Transformer grounding question

[ggunn]

I know that this is old hat to some of you but it's bit of a new wrinkle for me.

My client wants a 208Y/120V panel for a bank of EV chargers powered from a 480Y/277V panel via a breaker through a transformer. I can size the transformer and all the OCPD and conductors, and I figure I'll want the transformer to be delta on the 480V primary side and wye on the 208V secondary side. What I'm not quite sure of is the grounding.

If this were a PV system through a transformer I would not run a ground from the panel to the transformer on the primary side, and I would bond neutral to ground on the secondary side and bond them back to building ground. Should I do this the same way?

 

[augie47]

You will need a properly sized (250.122) EGC on the primary side.
The secondary would have a supply side bond jumper and a system bond jumper and grounding electrode system per 250.30

 

[ggunn]

You will need a properly sized (250.122) EGC on the primary side.

That doesn't seem right; the secondary of the transformer is separately derived, is it not?

 

[xformer]

That doesn't seem right; the secondary of the transformer is separately derived, is it not?

The EGC is for the circuit feeding the transformer.

 

[augie47]

This may help:

 

[electrofelon]

That doesn't seem right; the secondary of the transformer is separately derived, is it not?

The EGC run with the primary is just like you would have with any feeder, to cover ground faults to the frame.

 

[wwhitney]

A couple longstanding questions of mine:

- As I understand it, the primary side EGC and secondary side EGC will both be bonded to the transformer case and thus interconnected. As such, the secondary neutral and EGC are earthed via the primary side GEC and GES. So what is the upside of the additional secondary side GEC and GES connection? If it's just a question of different sizing for EGC vs GEC, is it sufficient to provide one adequately sized primary side bonding conductor for both primary EGC and secondary GEC, to take care of the secondary side GES connection?

- If I'm not mistaken, there's no NEC requirement to install the transformer as an SDS, and so one could bring the primary neutral to the delta-wye transformer and solidly bond the secondary neutral to the primary neutral. Then there would be no additional earthing, and the primary EGC would function as secondary EGC. This makes the secondary fault clearing path longer, as a secondary line-EGC fault has to travel back to the primary system bonding jumper and along the primary neutral supply to the transformer to reach the secondary neutral point. Is there any other downside to this arrangement?

Cheers, Wayne

 

[electrofelon]

A couple longstanding questions of mine:

- As I understand it, the primary side EGC and secondary side EGC will both be bonded to the transformer case and thus interconnected. As such, the secondary neutral and EGC are earthed via the primary side GEC and GES. So what is the upside of the additional secondary side GEC and GES connection? If it's just a question of different sizing for EGC vs GEC, is it sufficient to provide one adequately sized primary side bonding conductor for both primary EGC and secondary GEC, to take care of the secondary side GES connection?

- If I'm not mistaken, there's no NEC requirement to install the transformer as an SDS, and so one could bring the primary neutral to the delta-wye transformer and solidly bond the secondary neutral to the primary neutral. Then there would be no additional earthing, and the primary EGC would function as secondary EGC. This makes the secondary fault clearing path longer, as a secondary line-EGC fault has to travel back to the primary system bonding jumper and along the primary neutral supply to the transformer to reach the secondary neutral point. Is there any other downside to this arrangement?

Cheers, Wayne

Regarding #1, I absolutely agree: we should just use the primary EGC as the grounding reference and do away with this GEC nonsense. Actually the code does allow it, if the EGC meets all the requirements of a GEC

For #2, I think the code needs some work here. We can just change color and what we call a Conductor and now don't have an SDS?

 

[tom baker]

In one case (4 wire) you have seperate white and green wires, so they are sized as a neutral and EGC in the other (3 wire) its a white wire with an imaginary greens stripe, so its sized per a neutral and EGC

 

[ggunn]

This may help:
View attachment 2559964

It does; thanks.

 

[wwhitney]

In one case (4 wire) you have seperate white and green wires, so they are sized as a neutral and EGC in the other (3 wire) its a white wire with an imaginary greens stripe, so its sized per a neutral and EGC

If your comment is in reference to my questions, I don't follow. Say the primary system is a 3 phase wye system, and the transformer is delta-wye. Also say that conduit is used as the EGC on both sides, and that in the SDS case the system bonding jumper is at the transformer.

Then in both cases you just have the normal secondary conductors leaving the transformer, with the conduit as the EGC. In the SDS case, you have 4 wires going to the transformer: 3 primary phase conductors and 1 GEC (since the GEC has to be of the wire type, correct?). In the non-SDS case you also have 4 wires going to the transformer: 3 primary phase conductors and the primary neutral.

So for this scenario, either way it's 4 wires. The only difference I see is whether the 4th wire is a GEC, connected to the transformer case and secondary neutral (SDS case) or the 4th wire is a primary neutral conductor, connected only to the secondary neutral and insulated from the transformer case/EGC.

Cheers, Wayne

 

[tom baker]

My answer was based on the first transformer graphic.

 

[ggunn]

It does; thanks.

One more question: To review, I need to power a 208/120V panel from a breaker in a 480/277V panel. From the diagram I know what I need to do with the grounding on the secondary side of the transformer, but from the diagram it isn't clear to me what to do on the primary side. Do I need an EGC from the 480 panel to the transformer or not? I would have thought not but a jumper between the ground rod at the transformer and building ground.

 

[petersonra]

One more question: To review, I need to power a 208/120V panel from a breaker in a 480/277V panel. From the diagram I know what I need to do with the grounding on the secondary side of the transformer, but from the diagram it isn't clear to me what to do on the primary side. Do I need an EGC from the 480 panel to the transformer or not?

All power circuits require an egc.

 

[ggunn]

All power circuits require an egc.

I know that the grounds must be tied together, but with an EGC in the conduit from the 480V panel to the transformer or an external conductor between the electrodes?

 

[wwhitney]

Do I need an EGC from the 480 panel to the transformer or not? I would have thought not but a jumper between the ground rod at the transformer and building ground.

Yes, you need an EGC on the primary side with the primary conductors. If the primary supply faults to the transformer case, you need a fault return path, i.e. an EGC.

You don't need a ground rod at the transformer, you need to follow 250.30. If your primary side EGC meets the requirements for your secondary side GEC, that would suffice.

Cheers, Wayne

 

[ggunn]

Yes, you need an EGC on the primary side with the primary conductors. If the primary supply faults to the transformer case, you need a fault return path, i.e. an EGC.

You don't need a ground rod at the transformer, you need to follow 250.30. If your primary side EGC meets the requirements for your secondary side GEC, that would suffice.

Cheers, Wayne

I appreciate the help, I really do, but 250.30 is a fairly daunting piece of code. Without going into sizing, here's what I am trying to accomplish, which on the surface seems pretty simple: I have a 480/277V panel which will be part of a commercial PV system. The customer wants to feed a 208/120V panel from it so that he can install some EV charging stations. I plan to have a 480V Delta to 208/120V Wye transformer fed from a breaker in the 480/277V panel on the primary side and a 208/120V panel with a main breaker on the secondary side, i.e., OCP on both sides of the transformer.

Specifically what I would really appreciate is if someone could advise me on what I need to do with the grounding on the primary side of the transformer which I can take to 250.30 and see how it conforms rather than asking me to read 250.30 and figure it out for myself. If you tell me the what I can figure out the why.

 

[electrofelon]

I appreciate the help, I really do, but 250.30 is a fairly daunting piece of code. Without going into sizing, here's what I am trying to accomplish, which on the surface seems pretty simple: I have a 480/277V panel which will be part of a commercial PV system. The customer wants to feed a 208/120V panel from it so that he can install some EV charging stations. I plan to have a 480V Delta to 208/120V Wye transformer fed from a breaker in the 480/277V panel on the primary side and a 208/120V panel with a main breaker on the secondary side, i.e., OCP on both sides of the transformer.

Specifically what I would really appreciate is if someone could advise me on what I need to do with the grounding on the primary side of the transformer which I can take to 250.30 and see how it conforms rather than asking me to read 250.30 and figure it out for myself. If you tell me the what I can figure out the why.

Forget it is a transformer, call it a milkshake machine (yum) or a widget maker and just size your EGC with the
primary feeder using 250.122

 

[petersonra]

I appreciate the help, I really do, but 250.30 is a fairly daunting piece of code. Without going into sizing, here's what I am trying to accomplish, which on the surface seems pretty simple: I have a 480/277V panel which will be part of a commercial PV system. The customer wants to feed a 208/120V panel from it so that he can install some EV charging stations. I plan to have a 480V Delta to 208/120V Wye transformer fed from a breaker in the 480/277V panel on the primary side and a 208/120V panel with a main breaker on the secondary side, i.e., OCP on both sides of the transformer.

Specifically what I would really appreciate is if someone could advise me on what I need to do with the grounding on the primary side of the transformer which I can take to 250.30 and see how it conforms rather than asking me to read 250.30 and figure it out for myself. If you tell me the what I can figure out the why.

Normally, one would size the EGC coming to the 480V transformer in accordance with table 250.122, or use metal conduit or EMT.

Unfortunately, there are other aspects of this that are related that you will have to dig through 250.30 to decide what applies in your case, such as the system bonding jumper, GEC, and SSBJ if applicable.

 

[electrofelon]

Normally, one would size the EGC coming to the 480V transformer in accordance with table 250.122, or use metal conduit or EMT.

Unfortunately, there are other aspects of this that are related that you will have to dig through 250.30 to decide what applies in your case, such as the system bonding jumper, GEC, and SSBJ if applicable.

If you are NOT trying to use the EGC with the feeder also as the grounding electrode conductor, then I don't think there's anything else to think about other than a quick glance at table 250.122.