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#181297 - 09/30/08 08:29 PM Can I use a transformer ground as a bond?  
bigpapa  Offline
Member
Joined: Mar 2006
Posts: 81
canada
I am doing a store in a mall that has a 600V 3PH 4W feed to it from the main electrical room. I will be installing a transformer in the store and it will need a ground back to the main ER. The ground will have to go in the conduit that serves the 600V disconnect in the store. This conduit already has a bonding conductor in it.

Here is the question. Do I leave the bond in place and pull my ground wire in as well, or do I pull it out and replace it with the ground conductor and use that to bond the 600V disconnect in the store?

The 2nd option makes more sense to me but I wonder which is the best way and why. The first option would leave two grounded condutors in the conduit not counting the neutral which somehow seems wrong.
If this installation were in armoured cable then I would have a ground and a bond basically in parallel.

Which is best?



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#181310 - 10/02/08 10:03 PM Re: Can I use a transformer ground as a bond? [Re: bigpapa]  
jay8  Offline
Member
Joined: Jul 2005
Posts: 183
Vancouver, BC
Just curious about why there is a bonding conductor in the conduit, is it EMT?

My thought is that you would be able to use the ground as a bond conductor, that would be based on commmon grounding conductor rule 10-502.


#181311 - 10/02/08 11:26 PM Re: Can I use a transformer ground as a bond? [Re: jay8]  
twh  Offline
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Joined: Mar 2004
Posts: 915
Regina, Sask.
I agree with Jay8. I'm against two ground/bond conductors in the same conduit, and I'm not aware of a rule against grounding with the bond conductor.


#181342 - 10/04/08 02:47 PM Re: Can I use a transformer ground as a bond? [Re: twh]  
bigpapa  Offline
Member
Joined: Mar 2006
Posts: 81
canada
OK,
What about this. You run a 4 conductor teck cable and use one conductor as the transformer ground. What do you do with the bonding conductor in the cable.

This would be something you would run into when you upgrade a 208V sub panel to a 600V feed with a step down transformer and use the existing cable.

I would guess you would ground one one of the bond conductor and isolate the other?


#181384 - 10/06/08 12:58 PM Re: Can I use a transformer ground as a bond? [Re: bigpapa]  
mikesh  Offline
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Joined: Jun 2006
Posts: 613
Victoria, BC, Canada
I have been stewing over an answer to this
First see rule 10-206 (2) and the only transformer you can ground using the bonding conductor is 1000va or less.
Not a very big store at 8.3 amps.
Second the bonding conductor probably has a lot of splices.
A ground conductor must be continuous (use thermit welding or listed Hypress connectors.
Is the store fed via EMT, ACWU, Short section of PVC and EMT? Maybe the bonding conductor in that pipe is a ground and not a bonding conductor.
We have a mall where the distribution is 600 volts and every store has a transformer for the 120/208 volt requirements. The ground runs as a ring and is located in the dropped ceilings each transformer just taps off the 3/0 ground ring.
In this case a bonding method, either the wire in the pipe or the pipe itself makes the bonding done and add a ground that may take just about any route to the ground electrode as long as it is direct (IE don't go east to get to the west)
The transformer secondary is a new service and the grounding must be done as for a new service. Direct to the New neutral and terminated at the transformer or service entry Switch or combo panel.
The only time a bonding conductor may be used to ground anything is as I said in opening.

Last edited by mikesh; 10/06/08 01:00 PM. Reason: spelling

#181392 - 10/06/08 04:39 PM Re: Can I use a transformer ground as a bond? [Re: bigpapa]  
mikesh  Offline
Member
Joined: Jun 2006
Posts: 613
Victoria, BC, Canada
Originally Posted by bigpapa
OK,
What about this. You run a 4 conductor teck cable and use one conductor as the transformer ground. What do you do with the bonding conductor in the cable.

This would be something you would run into when you upgrade a 208V sub panel to a 600V feed with a step down transformer and use the existing cable.

I would guess you would ground one one of the bond conductor and isolate the other?

A Ground wire must be green or bare, not black or any other color in an ACWU or teck. Are there other stores nearby with a transformer? If it is 600 volt distribution then there must other transformers. Where do their grounds come from? Was the wire you found in the pipe a ground or bonding? If for example the original feeder was EMT all the way from the supply, then it likely that it is also the bonding conductor. The green wire in the pipe could be the ground but you'd need to see where it is connected and how.
If the original feeder was part pvc and part emt then you are missing a wire (the ground) look in the ceiling. Is there a big copper wire strapped up that looks like it goes past each store and has a bare or insulated copper wire hypressed or thermit welded down to the stores individual transformer. Ground wires are not required to follow the feeder so this could be coming from a less obvious route.
To use a cable to feed your transformer you are going to have to add a Ground wire that is continuous from the main service electrode to the transformer or panel neutral. Unless you can purchase an ISO ground cable or get permission from your AHJ then you can't ground with the bonding conductor in the cable or unused Other wire. From experience ISO (isolated ground) in a cable is pretty rare once over #12.
Now for another possibility if you are in a strip mall IE every store is on grade or you store is at grade you could install you own rods or plate but then you have to ensure this new electrode gets bonded to the other electrode. That can be done via the bonding wire in the feeder cable and a regular ground from the neutral of the new service to the plate or rods.
All this can be done in pipe but cables just don't have ground and bonding conductors inside them.


#181406 - 10/06/08 08:58 PM Re: Can I use a transformer ground as a bond? [Re: mikesh]  
twh  Offline
Member
Joined: Mar 2004
Posts: 915
Regina, Sask.
Quote
if you are in a strip mall IE every store is on grade or your store is at grade you could install your own rods or plate but then you have to ensure this new electrode gets bonded to the other electrode. That can be done via the bonding wire in the feeder cable and a regular ground from the neutral of the new service to the plate or rod
Good idea!



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