Best practices for a couple 15kV transformer scenarios

[Matt Donovan]

There may be multiple code-compliant ways to ground these. What are your opinions on best practices? I want to consider code compliance, and also just whatever electricians & inspectors are most accustomed to. And is anything I'm proposing below not actually code-compliant?

Scenario 1:
customer-owned transformer 2000kVA, 15kV delta:480V wye, pad mounted outside, 3000A disconnect on same pad.

• Option1A: ground Xo bushing, do not bond N-G in switchboard. Run 8 PVC conduits with phase, N, and SSBJ in each conduit.
• Option1B: ground Xo bushing, do not bond N-G in switchboard. Run 8 PVC conduits with phase and N only. Run one large SSBJ buried in the pad from Xo to switchboard G bus.
• Option 1C: do not ground Xo bushing, bond N-G in switchboard. Run 8 PVC conduits with phase, N, and SSBJ in each conduit.
• Option 1C: do not ground Xo bushing, bond N-G in switchboard. Run 8 PVC conduits with phase and N in each conduit. Run one large SSBJ buried in the pad from Xo to switchboard G bus.

Scenario 2:
Exactly the same as Scenario 1, except now its a service transformer. 3000A disco is now service disco and still outside on the same pad and xfmr. Does anything change?

I think most transformers do not come with a ground space large enough for 8 sets of SSBJ may be a problem, unless we're savvy when ordering the transformer (which can easily be missed)

 

[electrofelon]

There may be multiple code-compliant ways to ground these. What are your opinions on best practices? I want to consider code compliance, and also just whatever electricians & inspectors are most accustomed to. And is anything I'm proposing below not actually code-compliant?

Scenario 1:
customer-owned transformer 2000kVA, 15kV delta:480V wye, pad mounted outside, 3000A disconnect on same pad.

• Option1A: ground Xo bushing, do not bond N-G in switchboard. Run 8 PVC conduits with phase, N, and SSBJ in each conduit.
• Option1B: ground Xo bushing, do not bond N-G in switchboard. Run 8 PVC conduits with phase and N only. Run one large SSBJ buried in the pad from Xo to switchboard G bus.
• Option 1C: do not ground Xo bushing, bond N-G in switchboard. Run 8 PVC conduits with phase, N, and SSBJ in each conduit.
• Option 1C: do not ground Xo bushing, bond N-G in switchboard. Run 8 PVC conduits with phase and N in each conduit. Run one large SSBJ buried in the pad from Xo to switchboard G bus.

Scenario 2:
Exactly the same as Scenario 1, except now its a service transformer. 3000A disco is now service disco and still outside on the same pad and xfmr. Does anything change?

I think most transformers do not come with a ground space large enough for 8 sets of SSBJ may be a problem, unless we're savvy when ordering the transformer (which can easily be missed)

Regarding scenario 1, I think all options are potentially ok but a few points: 1) everytime you say "ground" or "bond" I think you mean "ground and bond". The GEC of an SDS connects at the same point as the SBJ. 2) you will require GFP, your bond arrangement will need to be compatible with that. 3) there is 250.30(A)(1) exception #2 which could throw in some more options.

For Scenario 2 typically I see the utility bond and ground the XO, but whether they do or not doesnt really effect anything I do, its their deal. It would just be phases and neutral run to disco, neutral bonded and grounded there.