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Re: Xfmr XO bond location preference [Re: sparkyinak] #177318 04/29/08 05:54 AM
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Roger Offline
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Assuming metalic raceways, bonding at both locations is not just a misunderstanding of the NEC, it is a violation of 250.30(A)(1)

Quote
(1) System Bonding Jumper An unspliced system bonding jumper in compliance with 250.28(A) through (D) that is sized based on the derived phase conductors shall be used to connect the equipment grounding conductors of the separately derived system to the grounded conductor. This connection shall be made at any single point on the separately derived system from the source to the first system disconnecting means or overcurrent device, or it shall be made at the source of a separately derived system that has no disconnecting means or overcurrent devices.


Exception No 2 to this article section would allow bonding at both locations if there were no parallel paths created in doing so but, this would mean a wooden structure with NM cable or NM raceways.

Roger

Last edited by Roger; 04/29/08 05:55 AM.
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Re: Xfmr XO bond location preference [Re: sparkyinak] #177319 04/29/08 05:54 AM
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Texas_Ranger Offline
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*headscratch* Main distribution is not exactly my field of expertise, but what about the public grid? Isn't the neutral of a regular supply transformer bonded to ground at the transformer and in any customer's panel too? Please help me if I'm way off!

Re: Xfmr XO bond location preference [Re: Redsy] #177320 04/29/08 06:46 AM
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SteveFehr Offline
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Originally Posted by Redsy
sparkyinak,

The particulat inspector on this job implied that it should be bonded at both places, which I believe to be a misunderstanding of the NEC.

I was thinking about bonding in the panel because he would be more likely to look there, as opposed in the transformer housing. (I know this approach may seem questionable, but I was just putting out feelers)

Yes, that is absolutely a violation of NEC (as well as safety in general), as it would provide the safety ground as an electrical return path for neutral current between the bonding points.

Roger- 250.30 only applies to seperately derived systems, like if you used a 4-pole switch to switch the neutral at a generator ATS which was bonded both at the generator and the service entry. If he bonded at both the transformer and panel but replaced the neutral wire between them with a ceramic insulator, the neutral current would simply flow between the ground points, and the problem would still exist.

Texas_Ranger - Yes, yes it is. The service neutral is bonded to ground at the transformer, at your house, and all your neighbors houses. Why this method persists to this day, I cannot fathom- the poco can do it, but it'd be a big fat violation if anyone tried to do it in a structure.

Re: Xfmr XO bond location preference [Re: Redsy] #177327 04/29/08 11:47 AM
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sparky25yr Offline
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In northwest Ohio
The XO is not bonded to the transformer.
As per the state inspectors.
The secondary panel is considered a new service and the green ground screw is installed. A ground rod is installed at the panel. all ground and neutrals attach to the same neutral bar.
The transformer is grounded from its primary panel with a secondary ground rod installed.
I have had only one inspector that wanted a ground wire added from the secondary panel back to the transformer to tie all the groundes together.

Re: Xfmr XO bond location preference [Re: SteveFehr] #177333 04/29/08 01:26 PM
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Roger Offline
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Originally Posted by SteveFehr



Roger- 250.30 only applies to seperately derived systems, like if you used a 4-pole switch to switch the neutral at a generator ATS which was bonded both at the generator and the service entry. If he bonded at both the transformer and panel but replaced the neutral wire between them with a ceramic insulator, the neutral current would simply flow between the ground points, and the problem would still exist.



Steven, I'd wager that Redsy is talking about an SDS and a switched neutral has nothing to do with it. If you bond at both locations and there is a parallel path as in metallic raceways you will have net currents flowing in the common GES resulting in EMF issues.


Roger


Re: Xfmr XO bond location preference [Re: Redsy] #177340 04/29/08 04:58 PM
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Redsy Offline OP
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Thanks guys.

I understand the NEC rules on transformer bonding, parallel paths, etc.

I am being polite when I said "misunderstanding". I know this misunderstanding results in a violation, but this is an inspector that I like and respect.

Maybe I misunderstood him, but he reminded me to treat this similsr to a "service" (as in Ohio?) and bond the neutral at the secondary panel. This was right after looking at the transformer connections and (I thought) noticing ny XO bond to the housing.

Re: Xfmr XO bond location preference [Re: Redsy] #177344 04/29/08 06:39 PM
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HotLine1 Offline
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Redsy:
A little late....but I bond at the XO terminal within the xfr. That's the 'way' for >35 years. Up here, that's the 'way' it is, & has been.

Now as an AHJ, I check terminations within the xfr, the primary source, the secondary panel(s), wireways, etc., & bond terminations.

Take care


John
Re: Xfmr XO bond location preference [Re: Redsy] #177348 04/29/08 08:27 PM
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Redsy Offline OP
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Thanks John.
Thanks everyone for your insight.

Re: Xfmr XO bond location preference [Re: Redsy] #177355 04/30/08 04:29 AM
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Texas_Ranger Offline
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I think the main difference between the PoCo and a structure is: the PoCo grounds the neutral in multiple locations but does not run a parallel ground wire. So, applied to the scenario described above there would be one ground rod at the transformer, one at the panel (ridiculous if the 2 are only 6 feet apart, but as far as PoCo distribution is concerned the distances are larger of course) and one at the panel but only the neutral wire connecting to both of them. A neutral-ground connection after the point where a separate ground wire is derived can lead to all sorts of trouble, especially downstream of a GFI or RCD.

Re: Xfmr XO bond location preference [Re: Texas_Ranger] #177356 04/30/08 06:56 AM
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SteveFehr Offline
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Originally Posted by Texas_Ranger
I think the main difference between the PoCo and a structure is: the PoCo grounds the neutral in multiple locations but does not run a parallel ground wire. So, applied to the scenario described above there would be one ground rod at the transformer, one at the panel (ridiculous if the 2 are only 6 feet apart, but as far as PoCo distribution is concerned the distances are larger of course) and one at the panel but only the neutral wire connecting to both of them. A neutral-ground connection after the point where a separate ground wire is derived can lead to all sorts of trouble, especially downstream of a GFI or RCD.
They poco doesn't need to run a ground wire, the earth itself is an excellent conductor over long distances. There is current flowing through the rod at each pole all the time, and through the earth back. It's no different than if we hooked up a ground wire in parallel with the neutral, and bonded them in every 2x4 box in the building. Only, for some reason it's a fire hazard and safety hazard for everyone else in the world, but OK for the poco?

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