250.32

[ccarr7]

Guys,

I have a situation where I have a 1200A 277/480V main switchboard in building 1 that includes a 400A breaker which feeds a panel with 400A main breaker in building 2. The conductors between the two breakers are direct buried in PVC and an equipment ground conductor is included with the three phase wires and a neutral. My interpretation of 250.32 is that at the 400A panel in building 2 the equipment ground wire, as well as the available ground electrode conductors (in this case from the water pipe and a ground rod) at the building are to be landed.
Downstream of the 400A panel (fed from a 200A breaker in the panel) in building 2 is a 480 to 120/208V step down transformer. An equipment ground conductor is being installed to it from the 200A breaker on the line side, and it was planned to have ground electrode conductors installed to it, as well.
My questions are...are separate ground electrode conductors required to be pulled to both the 400A panel in building 2, as well as to the step-down transformer? If the ground electrode conductors at building 2 are pulled to the 400A panel, can an appropriately sized ground electrode conductor be pulled from it to the transformer perhaps in a separate conduit. I would still look to have the equipment ground conductor pulled with the conductors between the 200A breaker and the transformer.

 

[LarryFine]

My interpretation of 250.32 is that at the 400A panel in building 2 the equipment ground wire, as well as the available ground electrode conductors (in this case from the water pipe and a ground rod) at the building are to be landed.

Correct. Other than the isolated neutral system, the grounding is done just like a service. There's just no MBJ.

My questions are...are separate ground electrode conductors required to be pulled to both the 400A panel in building 2, as well as to the step-down transformer?

Well, the panel definitely needs an electrode system, as the separate building's "service." The transformer's secondary, as an SDS, requires one electrode connection, which can be to any part of the building's electrode system.

If the ground electrode conductors at building 2 are pulled to the 400A panel, can an appropriately sized ground electrode conductor be pulled from it to the transformer perhaps in a separate conduit. I would still look to have the equipment ground conductor pulled with the conductors between the 200A breaker and the transformer.

As a higher-current system, the lower-voltage secondary electrode conductor must likely be larger than the primary's EGC, but I believe one large-enough conductor can serve as both.

 

[augie47]

You can do what you plan, but as Larry says the transformer GEC might be larger so you would have to increase the panel GEC if you connected to it. The transformer GEC would be better in PVC as you won't have to bond the conduit that way. The water pipe and building steel are listed in 250.30 as the preferred electrodes. Could you not connect to either of those without going back to your 400 amp panel ?

 

[ccarr7]

250.32

Guys,

Thanks for the response. The derived conductors from the transformer are the same size as the service entrance conductors to the disconnect on the line side of the transformer (one set of 600CU), so we will go with a minimum of a 1/0CU for the electrode conductors.

Creig