Using EGC as GEC for Transformers

[scbunker]

Hello all,

I have a memory of a conversation I had with an engineer that trained me years ago about using a transformer's primary-side equipment grounding conductor as the separately derived system's grounding electrode conductor as long as the conductor was sized appropriately.

Example: assume you have a 480-208V 75 kVA transformer fed from a 125A breaker. Assume secondary conductors are 300 kcmil copper. In this case, the transformer's EGC is required to be 6 AWG copper per 250.122. The GEC is required to be 2 AWG copper per 250.66. The system bonding jumper is in the transformer enclosure. Assume further that the panel feeding the transformer (containing the 125A breaker) is bonded to the grounding electrode system with a 2 AWG GEC.

In the above scenario, is it allowable to install a 2 AWG grounding conductor between the source panel and the transformer that serves both as the transformer's EGC and as the separately derived system's GEC?

Unless my memory is faulty (and it very well may be), my supervising engineer told me years ago that this was common practice in some institutions in order to avoid many multiple GEC's throughout the building, and I even think we found the code section where it was specifically allowed.

However, I cannot find in the 2020 NEC where such an installation is allowed.

I appreciate anyone's input on this.

Thank you.

 

[ptonsparky]

The NEC may prohibit but does not specifically allow.

In other words a certain path(s) may be closed, but you can take any other path to Grammas you want to.

 

[wwhitney]

See 250.121(A) and its exception.

In the example given, if the transformer's GEC is required to be unspliced (not so familiar with transformer requirements), then you'd need to extend the existing GEC or bonding jumper terminated in the supplying panelboard with an irreversible splice so that it could act as both the GEC and EGC for the transformer.

Cheers, Wayne

 

[don_resqcapt19]

See 250.121(A) and its exception.

In the example given, if the transformer's GEC is required to be unspliced (not so familiar with transformer requirements), then you'd need to extend the existing GEC or bonding jumper terminated in the supplying panelboard with an irreversible splice so that it could act as both the GEC and EGC for the transformer.

Cheers, Wayne

Yes, that is the biggest issue with using the EGC as both an EGC and a GEC for a SDS.

I have a PI in to permit the EGC for the primary to serve as the GEC for the secondary under some specific conditions, but doubt it will be accepted.

 

[scbunker]

Thank you for your replies:

On the continuous/unspliced requirement: 250.64(C) requires continuous GEC except as provided in 250.30(A)(5). Section 250.30(A)(5) is the section dealing with GEC for SDS. What exactly is intended by Exceptions 1 and 2 to 250.30(A)(5)? I have looked for a diagram illustrating the exceptions but have found none.

Exception 2 seems like it may allow the scenario I described, albeit only if the source panel is listed SUSE.

 

[tortuga]

Example: assume you have a 480-208V 75 kVA transformer fed from a 125A breaker. Assume secondary conductors are 300 kcmil copper. In this case, the transformer's EGC is required to be 6 AWG copper per 250.122. The GEC is required to be 2 AWG copper per 250.66. The system bonding jumper is in the transformer enclosure. Assume further that the panel feeding the transformer (containing the 125A breaker) is bonded to the grounding electrode system with a 2 AWG GEC.

Is the source panel also the service equipment with the main breaker?
What type of raceway is between the source panel and the transformer?

 

[scbunker]

Is the source panel also the service equipment with the main breaker?
What type of raceway is between the source panel and the transformer?

Assume the source panel is not service equipment.
Assume PVC conduit.

 

[Tulsa Electrician]

Is the source panel in a separate building fed with a feeder.

 

[tortuga]

Assume the source panel is not service equipment.
Assume PVC conduit.

Ahh then I'd be running them separate.

If it was a Main Distribution Panel that was the service I say go for it.
And with PVC conduit you would not have to worry about bond bushings at each end.
I would be tempted to tag or label the wire GEC/EGC at the service end.

 

[__dan]

I have seen it done and the times I saw it done ,it was done it was done wrong (spliced and did not go to the electrode or the electrode common busbar. They ran it hundreds of feet while the facility earth grounding system was right there in the floor and the building steel was less than 50 ft away.

There are time it's the best option as long as you comply with the stricter requirements for a GEC. But there are times the GEC is simply going to a different point B, or there are times the different point B is preferable or an advantage. The EGC is or can be normally noisy. The GEC should be a way to clean that up.

Sometimes there is no other point B and they both do go to the same place. But in a large facility with the building steel very close, that would be likely easier and perform better.

 

[wwhitney]

Seems like the upshot is this: if you install the transformer as an SDS, it needs a GEC. If it's convenient to route the GEC through the conduit carrying the primary-side feeder to the transformer, obeying all the GEC rules, and that GEC is large enough, then it can be your feeder EGC. If not, then you just run a separate EGC with your primary-side feeder as typical.

Cheers, Wayne

 

[Seekthetruth]

Wayne, Just to confirm your statement. As long as the GEC is installed per code it can also be used as the EGC in the primary conduit to the transformer? Especially if the building steel and water pipes were not available.

 

[wwhitney]

As long as the GEC is installed per code it can also be used as the EGC in the primary conduit to the transformer?

My understanding of the 250.121(A) exception is that if a single conductor meets all the requirements to be the GEC, and it meets all the requirements to be the EGC, it can fulfill both roles.

Cheers, Wayne

 

[Seekthetruth]

My understanding of the 250.121(A) exception is that if a single conductor meets all the requirements to be the GEC, and it meets all the requirements to be the EGC, it can fulfill both roles.

Cheers, Wayne

Thanks Wayne, Is the secondary sized according to the derived service (secondary)?

 

[kwired]

Thanks Wayne, Is the secondary sized according to the derived service (secondary)?

The GEC is for the separately derived system.

In step down situations this almost always will mean the required GEC for the secondary will be larger than the minimum required EGC for the primary circuit. In step up situations the EGC for the primary circuit will likely be larger than the needed GEC for the secondary. Either way if wanting to use a single conductor, where it is allowed, pick the larger of the two minimums.

 

[Seekthetruth]

In a step down transformer the EGC for the primary would have to be sized by the GEC required for the secondary (larger) conductors I believe you are saying?

 

[__dan]

In a step down transformer the EGC for the primary would have to be sized by the GEC required for the secondary (larger) conductors I believe you are saying?

It depends on the path the GEC takes. If you intend the GEC to take the path of the primary EGC, that conductor is sized for the larger of the two requirements (the GEC) and must comply with all of the GEC conductor requirements, no splices, grounding bushings bonding jumpers entering and leaving pull boxes ...

If that is the easiest way to get to the grounding electrode system, it can be done that way and there is no need for a redundant EGC on the same path.

 

[Seekthetruth]

Thank you all for your input. This is a great trade. Have a Great Week.

 

[electrofelon]

I have a PI in to permit the EGC for the primary to serve as the GEC for the secondary under some specific conditions, but doubt it will be accepted.

I hope it gets accepted . It is baffling the NEC's refusal to back down from the irrational importance they put on the GEC. I guess they just refuse to entertain any common sense or electrical theory. I think it was ActionDave who said the GEC is the least important conductor, but it has by far the most strict requirements.

 

[don_resqcapt19]

I hope it gets accepted . It is baffling the NEC's refusal to back down from the irrational importance they put on the GEC. I guess they just refuse to entertain any common sense or electrical theory. I think it was ActionDave who said the GEC is the least important conductor, but it has by far the most strict requirements.

I think it all traces back to when metal underground water systems were the used almost exclusively. In that case, the GEC is actually an effective fault clearing path, and can easily prevent the problems that result from an open service neutral. In most areas, new metal underground water pipes are not being installed and the GEC is almost never part of the fault clearing path, or serve as an alternate neutral.