Objectionable Current on Rigid Conduit

[yesterlectric]

I wonder if anyone has been faced with this situation before. 250.24(2) would require an additional bond from the transformer XO to ground, and then a bond at the main disconnect. What about a circumstance where an engineer has specified metal conduit between a transformer and main disconnect center? Wouldn't this be a problem?

 

[chris1971]

I wonder if anyone has been faced with this situation before. 250.24(2) would require an additional bond from the transformer XO to ground, and then a bond at the main disconnect. What about a circumstance where an engineer has specified metal conduit between a transformer and main disconnect center? Wouldn't this be a problem?

If bonding & grounding is done properly, then it shouldn't be an issue.

 

[yesterlectric]

If bonding & grounding is done properly, then it shouldn't be an issue.

Undoubtedly though, the conduit would be a parallel neutral conductor, for which it is not made. Excessive heating could result. This concern is the same reason it is made a violation to bond at the main disconnect, and then at a sub panel.

 

[iwire]

Undoubtedly though, the conduit would be a parallel neutral conductor, for which it is not made. Excessive heating could result. This concern is the same reason it is made a violation to bond at the main disconnect, and then at a sub panel.

How is it a parallel neutral?

XO is a new neutral, not the same neutral as the one at the service disconnect.

 

[Smart $]

I wonder if anyone has been faced with this situation before. 250.24(2) would require an additional bond from the transformer XO to ground, and then a bond at the main disconnect. What about a circumstance where an engineer has specified metal conduit between a transformer and main disconnect center? Wouldn't this be a problem?

250.24(A)(2) requires an additional connection from X0 to a grounding electrode... not the supply-side bonding jumper (SSBJ) nor the enclosure if the system bonding jumper is located in the system disconnecting means.

Or you could bond X0 to the electrode, the case, conduit, and SSBJ at the outdoor transformer and not have a system bonding jumper in the system disconnecting means. Still needs a building GES connection to the SSBJ/EGC at the disconnect, assuming it's at building entrance.

 

[electrofelon]

How is it a parallel neutral?

XO is a new neutral, not the same neutral as the one at the service disconnect.

I dont follow you. Which XO is a "new neutral"? I think we only have one X0, the one on the utility transformer. Whether the conduit is a parallel path depends on if the conduit is bonded to the transformer....

 

[Fulthrotl]

Undoubtedly though, the conduit would be a parallel neutral conductor, for which it is not made. Excessive heating could result. This concern is the same reason it is made a violation to bond at the main disconnect, and then at a sub panel.

.
ground bushing to properly bond the raceway?

i'm not getting how the raceway is a parallell neutral conductor.

 

[chris1971]

.
ground bushing to properly bond the raceway?

i'm not getting how the raceway is a parallell neutral conductor.

My thoughts exactly. Not sure what the OP is getting at with his post?

 

[GoldDigger]

If it is the utility transformer, then additional bonds on the POCO side of the service disconnect are by definition not objectionable since they are not covered by the NEC.
If it is a customer owned transformer and the service disconnect is on the primary side, then it is a. SDS and specific bonding rules apply.

Sent from my XT1585 using Tapatalk

 

[Smart $]

My thoughts exactly. Not sure what the OP is getting at with his post?

He's thinking bonding jumper at both transformer and system disconnect... which is fine with an outdoor transformer that has no metallic paths other than the grounded conductor, which is actually part of the exception conditions in order to install a system bonding jumper at both locations.

Appears to be an SDS installation.

 

[Fulthrotl]

My thoughts exactly. Not sure what the OP is getting at with his post?

.
i'm perplexed, however that's not new.

you have a ground wire in the pipe. you have
a ground bushing on each end of the pipe.
you strip an inch of the insulation, loop it
thru the finger lugs on each bushing, and
ground it at both ends. it's not going to
be any more of a parallell neutral than the
ground wire is.

 

[yesterlectric]

He's thinking bonding jumper at both transformer and system disconnect... which is fine with an outdoor transformer that has no metallic paths other than the grounded conductor, which is actually part of the exception conditions in order to install a system bonding jumper at both locations.

Appears to be an SDS installation.

Yes that is the issue. The code would require bonding at the main disconnect, and an additional bond at the outdoor transformer. It is fine as long as metal conduit isn't used. What if it is to be used?

 

[Smart $]

Yes that is the issue. The code would require bonding at the main disconnect, and an additional bond at the outdoor transformer. It is fine as long as metal conduit isn't used. What if it is to be used?

What Code edition is the project under? There's been a couple, three changes from 2005 to 2014 in this area.

An second bond (system bonding jumper, aka SBJ) is never required. One at either the transformer (XFMR) or one at the building/system disconnecting means (B/SDM) is all that is required. The grounding electrode conductor connection must also be in the same enclosure. Some look at this as requiring a second SBJ because you have a GEC connection at both the transformer and at the B/SDM. That is simply not true. The GE/GEC at the XFMR establishes a separate grounding electrode system (GES) from the one at the building. Only one GES can provide the GE required by 250.30(A)(4).

 

[yesterlectric]

What Code edition is the project under? There's been a couple, three changes from 2005 to 2014 in this area.

An second bond (system bonding jumper, aka SBJ) is never required. One at either the transformer (XFMR) or one at the building/system disconnecting means (B/SDM) is all that is required. The grounding electrode conductor connection must also be in the same enclosure. Some look at this as requiring a second SBJ because you have a GEC connection at both the transformer and at the B/SDM. That is simply not true. The GE/GEC at the XFMR establishes a separate grounding electrode system (GES) from the one at the building. Only one GES can provide the GE required by 250.30(A)(4).

This is under the 2011 edition. 250.24(2) says an additional bond is required at the outdoor transformer.

 

[electrofelon]

Can we clarify whether this is a service or an sds? Lots of sds jargon being thrown around. 250.24 (A)(2) is for utility owned transformers and the conductors it supplies would be SEC. An additional grounding connection from the grounded conductor to electrode does not make the conduit a parallel path, but there would be a parallel path on the GEC/bonding jumpers if its all one GES.

 

[Smart $]

This is under the 2011 edition. 250.24(2) says an additional bond is required at the outdoor transformer.

Are we talking service or SDS?

While 250.24 is for service grounding, the same is required of an outdoor SDS transformer in 250.30(C)

The required connection is from X0 to a grounding electrode, nothing else. You do not bond X0 to the enclosure where the SBJ is located in the B/SDM.

Alternatively, you can [SBJ] bond X0 to this GE and to the enclosure and subsequently the supply-side bonding jumper (SSBJ; the conduit and the wire-type SSBJ, if run)... but then there is no SBJ in the B/SDM.

 

[don_resqcapt19]

Yes that is the issue. The code would require bonding at the main disconnect, and an additional bond at the outdoor transformer. It is fine as long as metal conduit isn't used. What if it is to be used?

That only occurs with service conductors. It is not permitted for equipment on the load side of the service equipment. There are multiple code required/permitted paths for neutral current on the line side of the service disconnect.

 

[yesterlectric]

That only occurs with service conductors. It is not permitted for equipment on the load side of the service equipment. There are multiple code required/permitted paths for neutral current on the line side of the service disconnect.

I don't follow where the code allows it. 250.6 says grounding of (ALL) electrical systems shall be installed in a manner arranged to prohibit objectionable current. It would seem that the hazards associated with this exist wherever it is in a service.

In the event one bonded on the transformer per 250.24(2) (additional bonding jumper required), then installed a main service bonding jumper, and used rigid metal conduit between the transformer and service disconnect, there would be be objectionable neutral current which could present a fire or other hazard.

 

[Smart $]

Code does not define objectionable current, so how do you know if there is any or not?

And as I said earlier, only an additional grounding electrode and GEC are required. An additional bonding jumper is not.

 

[electrofelon]

Code does not define objectionable current, so how do you know if there is any or not?

But with 250.24(A)(2) dont we also need to discuss whether the "additional grounding connection" is made to the premises GES or a new GES? I generally would think of this as being a new standalone electrode. If it was a common electrode, than we would certainly have a parallel path and perhaps more likely to be defined as objectionable.