form, the wiring method should be modified to eliminate it.

Rizer, in your last question you could treat this the same as the right hand diagram and eliminate the green wire conductor shown. The bonding bushing would only be necessary on one end, with the other bonding jumpers as shown.

Rizer, I also apologize for the added confusion to your question.

Roger

[This message has been edited by Roger (edited 03-21-2004).]

Rizer, I just noticed you are in Btown, I was born there, and still get down there once or twice a year.

Who are you working for?

I worked for quite a few Bradenton and Sarasota companies in the 70's and 80's, to name a few, Parrish, Pearson, Bridgeman, Dixie lighting, Otis, and Lantern.

I don't know how many of these companies are still in business and do know some are not.

Roger

Roger
When it comes to the theory and technical ends of this business, you have a wonderful knowledge level - hence your posts on this topic are confusing me as to how I understand the theory and code rules for a transformer installation within a building (SDS).

As exception 1 to 250.30(A)(1) would seem very restrictive (almost an arbritary inclusion to this section), I have had the understanding that treating a SDS in a building is much more restrictive than treating a service to the same building. As in not permitting the bonding in both the transformer and the first point of disconnect. It would seem to me that the bonding and grounding performed at the transformer would be the only place permitted (in actuality) and separating the equipment ground and the neutral at the first point of disconnect would be mandatory or the bonding and grounding at the disconnect, but not at the transformer - never at both for this installation. ... Am I incorrect in this thought process?

Pierre

Pierre, thank you for the complimentary words.

I like the way you put this.

You are absolutly correct and I agree. This will be the last time I bring this up.

Even though the exception allows this, it would be a rare situation where it could be used, and by me bringing it into the "allowed by exception" limelight
I am confussing others and potentially creating some possible violations and or dangerous situations.

Now, let me say that I agree with all of you, and due to the rare situations where bonding at both locations could be done per 250.30(A)(1) exception 1, I think it would be advantagious to remove this exception from the NEC. (If for no other reason but to keep me from confussing people. )

I appologize to all and especially Rizer.


Roger





[This message has been edited by Roger (edited 03-22-2004).]

roger, i work out of the lu 915. i have worked almost exclusively for dale c. rossman electric at the bradenton tropicana facility. i apologize for not reading your responses sooner but i have been working on a "shut down" and as the forman on the job and have had to see it through with all the inherent headaches (like 3 revisions in threedays from the engineer). I cut my teeth in the non-union sector in 5 different states across america. when i moved to fl. it became apparent that i would have to join the union to get even close to the pay i was used to. by the way its still beautiful during the winter and hellish during the summer. just 2 more weeks till the snowbirds leave and traffic returns to normal.
during the five years i have worked on projects at the tropicana plant i have had the opportunity to learn a great deal about industrial electric and would never(if i can help it) go back to the commercial end of the trade. the constant repetition and wear and tear of commercial work has taken its toll on me.
i would like to thank everyone who has responded to my questions.
thanks again!

[This message has been edited by rizer (edited 03-27-2004).]