ECN Forum
Posted By: Redsy Xfmr XO bond location preference - 04/26/08 08:31 PM
I would prefer to bond the XO terminal in the transformer (120/208)housing. Doing so, I would need to float the neutral in the panelboard, but I know that semi-qualified staff will use this panel and they always seem to assume that neutral and ground busses are the equivalent.

Is there a disadvantage to bond the neutral in the panel instead?
The secondary raceway is about 3' of 2" greenfield.

Please discuss any pros & cons (if any).

Thanks

John (Redsy)
Posted By: WireNuts29 Re: Xfmr XO bond location preference - 04/27/08 01:19 AM
John,
It would seem to me, if somehow, these semi- qualified individuals were to somehow add a second panel. Either add double lugs at the transformer or a feed through from your setup, they would be relying completely on that bond in the original panel to create the neutral. It is in my opinion the most practical, and safest method to bond your XO at the transformer. Suppose you do you bond at the first panel, and one of these qualified people remove the bonding jumper, because it should be bonded at the transformer. you may be opening up a can of worms.
Posted By: Redsy Re: Xfmr XO bond location preference - 04/27/08 12:36 PM
Thanks Nuts!
I know the facility and their people. They would not add a subpanel, but they will add branch circuits. And they will indiscriminately tie ground and neutral to either buss. I see it everywhere.

I know that the bonding can be done at either location as far as NEC goes, but other than my concern about the staff, I'm wondering what others prefer, and why.

Thanks,

John

Posted By: electure Re: Xfmr XO bond location preference - 04/27/08 01:50 PM
I much prefer bonding XO in the transformer.

Could you possibly label the busses in respect to their usages? That might get the ball rolling for their education, too.
Posted By: Redsy Re: Xfmr XO bond location preference - 04/27/08 07:18 PM
Thanks guys.

Is there any(technical,or functional)reason you prefer bonding in the Xfmr?

Other than the cost to run additional ground wire to the panel,(which in this case is only about 6' of wire) is there any reason to choose bonding in the panel?

Just trying to get different opinions.
Posted By: SteveFehr Re: Xfmr XO bond location preference - 04/28/08 11:21 AM
It's generally best to ground the neutral as close to the source as possible, as this provides the most efficient path to ground. This will also keep voltage drop between the grounding point and X0 terminal as low as possible, which will keep the 3 phases better centered around ground. In practice, it's only going to make a few volts worth of difference, especially if you're only talking about 6', but it's still more efficient to bond at the transformer.

I would recommend bonding at the transfomer.
Posted By: sparkyinak Re: Xfmr XO bond location preference - 04/28/08 02:00 PM
Just of curiousity, why woundn't you bond at the tranny?
Posted By: Redsy Re: Xfmr XO bond location preference - 04/29/08 01:50 AM
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)

Posted By: twh Re: Xfmr XO bond location preference - 04/29/08 02:13 AM
Bonding in two places is evil. It puts the neutral wire in parallel with the ground wire.
Posted By: sparkyinak Re: Xfmr XO bond location preference - 04/29/08 02:31 AM
A double bond is double trouble. I'm carious the inspector woud implied that.
Posted By: Roger Re: Xfmr XO bond location preference - 04/29/08 09:54 AM
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
Posted By: Texas_Ranger Re: Xfmr XO bond location preference - 04/29/08 09:54 AM
*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!
Posted By: SteveFehr Re: Xfmr XO bond location preference - 04/29/08 10:46 AM
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.
Posted By: sparky25yr Re: Xfmr XO bond location preference - 04/29/08 03:47 PM
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.
Posted By: Roger Re: Xfmr XO bond location preference - 04/29/08 05:26 PM
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

Posted By: Redsy Re: Xfmr XO bond location preference - 04/29/08 08:58 PM
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.
Posted By: HotLine1 Re: Xfmr XO bond location preference - 04/29/08 10:39 PM
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
Posted By: Redsy Re: Xfmr XO bond location preference - 04/30/08 12:27 AM
Thanks John.
Thanks everyone for your insight.
Posted By: Texas_Ranger Re: Xfmr XO bond location preference - 04/30/08 08:29 AM
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.
Posted By: SteveFehr Re: Xfmr XO bond location preference - 04/30/08 10:56 AM
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?
Posted By: gfretwell Re: Xfmr XO bond location preference - 04/30/08 01:45 PM
Steve, I am not sure the idea of the PoCo using the earth as a conductor is really that good an idea. I bet that is a major source of "stray voltage". Any place that uses wye distribution will see some voltage imposed on the dirt ... plus the parallel path from the building ground to the pole ground. I actually had a guy who should know better tell me that was why "all" PoCos used delta distribution. Evidently he doesn't get around much.
Posted By: homer Re: Xfmr XO bond location preference - 05/02/08 12:14 PM
It all goes back to who has the power over whom. The utilities have long operated in their own world with rules that only they develop. The situation is very similar to elevators, mines, etc. If they spend the appropriate amount of lobby $ every year, especially every election cycle, nobody can touch them. Can you imagine the cost to install a fourth wire to carry the unbalanced load back to the source?
Posted By: Tesla Re: Xfmr XO bond location preference - 05/05/08 12:13 AM
Best practice is to bond everything at the XO tab with quad bbl mechanical lug/ double bbl lugs plus some singles.

It's easier to inspect and understand.

It's easier to teach.
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