Multiple Service Disconnects Main Bonding Jumper Creating Objectionable Current

[fun ee]

Ladies and Gents,

Please see the attached marked up image that supposedly represents an acceptable service installation with multiple disconnects. It appears the main bonding jumper bonded to the enclosure could cause a parrallel path for neutral current as indicated in the red highlighted path (others could be drawn). Is there some exemption to 250.6 that I am not aware of for service equipment. If the main bonding jumper was brought to the neutral bus in the gutter then I don't believe there would be objectionable current. Please advise.





Thanks

 

[hurk27]

Ahead of the main service disconnect or disconnects in the case of multiple, you will sometimes have parallel neutral paths, in most cases this is unavoidable especially if the wiring method is in a metal raceway, many meter bases will have the neutral mounted directly to the can and if there is a metal raceway between the meter and the cabinet that holds the main service disconnect you will have a parallel path, this has been allowed for as long as I can remember, but it is after the main service disconnect that you must keep the neutral separate from the EGC and no parallel paths can exist and is not allowed by the NEC.

Remember that ahead of the main service disconnect the neutral is the fault current path for the return of fault current to the source, this is also why the NEC requires that the electrode conductor to be connected to the service neutral at or before the main service disconnect and must be a connection by wire or buss bar not through a screw.

 

[hurk27]

Ahead of the main service disconnect or disconnects in the case of multiple, you will sometimes have parallel neutral paths, in most cases this is unavoidable especially if the wiring method is in a metal raceway, many meter bases will have the neutral mounted directly to the can and if there is a metal raceway between the meter and the cabinet that holds the main service disconnect you will have a parallel path, this has been allowed for as long as I can remember, but it is after the main service disconnect that you must keep the neutral separate from the EGC and no parallel paths can exist and is not allowed by the NEC.

Remember that ahead of the main service disconnect the neutral is the fault current path for the return of fault current to the source, this is also why the NEC requires that the electrode conductor to be connected to the service neutral at or before the main service disconnect and must be a connection by wire or buss bar not through a screw.

Edit to add:

The red supply side bonding jumper if it was installed is un-necessary since the service neutral is already landed on the same buss, and it would be a violation of 310.4 because it is a wire pathway, and would hide a bad neutral connection that could cause a fire if the neutral connection is lost since this conductor is not sized for the rating of the main service and is not the same size as the main neutral which is required for paralleling of conductors 1/0 and larger, having a raceway that ends up paralleling a neutral is a big difference then having a conductor paralleling the neutral.

 

[don_resqcapt19]

The code rules, in a number of cases, actually require parallel paths for the grounded conductor current on the line side of the service disconnect.

 

[Smart $]

Another perspective is the resistance of the parallel pathways is minimal at near the service disconnects. Current is only dangerous to personnel where it creates a voltage gradient (i.e. touch potential). With grounding of the system occurring in this area, touch potential is at a minimum.

 

[fun ee]

Edit to add:

The red supply side bonding jumper if it was installed is un-necessary since the service neutral is already landed on the same buss, and it would be a violation of 310.4 because it is a wire pathway, and would hide a bad neutral connection that could cause a fire if the neutral connection is lost since this conductor is not sized for the rating of the main service and is not the same size as the main neutral which is required for paralleling of conductors 1/0 and larger, having a raceway that ends up paralleling a neutral is a big difference then having a conductor paralleling the neutral.

Hurk,

Please advise on 310.4. That section is not found in mine. Is it some other section.

Thanks Again

 

[fun ee]

Another perspective is the resistance of the parallel pathways is minimal at near the service disconnects. Current is only dangerous to personnel where it creates a voltage gradient (i.e. touch potential). With grounding of the system occurring in this area, touch potential is at a minimum.

This seems reasonable and handles one of the reasons to avoid objectionable current. Yet what about heating concerns on equipment enclosures and such is this thought to be small as well and acceptable? Has NEC board ever thought about stating an exemption for service disconnecting means because 250.6 doesn't speak to any exclusions for service equipment. Per your understanding, 250.6 is ignored by inspectors and engineers when it comes to service equipment?

Thanks Again

 

[don_resqcapt19]

Hurk,

Please advise on 310.4. That section is not found in mine. Is it some other section.

Thanks Again

If using the 2011 or newer, look at 310.10(H).

That being said, I think his comment was based on the "red line" in the drawing being an actual installed conductor and not just an indication of a current path.

 

[jxofaltrds]

If using the 2011 or newer, look at 310.10(H).

That being said, I think his comment was based on the "red line" in the drawing being an actual installed conductor and not just an indication of a current path.

Don

He is just showing a path from the 4AWG jumper to the 2/0AWG jumper. I do not believe that he is saying that they are connect by an 'additional' conductor.

I think that he is asking if current runs on the case(s) and conduit(s).

 

[texie]

Don

He is just showing a path from the 4AWG jumper to the 2/0AWG jumper. I do not believe that he is saying that they are connect by an 'additional' conductor.

I think that he is asking if current runs on the case(s) and conduit(s).

I agree. I think the added red line obscured the fact that the 2/0 jumper on the right end of the neutral bar just connects to the wireway. If that is the case then this drawing looks correct and compliant to me. The only thing missing are bonding jumpers for the nipples from the wireway to disconnects (assuming they are metallic).
And, yes, with services there often parallel paths.

 

[Smart $]

This seems reasonable and handles one of the reasons to avoid objectionable current. Yet what about heating concerns on equipment enclosures and such is this thought to be small as well and acceptable? Has NEC board ever thought about stating an exemption for service disconnecting means because 250.6 doesn't speak to any exclusions for service equipment. Per your understanding, 250.6 is ignored by inspectors and engineers when it comes to service equipment?

Thanks Again

Heating from hysteresis occurs where magnetic fields cause eddy currents to flow in opposing directions?the reason we have to cut slots between ferromagnetic enclosure entries of same-circuit conductors.

For the issue we are discussing, opposing magnetic fields are kept at a minimum because the wire conductors enter through the same raceway, same hole.

 

[fun ee]

Heating from hysteresis occurs where magnetic fields cause eddy currents to flow in opposing directions?the reason we have to cut slots between ferromagnetic enclosure entries of same-circuit conductors.

For the issue we are discussing, opposing magnetic fields are kept at a minimum because the wire conductors enter through the same raceway, same hole.

Thanks for the feedback. The heating I was mentioning was plain old I^2R losses through main bonding jumpers and panel enclosures. Don't believe it to be significant but what is significant. 250.6 seems to take that judgement of what is significant from the engineer's hands by disallowing all objectionable current. Yet the community agrees that there will current service enclosures and jumpers in normal operating conditions, even though 250.6 says this is not allowed and states no exemption for service enclosures or jumpers. Is the NEC code in conflict here (not suprising most large specifications are) or just not explicit enough?

Thanks Again

 

[fun ee]

Don

He is just showing a path from the 4AWG jumper to the 2/0AWG jumper. I do not believe that he is saying that they are connect by an 'additional' conductor.

I think that he is asking if current runs on the case(s) and conduit(s).

Yes you are correct. Case and conduit and jumpers as well. I apologize for the poor sketch.

 

[Smart $]

Thanks for the feedback. The heating I was mentioning was plain old I^2R losses through main bonding jumpers and panel enclosures. Don't believe it to be significant but what is significant. 250.6 seems to take that judgement of what is significant from the engineer's hands by disallowing all objectionable current. Yet the community agrees that there will current service enclosures and jumpers in normal operating conditions, even though 250.6 says this is not allowed and states no exemption for service enclosures or jumpers. Is the NEC code in conflict here (not suprising most large specifications are) or just not explicit enough?

Thanks Again

I think the Code is explicit enough... but regarding service and service entrance equipment, often ignored because of well established practices. The depiction you posted is one such example, but the nipples between the wireway and disconnects could have been PVC, or one end could have been a PVC FA/BA raceway termination.

 

[hurk27]

I agree. I think the added red line obscured the fact that the 2/0 jumper on the right end of the neutral bar just connects to the wireway. If that is the case then this drawing looks correct and compliant to me. The only thing missing are bonding jumpers for the nipples from the wireway to disconnects (assuming they are metallic).
And, yes, with services there often parallel paths.

Yep I missed that about the red line, Opps

But there is no requirement for a raceway containing service conductors to be bonded on both ends of the raceway, as long as one end has been bonded the requirements for a fault path have been met, many get the requirements of metal raceways enclosing a GEC mixed up here, but one is for fault current and the other is to prevent creating a high frequency choke from a lightning strike.

 

[hurk27]

Thanks for the feedback. The heating I was mentioning was plain old I^2R losses through main bonding jumpers and panel enclosures. Don't believe it to be significant but what is significant. 250.6 seems to take that judgement of what is significant from the engineer's hands by disallowing all objectionable current. Yet the community agrees that there will current service enclosures and jumpers in normal operating conditions, even though 250.6 says this is not allowed and states no exemption for service enclosures or jumpers. Is the NEC code in conflict here (not suprising most large specifications are) or just not explicit enough?

Thanks Again

Generally the size of the raceway needed for the size of the conductors of a given amperage service takes care of the raceway being large enough that the little parallel current from the unbalanced neutral that there would be virtually no I^2R heating even if the neutral connection was lost, but as I said before with a parallel conductor you wouldn't have this added protection, of course both methods can mask a bad neutral connection which could be a problem if the raceway connections to the cabinets are not made wrench tight and kept clean from corrosion.

 

[texie]

Yep I missed that about the red line, Opps

But there is no requirement for a raceway containing service conductors to be bonded on both ends of the raceway, as long as one end has been bonded the requirements for a fault path have been met, many get the requirements of metal raceways enclosing a GEC mixed up here, but one is for fault current and the other is to prevent creating a high frequency choke from a lightning strike.

No disagreement from me.