Bonding/Grounding Utility transformer to 480v panel, then from step-down transformer to 208v panel

[Eeyore's Tail]

I’m a bit confused on how to correctly do all this and neutral sizing seems contradictory depending on the author.
I’m installing a 400amp 480V Panel from a CT can which comes from a new utility transformer that’s yet to be installed. Utility wanted 2 parallel runs of 500kcmil copper for 800amps at the can. Pretty unlikely we’ll add anything else to the can. Inspector says this requires a neutral, any issues with sizing that at a parallel 1/0?
I drove 2 ground rods for our transformer, since the 480v is considered a service he wants the ground rod wire (EGC) installed at the 480v panel.

What do I now do with the step-down 112.5 kva transformer I’m installing? Neutral goes to X0 already and lands on neutral bar in my 208 panelScribbled one-line

 

[tom baker]

Interesting the poco is dictating the size of service conductors. They won’t allow AL?
Yes the minimum size neutral is 1/0 for conductors in parallel. If you most of your loads are line to line that will be fine, but the minimum size is the larger of unbalanced loads or based on ungrounded conductors
For your transformer, the GEC goes to the same location as its SBJ, which typically at the XO connection. Run a #6 to to your service ground rods, and bond to any metal/water lines in the area served

 

[don_resqcapt19]

If the utility is supplying you with a grounded service, and it would be pretty rare for a utility to supply a new ungrounded service, the inspector is correct. See 25024(D). The minimum sizing rules for the grounded conductor are also found in that section.
The step down transformer should be a 480 volt delta, to 208 volt wye. The feeder will be the three 480 volt phase conductors and an EGC. There will be no neutral in the feeder to the transformer.
The 208Y/120 volt system will have its neutral originate at the secondary XO terminal.

 

[augie47]

Seemingly missing from your sketch (possibly just not shown) would be grounding electrode conductors from your 480v disconnect to your building grounding electrode system, a EGC to your transformer and grounding electrode connections from you transformer secondary to the building GES.

 

[Eeyore's Tail]

Interesting the poco is dictating the size of service conductors. They won’t allow AL?
Yes the minimum size neutral is 1/0 for conductors in parallel. If you most of your loads are line to line that will be fine, but the minimum size is the larger of unbalanced loads or based on ungrounded conductors
For your transformer, the GEC goes to the same location as its SBJ, which typically at the XO connection. Run a #6 to to your service ground rods, and bond to any metal/water lines in the area served

They would allow AL, we pulled Cu for no good reason
Normally I’d run the GEC the way you mentioned and that’s how I did it to begin with. Never had to run a GEC at the 480v side first as he says but this is my first time the utility’s installed a transformer vault in an open field for me too. Again does that mean I’ll have to drive 2 more rods to establish X0?

Interesting the poco is dictating the size of service conductors. They won’t allow AL?
Yes the minimum size neutral is 1/0 for conductors in parallel. If you most of your loads are line to line that will be fine, but the minimum size is the larger of unbalanced loads or based on ungrounded conductors
For your transformer, the GEC goes to the same location as its SBJ, which typically at the XO connection. Run a #6 to to your service ground rods, and bond to any metal/water lines in the area served

 

[Eeyore's Tail]

If the utility is supplying you with a grounded service, and it would be pretty rare for a utility to supply a new ungrounded service, the inspector is correct. See 25024(D). The minimum sizing rules for the grounded conductor are also found in that section.
The step down transformer should be a 480 volt delta, to 208 volt wye. The feeder will be the three 480 volt phase conductors and an EGC. There will be no neutral in the feeder to the transformer.
The 208Y/120 volt system will have its neutral originate at the secondary XO terminal.

250.24 (c)(3) says the grounded conductor must handle the ampacity of ungrounded conductors effectively meaning the same size. CT can neutral lugs are sized at 250kcmil x2. Does that jive? Damn poco wouldn’t just let us set this up for 400amp

For X0 - I originally had that set up from 2 grounds rods through transformer ground bar to X0 with #6 bare copper. Inspector said I could reroute to the 480v panel - I assume through ground bar to the neutral lug.
H1,2,3 with egc and X1,2,3 with neutral and egc are already pulled

What would I add to my step down transformer to establish X0 since inspector said to reroute the bare copper?

 

[Eeyore's Tail]

Seemingly missing from your sketch (possibly just not shown) would be grounding electrode conductors from your 480v disconnect to your building grounding electrode system, a EGC to your transformer and grounding electrode connections from you transformer secondary to the building GES.

There’s no building GES, it’s in a parking lot field. 480v panel gec to transformer and from transformer to 208v panel are pulled along with 208v neutral. Trying to figure out ct can neutral size and X0 requirements since he said I could reroute the bare copper from my ground rods.

Is it better to add another ground rod at the 480v panel with bare copper and keep my 2 ground rods going to the transformer?

 

[jaggedben]

250.24 (c)(3) says the grounded conductor must handle the ampacity of ungrounded conductors effectively meaning the same size. .

That only applies to a 3-wire delta, which if it has a grounded conductor would mean that it's corner grounded. That would not be a neutral and that rule would not apply to you if the utility service is a wye.

For X0 - I originally had that set up from 2 grounds rods through transformer ground bar to X0 with #6 bare copper. Inspector said I could reroute to the 480v panel - I assume through ground bar to the neutral lug.
H1,2,3 with egc and X1,2,3 with neutral and egc are already pulled

What would I add to my step down transformer to establish X0 since inspector said to reroute the bare copper?

The XO on the *secondary* 208/120 side of your transformer should be grounded to the same electrodes as the neutral at your 480V disconnect. (The neutral of the 480V system should *not* be grounded except at the service. Typically at the 480V service disconnect.)

I personally don't like the terminology of you 'establishing XO'. The transformers (each of them) establish a neutral point for each system. You are just grounding them. You ground the secondary XO for a transformer that the customer owns (typically). You ground the service neutral when the utility owns the transformer.

 

[jaggedben]

There’s no building GES, it’s in a parking lot field. 480v panel gec to transformer and from transformer to 208v panel are pulled along with 208v neutral. Trying to figure out ct can neutral size and X0 requirements since he said I could reroute the bare copper from my ground rods.

Is it better to add another ground rod at the 480v panel with bare copper and keep my 2 ground rods going to the transformer?

Sorry, missed this part.

If your transformers are some distance apart the code basically says to treat them as separate structures and drive a separate set of rods (or other electrode) at each location. (250.32). If they are close enough to sensibly use the same set of rods, you can do that.

You typically do *not* pull a grounding electrode conductor with circuit conductors. That could be an EGC.

 

[don_resqcapt19]

250.24 (c)(3) says the grounded conductor must handle the ampacity of ungrounded conductors effectively meaning the same size. CT can neutral lugs are sized at 250kcmil x2. Does that jive? Damn poco wouldn’t just let us set this up for 400amp

For X0 - I originally had that set up from 2 grounds rods through transformer ground bar to X0 with #6 bare copper. Inspector said I could reroute to the 480v panel - I assume through ground bar to the neutral lug.
H1,2,3 with egc and X1,2,3 with neutral and egc are already pulled

What would I add to my step down transformer to establish X0 since inspector said to reroute the bare copper?

The grounding electrode conductor from the separately derived system must go to a grounding electrode. It can't go to the neutral of the 480 volt service. It could go to the same grounding electrode that is being used for the service.

 

[Eeyore's Tail]

The grounding electrode conductor from the separately derived system must go to a grounding electrode. It can't go to the neutral of the 480 volt service. It could go to the same grounding electrode that is being used for the service.

Ok, would something like this work? I ran through acorns for the transformer, acceptable to add a second acorn on the same rod for the 480v ground bar?Ground rod/grounding setup

 

[jaggedben]

Ok, would something like this work? I ran through acorns for the transformer, acceptable to add a second acorn on the same rod for the 480v ground bar?Ground rod/grounding setup

Yes, that's great.

Actually one nitpicky detail: the GEC to the 480V panel technically needs to go to the neutral, not the groundbar. You will also bond the neutral to the can.

 

[Eeyore's Tail]

That only applies to a 3-wire delta, which if it has a grounded conductor would mean that it's corner grounded. That would not be a neutral and that rule would not apply to you if the utility service is a wye.



The XO on the *secondary* 208/120 side of your transformer should be grounded to the same electrodes as the neutral at your 480V disconnect. (The neutral of the 480V system should *not* be grounded except at the service. Typically at the 480V service disconnect.)

I personally don't like the terminology of you 'establishing XO'. The transformers (each of them) establish a neutral point for each system. You are just grounding them. You ground the secondary XO for a transformer that the customer owns (typically). You ground the service neutral when the utility owns the transformer.

Gotcha that makes some sense. To use the inspector’s lingo though my 400amp main breaker is considered the service disconnect which he wants a phenolic label saying.

Mostly I’m trying to wade through the language to see how I’m supposed to physically install this. Poco’s been pretty unhelpful and I think the inspector’s just handing me a rope to hang myself with.

 

[Eeyore's Tail]

Yes, that's great.

Actually one nitpicky detail: the GEC to the 480V panel technically needs to go to the neutral, not the groundbar. You will also bond the neutral to the can.

So N-G bonding at the service enclosure (CT can) and GEC wire to the Neutral in the 480V panel? There's a removable lug on the CT's Neutral bar, would make an easy for a mechanical lug for bonding if so. My understanding is it must be at least as big as the EGC which we used 3/0 for