Service Grounding for a Client Owned Transformer

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Kartracer087

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Location
Chicago
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Consulting Engineer
Good morning,

I was wondering if you can provide your interpretation of code for the following situation (we are planning on installing this):

We will have a 1500/2000kVA outdoor transformer that is in a supervised industrial installation. The transformer secondary conductors will leave the unit and go underground and will stub up into an indoor switchboard. The service voltage is 480Y/277V but the switchboard is going to be straight 480V with no neutral. For my grounding conductor size which will run from the transformer secondary to the switchboard, does that need to be sized per NEC 250.66 or NEC 250.122? I am bonding the neutral to the case ground at the transformer, so no neutral will be run with the service conductors. However, I will need a grounding conductor run with the conductors since it is PVC conduit. I just don't know if that falls into a feeder ground or a grounding electrode in terms of size. If it falls into being considered a feeder ground, then there is no overcurrent device at the transformer, so how would you size it in that case? Would it be based on the main overcurrent device inside the indoor switchboard?

Thanks,
 
Kartracer087 said:
Good morning,

I was wondering if you can provide your interpretation of code for the following situation (we are planning on installing this):

We will have a 1500/2000kVA outdoor transformer that is in a supervised industrial installation. The transformer secondary conductors will leave the unit and go underground and will stub up into an indoor switchboard. The service voltage is 480Y/277V but the switchboard is going to be straight 480V with no neutral. For my grounding conductor size which will run from the transformer secondary to the switchboard, does that need to be sized per NEC 250.66 or NEC 250.122? I am bonding the neutral to the case ground at the transformer, so no neutral will be run with the service conductors. However, I will need a grounding conductor run with the conductors since it is PVC conduit. I just don't know if that falls into a feeder ground or a grounding electrode in terms of size. If it falls into being considered a feeder ground, then there is no overcurrent device at the transformer, so how would you size it in that case? Would it be based on the main overcurrent device inside the indoor switchboard?

Thanks,
Click to expand...
Ok, just to be clear on some terminology: There are no service conductors here. What you have is a separately derived system with transformer secondary conductors (see 240.21(C) and 450.3 for transformer protection). The "ground" run with the conductors is a supply side bonding jumper. See 250.30. You will still need a grounding electrode system at the building. Perhaps you might consider just running a neutral conductor, then you could use 250.30(A)(1) exception #2. ITs really the same thing, all that changes is the color of that conductor and your gear in the building would be bonded like a service.
 
I am not going to look up and cite the codes, but I will answer you question. First, there is a code that says the neutral must run with the line conductors to the first means of disconnect, and then be bonded to the GES. You are not required to run a supply side bonding jumper from the transformer to the first means of disconnect. All metal enclosures, including the transformer case, however, must be bonded to the grounded conductor anywhere prior to the first means of disconnect. I actually did a one line of a similar situation recently. At the time I researched all the applicable codes.
 

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Strathead said:
I am not going to look up and cite the codes, but I will answer you question. First, there is a code that says the neutral must run with the line conductors to the first means of disconnect,
Click to expand...
That would be for a grounded service. This is an SDS and although you do not have to run a neutral/grounded conductor, I would run it instead of the SSBJ and use that exception I mentioned to bond at both ends.
 
Strathead said:
I am not going to look up and cite the codes, but I will answer you question. First, there is a code that says the neutral must run with the line conductors to the first means of disconnect, and then be bonded to the GES. You are not required to run a supply side bonding jumper from the transformer to the first means of disconnect. All metal enclosures, including the transformer case, however, must be bonded to the grounded conductor anywhere prior to the first means of disconnect. I actually did a one line of a similar situation recently. At the time I researched all the applicable codes.
Click to expand...
For this SDS (not a service), we typically run 3 wire (no neutral) and a SSBJ per 250.102
 
electrofelon said:
That would be for a grounded service. This is an SDS and although you do not have to run a neutral/grounded conductor, I would run it instead of the SSBJ and use that exception I mentioned to bond at both ends.
Click to expand...
Yeah, I missed this. The Neutral, or technically the grounded conductor must be run to the first means of disconnect. I first got busted on this when running power to a fire pump without a neutral since it wasn't needed.
 
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