service grounding

[autecbernie]

I have a 3 phase 4 wire transformer feeding a service disconnect panel that supplies 4 buildings. This is all outdoors.The buildings are 200' away and feed through underground PVC.There is a ground rod at the transformer and at each building. Do I have to run a grounding conductor from the transformer to each building or can I just run 3 ungrounded conductors 1 grounded conductor and bond the service panel in each building.This panel supplies 10 apartments

 

[charlie]

Re: service grounding

Need some clarification. I assume this is a transformer that is owned, controlled, and maintained by the serving electric utility. Is that statement correct?

Do you have free standing service equipment that is being fed from the transformer or is just the metering equipment in the transformer? In other words where is the first overcurrent device?

I am trying to determine if we are working with feeders or service to the buildings.

 

[autecbernie]

Re: service grounding

this is on private property. our 4160 Pri and our transformer and our buildings

 

[bphgravity]

Re: service grounding

I believe what you have is a separately derived system which will require you to comply with 250.30 of the NEC. So, your bonding should be made at either the transformer or the main service disco. After that, you have feeders supplying the buildings that must comply with 250.32(B) 1 or 2.

 

[jtester]

Re: service grounding

I think the proper grounding description still needs more info on the primary system grounding. Is the 4160 system grounded in each box, transformer, pole if overhead, etc.?
If your 4160 is multi-grounded, you should bond the primary (4160) neutral at the transformer to the secondary neutral. When the two are bonded, by definition, you will not have a separately derived system. It will function as a utility transformer. You would pull phase and neutral wires from the transformer to the service disconnect, bond neutral and ground, and run a ground wire from there.

 

[don_resqcapt19]

Re: service grounding

As long as there are no conductive paths other than the circuit conductors, you can bond at both the transformer and the buildings just like you would do if the transformer was utility owned. See Exception 1 to 250.30(A)(1).
Don

 

[don_resqcapt19]

Re: service grounding

jtester,

When the two are bonded, by definition, you will not have a separately derived system. It will function as a utility transformer.

How is this any different from other transfomers where there is a bond between the primary and secodary via the primary EGC? In my opinion, this is an SDS per the NEC.
Don

 

[jtester]

Re: service grounding

don
quote
How is this different from any other transformer where there is a bond from primary to secondary via the EGC?

By definition in Art.100 a separately derived system has no direct electrical connection, including a solidly connected grounded circuit conductor, to supply conductors originating in another system.
Most, but not all, transformers that provide SDS's are 480 delta to 120/208 wye, or at least something delta on the high side. There's no neutral, and no tie between separate system supply conductors.
I guess you could argue that the ground wire is a part of both systems, but I don't think it is a supply conductor.
Jim T

 

[kiloamp7]

Re: service grounding

Treat it as, & wire it as, a SDS. Sounds like it is no different than our privately-owned facility. We own & maintain all of our xfmrs.

The only service is at 69KV from POCO.
We have many, many, SDS with primary voltages of 69KV, 13.8KV, 13.2KV, 4.16KV, 2.4KV. 480V., etc.

For whatever it's worth, all xfmrs have delta primaries.