250.30(A)(1) exception #2

[Isaiah]

Confused by this exception. How does having a system bonding jumper at both ends, source and first disconnecting means, create a parallel neutral connection?


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[dkidd]

Something like this, where the earth is conductive

 

[Isaiah]

Something like this, where the earth is conductive

View attachment 2567358

Looks like two GECs connected to earth - at both ends correct?


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[jaggedben]

Looks like two GECs connected to earth - at both ends correct?


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But also system bonding jumpers connected to enclosure at both ends.

 

[jaggedben]

Confused by this exception. How does having a system bonding jumper at both ends, source and first disconnecting means, create a parallel neutral connection?

Typically it wouldn't create a parallel path if the transformer is outside at a separate pad. But they are throwing in that language just to make it clear that in a less usual situation where you had a parallel path - say, from structural steel or rebar connecting the pad to the main structure, or perhaps underground metal water pipe system used as electrode at both ends - then this method would be prohibited.

 

[Isaiah]

Typically it wouldn't create a parallel path if the transformer is outside at a separate pad. But they are throwing in that language just to make it clear that in a less usual situation where you had a parallel path - say, from structural steel or rebar connecting the pad to the main structure, or perhaps underground metal water pipe system used as electrode at both ends - then this method would be prohibited.

Is this why a service entrance can have two GECs - one at XFMR and one at the disconnecting means - but only has a grounded conductor without the SSBJ?


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[jaggedben]

Is this why a service entrance can have two GECs - one at XFMR and one at the disconnecting means - but only has a grounded conductor without the SSBJ?

I mean that's what this exception says. Yes.

Note that almost all services are done this way, except that in most cases the transformer and its GEC are owned by the utility and therefore not covered by the NEC. This exception allows it to be done similarly where the transformer is owned by the facility.

 

[Isaiah]

I mean that's what this exception says. Yes.

Note that almost all services are done this way, except that in most cases the transformer and its GEC are owned by the utility and therefore not covered by the NEC. This exception allows it to be done similarly where the transformer is owned by the facility.

Many thanks for the clarification!


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[wwhitney]

Something like this, where the earth is conductive

No, the text of 250.30(A)(1) Exception 2 explicitly says "connection through the earth shall not be considered as providing a parallel path."

The parallel path this exception is discussing would be, for example, metallic conduit containing the conductors and bonded to the enclosures at both ends. Then if you have a system bonding jumper at both ends, the metallic conduit will be carrying grounded conductor current.

Cheers, Wayne

 

[Isaiah]

No, the text of 250.30(A)(1) Exception 2 explicitly says "connection through the earth shall not be considered as providing a parallel path."

The parallel path this exception is discussing would be, for example, metallic conduit containing the conductors and bonded to the enclosures at both ends. Then if you have a system bonding jumper at both ends, the metallic conduit will be carrying grounded conductor current.

Cheers, Wayne

But typically on SDS the metallic conduit is bonded at both ends - and just one GEC at either the transformer or first disconnecting means
I believe the exception is directed at service entrance conductors where you have two GEC’s and therefore have parallel paths if the SSBJ is included with the phase conductors.
Correct me if I’m wrong here—-I’ve been wrong in the past

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[electrofelon]

But typically on SDS the metallic conduit is bonded at both ends - and just one GEC at either the transformer or first disconnecting means
I believe the exception is directed at service entrance conductors where you have two GEC’s and therefore have parallel paths if the SSBJ is included with the phase conductors.
Correct me if I’m wrong here—-I’ve been wrong in the past

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No it is not directed at service entrance conductors because that section only covers an SDS. As Wayne said it would be something like a metallic conduit, metallic water line etc, that would preclude its use.

 

[wwhitney]

Say you have an outdoor SDS and you run your secondary conductors to a building in non-metallic conduit. Any GES at the SDS is separate from the GES at the building (only connected via earth). And there's no SSBJ run with the secondary conductors.

Then 250.30(A)(1) Exception lets you have SBJs at both the outdoor SDS and at the building. The secondary grounded conductor is the fault clearing path for ground faults at the building. A hot to case fault at the SDS clears through the SBJ at the SDS. This looks a lot like a service.

Now suppose you add a parallel path, which could be (a) metallic conduit (b) an SSBJ (c) a common GES with GECs connected at both the SDS and the building or (d) something else. Then with two SBJs you'd have grounded conductor current on this parallel path, so the Exception text rules out two SBJs in this case.

Cheers, Wayne

 

[Isaiah]

No it is not directed at service entrance conductors because that section only covers an SDS. As Wayne said it would be something like a metallic conduit, metallic water line etc, that would preclude its use.

Ok thanks. So by simply bonding the conduit at both ends to the cases a parallel path is created? Is there a sketch you could post


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[Isaiah]

Say you have an outdoor SDS and you run your secondary conductors to a building in non-metallic conduit. Any GES at the SDS is separate from the GES at the building (only connected via earth). And there's no SSBJ run with the secondary conductors.

Then 250.30(A)(1) Exception lets you have SBJs at both the outdoor SDS and at the building. The secondary grounded conductor is the fault clearing path for ground faults at the building. A hot to case fault at the SDS clears through the SBJ at the SDS. This looks a lot like a service.

Now suppose you add a parallel path, which could be (a) metallic conduit (b) an SSBJ (c) a common GES with GECs connected at both the SDS and the building or (d) something else. Then with two SBJs you'd have grounded conductor current on this parallel path, so the Exception text rules out two SBJs in this case.

Cheers, Wayne

Ok thanks


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[wwhitney]

Ok thanks. So by simply bonding the conduit at both ends to the cases a parallel path is created? Is there a sketch you could post

Just look at the sketch in post #2, and add a fifth horizontal line connecting the two cases. That could be a conduit or an SSBJ. Either way it makes a loop for neutral current.

Cheers, Wayne