Service, Separately Derived or feeding one structure from another?

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KuhnsEngineering

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Interesting question.

We have a large building that is served by the utility with a 13.2KV primary. We have 15KV switchgear and a number of unit substations around the structure. Pretty straight forward. But...

In a couple of cases there was no room for an interior substation so primary MV conductors were run out of the building to pad-mount switch gear and pad-mount transformers. Secondary conductors then come from the transformers back into the building and terminate in 480 or 208 switchboards.

As far as bonding should those secondaries be treated as service entrance conductors, separately derived systems, or feeding one structure from another?

In the case of serivce entrance there would be no grounds run with the secondary and we would have a main bonding jumper in the switchboard.

In the case of a separately derived system we would normally have the option of making the bonding jumper in the transformer or in the first disconnect. In this case the pad-mounts are typically bonded and we would need to pull grounds in with the secondary and not have bonding jumpers installed in the switchboards .

In the case of feeding one structure from another we would run the equipment grounding conductors with the secondary and not have bonding jumpers in the switchboards.

Any words of wisdom appreciated.
 
I agree with David.

Since these are customer owned transformers after the utility service (13.2KV). The transformers would be separately derives systems that supply the different buildings and need to be grounded and bonded in accordance with 250.30.

Chris
 
Yea, I'm coming to the same conclusion but struggling with the ground fault path. The mains on the 480V switchboards are required by 215.10 to have GFP. The closest connection to ground is a driven ground (actually a triad of 3/4 x 15' copper) at the pad-mount which is about 75' away. The building steel and metallic piping is bonded but that is far away at the service MV switchgear. The substations that are within the building are bonded to building steel. The primary feeder to the padmounts do have the concentric shielding tape ground on the 133% EPR 15KV conductors and a 1/0 ground that runs with the feeder.

:roll:

The contractor is pulling secondary conductors to a new 480V switchboard on Saturday. I'll have him pull in the EGC's.
 
Yea, I'm coming to the same conclusion but struggling with the ground fault path. The mains on the 480V switchboards are required by 215.10 to have GFP. The closest connection to ground is a driven ground (actually a triad of 3/4 x 15' copper) at the pad-mount which is about 75' away. The building steel and metallic piping is bonded but that is far away at the service MV switchgear. The substations that are within the building are bonded to building steel. The primary feeder to the padmounts do have the concentric shielding tape ground on the 133% EPR 15KV conductors and a 1/0 ground that runs with the feeder.

:roll:

The contractor is pulling secondary conductors to a new 480V switchboard on Saturday. I'll have him pull in the EGC's.
 
KuhnsEngineering said:
Yea, I'm coming to the same conclusion but struggling with the ground fault path. The mains on the 480V switchboards are required by 215.10 to have GFP. The closest connection to ground is a driven ground (actually a triad of 3/4 x 15' copper) at the pad-mount which is about 75' away. The building steel and metallic piping is bonded but that is far away at the service MV switchgear. The substations that are within the building are bonded to building steel. The primary feeder to the padmounts do have the concentric shielding tape ground on the 133% EPR 15KV conductors and a 1/0 ground that runs with the feeder.

:roll:

The contractor is pulling secondary conductors to a new 480V switchboard on Saturday. I'll have him pull in the EGC's.
Click to expand...

If the structural metal frame is grounded per 250.52(2), then it can serve at the grounding electrode for your separately derived system. You can bond the neutral and ground at the 480v switchgear (per 250.30(A)(1)) and can run the grounding electrode conductor to nearest point of connection to the metal structure.
 
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