Transformer and Panel Grounding

[cripple]

I have a 3-phase pad mounted utility transformer primary rated 13.2KV and 480Y/277 on the secondary. The transformer and the panel serviced are both on the same pad and located about 4 feet apart. The print shows the transformer to be grounded to a 4/0 copper conductor that is installed in the pad, and the print shows to install the three phase conductors and a grounded conductor to the panel, which is be grounded at the panel to the same 4/0 copper conductor the transformer is grounded.

QUESTION: Would this installation violate 250.6, since by grounding both the transformer and the panel to the same grounding electrode, we will be creating a parallel path with the grounded conductor and the grounding electrode system or does the NEC address this type of installation?

 

[websparky]

Re: Transformer and Panel Grounding

No.

2002 NEC 250.24 (A)(2) Outdoor Transformer. Where the transformer supplying the service is located outside the building, at least one additional grounding connection shall be made from the grounded service conductor to a grounding electrode, either at the transformer or elsewhere outside the building.

 

[hurk27]

Re: Transformer and Panel Grounding

Websparky
Earth is not considered as a parallel path. if the outside connection is to a GE there would be no parallel path. but the way he is bonding the neutral there would be. If the transformer is on the same pad as the service equipment I don't see a reason for the extra grounding wire. as the neutral will serve both.

 

[gwz2]

Re: Transformer and Panel Grounding

Another Catch-22 ?

If this were a pole mounted Poco Transformer or a Pad mounted Poco transformer with a definite seperate structure for the disconnect, all using a grounded service conductor, the Disconnect is required to have a bonding jumper and a GEC.
250-24(A), 250.28, 250.50.

gwz2

 

[websparky]

Re: Transformer and Panel Grounding

Hi Wayne,

So if I understand you correctly;

The 3 phase conductors and the grounded conductor (not a GEC) is installed from the transformer seconary through a conduit to a service disconnect.

At the sevice disconnect, the grounded conductor is landed on the bonded lug of the enclosure.

The load side conductors leaving the disconnect will be; 3 phase conductors, 1 grounded conductor and 1 grounding conductor.

The service will have only 1 GE and 1 GRC located inside of the utility owned transformer.

I believe I answered "no" to his question of parallel paths. However, you are saying there would be a parallel path. If this is so, please explain how we would satisfy the code requirement of 250.24 and other code references for service equipment bonding to the GEC.

 

[hurk27]

Re: Transformer and Panel Grounding

I think like Glen said this is a catch 22. I would ask the utility eng what he would prefer. as the wires leaving this transformer should be under the utility's control. Matter of fact I'm not sure you would even be allow access to this transformer.

I would drop two rods as close to the pad as you could get and just bond to them this would be just like if the panel was at the building. this would allow the earth to act as a buffer inbetween the POCO's rod and your rod's. I think this would satifiy both sides as the NEC does not consider the earth as a parallel path when it comes to bonding.

But maybe others have a better idea than this as It's been awhile since I've done this type of instlation

[ December 12, 2003, 01:08 AM: Message edited by: hurk27 ]