250.30

[Isaiah]

We have several SDS, delta-wye, solidly grounded power transformers around the site outdoors that feed into switchgear inside of buildings. The inspector says the GEC should be grounded at the switchgear but I thought if the transformer was installed outside the building the GEC had to be at the XFMR.

 

[augie47]

As I understand it you need a grounding electrode at both;;;;
250.30(C) Outdoor Source. If the source of the separately derived system is located outside the building or structure supplied, a grounding electrode connection shall be made at the source location to one or more grounding electrodes in compliance with 250.50. In addition, the installation shall comply with 250.30(A) for grounded systems or with 250.30(B) for ungrounded systems.

The system bonding jumper should follow the rules in 250.30(A)(1) but note Exception 2 for outside transformers.
Likewise the SSBJ should be installed per 250.30(A)(2) but there is also an exception there that references (A)(1)

 

[Isaiah]

As I understand it you need a grounding electrode at both;;;;
250.30(C) Outdoor Source. If the source of the separately derived system is located outside the building or structure supplied, a grounding electrode connection shall be made at the source location to one or more grounding electrodes in compliance with 250.50. In addition, the installation shall comply with 250.30(A) for grounded systems or with 250.30(B) for ungrounded systems.

The system bonding jumper should follow the rules in 250.30(A)(1) but note Exception 2 for outside transformers.
Likewise the SSBJ should be installed per 250.30(A)(2) but there is also an exception there that references (A)(1)

But if we bond the GEC at both the source and the first disconnecting means we could run the risk of a parallel path for the grounded conductor.


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[ptonsparky]

But if we bond the GEC at both the source and the first disconnecting means we could run the risk of a parallel path for the grounded conductor.


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You will. Earth. Almost every service in the country.

 

[Isaiah]

You will. Earth. Almost every service in the country.

Ours is not a service- it is a SDS.


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[ptonsparky]

Ours is not a service- it is a SDS.


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Doesn't matter.

 

[Isaiah]

Doesn't matter.

So you’re advocating to ground the GEC at one location only….correct?


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[augie47]

I think what he is pointing out is every service has a grounding electrode at the transformer and the building (required by 250.24) so a outside SDS would not be any different.
The parallel path is thru the earth so negligible current flow.
If you can apply the exceptions I noted the install is identical to a service transformer.

 

[Isaiah]

I think what he is pointing out is every service has a grounding electrode at the transformer and the building (required by 250.24) so a outside SDS would not be any different.
The parallel path is thru the earth so negligible current flow.
If you can apply the exceptions I noted the install is identical to a service transformer.

The SDS has both a grounded conductor and the SSBJ to panelboard or disconnect.
But a service transformer only has the grounded conductor incoming to the panelboard or disconnect.

 

[augie47]

The exception allows you to omit the SSBJ on an outside transformer IF you meet the "no path" requirements.

 

[hacim23]

We have several SDS, delta-wye, solidly grounded power transformers around the site outdoors that feed into switchgear inside of buildings. The inspector says the GEC should be grounded at the switchgear but I thought if the transformer was installed outside the building the GEC had to be at the XFMR.

Ran into a similar situation where I'm at only the inspector is saying the MBJ needs to be outside at the XFMR. We've been leaving it inside at the MB. I thought it could be at either place.

 

[augie47]

Ran into a similar situation where I'm at only the inspector is saying the MBJ needs to be outside at the XFMR. We've been leaving it inside at the MB. I thought it could be at either place.

250.30(A)(1) shows it can be at either and, in the case of an outside transformer, under certain conditions, i can be at both.

 

[ptonsparky]

Are you using metallic raceway or cable tray between the transformers and the buildings?

 

[hacim23]

Are you using metallic raceway or cable tray between the transformers and the buildings?

cable tray here

 

[ptonsparky]

cable tray here

If metallic than you have a potential parallel path that should be avoided. Your choice as to where you make the bond.

 

[Isaiah]

250.30(A)(1) shows it can be at either and, in the case of an outside transformer, under certain conditions, i can be at both.

It seems to me that bonding at one location only is the better choice to avoid a parallel scenario.
Is there a technical advantage of having the SJB and/or GEC at two locations?


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[electrofelon]

It seems to me that bonding at one location only is the better choice to avoid a parallel scenario.
Is there a technical advantage of having the SJB and/or GEC at two locations?


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The advantage is you need one less conductor. The grounded conductor also serves as the EGC. Its basically set up just like a service. I Cant think of a disadvantage.

 

[RCD]

Article 250.32(A) GEC be provided at each building or structures disconnecting means.

 

[Isaiah]

The advantage is you need one less conductor. The grounded conductor also serves as the EGC. Its basically set up just like a service. I Cant think of a disadvantage.

Makes sense. Thanks


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[electrofelon]

Article 250.32(A) GEC be provided at each building or structures disconnecting means.

Just note than an outdoor transformer and/or other equipment is unlikely to be a structure, but there are other sections that require a grounding electrode there.