Another question about Main Bonding Jumper

M

munisparki

Member
Location
Texas
Occupation
Sparky
If the service panel enclosure is bonded to the neutral bus bar using a MBJ, with a metal conduit back to the meter...

And the neutral is also bonded to the meter enclosure..

Why doesn't current travel along the path of the metal enclosure?

I read that the steel enclosure and conduit is something like 1000 percent more resistive than the copper service neutral but isn't it true that some current at least will take that path? Isn't that a parallel pathway?

I have trouble wrapping my head around this issue
 
munisparki said:
Why doesn't current travel along the path of the metal enclosure?
Click to expand...
It does.

munisparki said:
I read that the steel enclosure and conduit is something like 1000 percent more resistive than the copper service neutral ...
Click to expand...
It isn't.

munisparki said:
but isn't it true that some current at least will take that path? Isn't that a parallel pathway?
Click to expand...
Yes and yes.

munisparki said:
I have trouble wrapping my head around this issue
Click to expand...
The meter and service enclosures and raceways are considered part of the neutral itself.

It's not a big deal because it's all (re-)established as the zero-volts-to-ground point.
 
munisparki said:
If the service panel enclosure is bonded to the neutral bus bar using a MBJ, with a metal conduit back to the meter...

And the neutral is also bonded to the meter enclosure..

Why doesn't current travel along the path of the metal enclosure?

I read that the steel enclosure and conduit is something like 1000 percent more resistive than the copper service neutral but isn't it true that some current at least will take that path? Isn't that a parallel pathway?

I have trouble wrapping my head around this issue
Click to expand...


I have a slightly different take and would say that yes the code does not want objectionable current and it is avoidable, but it is not enforced. Read thru 250.6. all one would have to do in your situation is transition to a PVC TA for the termination at one end.
 
electrofelon said:
I have a slightly different take and would say that yes the code does not want objectionable current and it is avoidable, but it is not enforced. Read thru 250.6. all one would have to do in your situation is transition to a PVC TA for the termination at one end.
Click to expand...
If it were required to completely eliminate it then metallic raceways would not be permitted with service conductors. Since they are as Dave stated there are instances where it is allowed because it is unavoidable when using code compliant metal raceways.

You're correct that you could use PVC to mitigate the neutral current being carried on the raceway but there is no requirement to do so.
 
infinity said:
If it were required to completely eliminate it then metallic raceways would not be permitted with service conductors. Since they are as Dave stated there are instances where it is allowed because it is unavoidable when using code compliant metal raceways.

You're correct that you could use PVC to mitigate the neutral current being carried on the raceway but there is no requirement to do so.
Click to expand...
Like I said, it is my opinion that the code is clear and provides the necessary remediations such as different bonding jumper arrangement or isolating metal raceways, however clearly they are never enforced.
 
Ok imagine you are an alien electrical inspector and you just came to earth from planet mars. You see a service with a three phase meter socket or CT cabinet, and rigid metal conduit between it and the service disconnect (single phase meter sockets are typically factory bonded so that eliminates one option laid out in 250.6(B)) . Let's say the meter socket or CT cabinet is bonded to the neutral. If you can look past what we are used to and just focus on what the NEC says, it seems clear that the inspector is within his rights to require one of the remediations in 250.6(B) such as bonding the enclosure with a jumper back to the service disconnect, or putting a non-metallic fitting in the raceway.
 
electrofelon said:
If you can look past what we are used to and just focus on what the NEC says, it seems clear that the inspector is within his rights to require one of the remediations in 250.6(B) such as bonding the enclosure with a jumper back to the service disconnect, or putting a non-metallic fitting in the raceway.
Click to expand...
There is no direct wording in that section that could require an inspector to make you break the metallic path of a service raceway. If there were I'm pretty sure someone would have enforced it by now.
 
infinity said:
There is no direct wording in that section that could require an inspector to make you break the metallic path of a service raceway. If there were I'm pretty sure someone would have enforced it by now.
Click to expand...
"interrupting the conductive path responsible for the objectionable current" would seem to cover it. Also except for the odd case of a single phase meter socket which can't be unbonded, one would discontinue/reorganize bonding jumpers.
 
ActionDave said:
You have identified the dilemma. The code prohibits objectionable currents and then requires them. I call it the Unavoidable Objectionable. Not all sparkys expend enough thought about it to realize that it exists. Congratulations.
Click to expand...
Those paths also exist via the TV cable and in areas that still have a metal underground water piping system, there will be neutral current on the underground water piping system.
 
electrofelon said:
"interrupting the conductive path responsible for the objectionable current" would seem to cover it. Also except for the odd case of a single phase meter socket which can't be unbonded, one would discontinue/reorganize bonding jumpers.
Click to expand...
It is not objectionable to me, so I will be taking no action to remedy a non-existent problem. Also take a look at 250.6(C) in the 2023 code.
(C) Currents Not Classified as Objectionable Currents.
Currents resulting from abnormal conditions such as ground faults, and from currents resulting from required grounding and bonding connections shall not be classified as objectionable current for the purposes specified in 250.6(A) and (B).
Click to expand...
This is the case for every service in my area, where the meter neutral is bonded to the meter enclosure as local code requires IMC or RMC for service conductors. All of the self contained meter cans on the approved list for the local utility have the neutral bonded.

I have never seen where this parallel path via metal service raceways has caused any real world issues.

However the code required path via the metal underground water pipe can be a serious hazard for plumbers in the case where the service neutral is open. The path via the water piping system is often good enough that the building occupants do not see any of the typical signs of an open service neutral, but when the plumber cuts the underground water service pipe to make a repair, and touches both ends, there is a serious shock hazard.
 
don_resqcapt19 said:
I have never seen where this parallel path via metal service raceways has caused any real world issues.
Click to expand...
I haven't either. A meter enclosure and the service disconnect mounted next to each other on aluminum siding would have some current flowing on the siding even with a non-metallic raceway. In fact a low resistance path on a metal raceway would mitigate most of the objectionable current on the siding.
 
ActionDave said:
You have identified the dilemma. The code prohibits objectionable currents and then requires them. I call it the Unavoidable Objectionable. Not all sparkys expend enough thought about it to realize that it exists. Congratulations.
Click to expand...

No dilemma. The code prohibits objectionable current without defining it, but then explicitly permits current on raceways on the line side of the service disconnect, thus defining the latter current as not objectionable.
 
electrofelon said:
I have a slightly different take and would say that yes the code does not want objectionable current and it is avoidable, but it is not enforced. Read thru 250.6. all one would have to do in your situation is transition to a PVC TA for the termination at one end.
Click to expand...
Are you sure you're not just trying to keep from getting credit for another cool catch phrase like I came up with when I coined the terms- The Cult of the Green Wire and The Dirt Worshipers?
 
jaggedben said:
but then explicitly permits current on raceways on the line side of the service disconnect
Click to expand...
I think I missed this allowance? An allowance for installation methods that would result on current on metal raceways on the line side of the service disconnect is not the same thing.

Cheers, Wayne
 
don_resqcapt19 said:
It is not objectionable to me
Click to expand...

Yes it would certainly help if they defined it. My theory (again, just going by what the words say and letting go of common practice) is that neutral current flowing on service raceways must be objectionable . I say this because: 1) the degree to which the NEC tries to eliminate current flowing on things other than intended conductors. 2) SDS's already have language prohibiting parallel paths. 3) I can't think of what else it could mean. Can you think of an otherwise code compliant installation that "objectionable current" could be referring to?


Also take a look at 250.6(C) in the 2023 code.
Click to expand...

That is a new section I was not aware of. I am torn between that being an improvement that can live in harmony with "rearranging or interrupting", and it being completely contradictory.
 
wwhitney said:
I think I missed this allowance? An allowance for installation methods that would result on current on metal raceways on the line side of the service disconnect is not the same thing.

Cheers, Wayne
Click to expand...
I shouldn't have said 'explicit'. It's implicit in various sections. Although I think the sections are pretty explicit that you can do the things that cause the supposedly objectionable current which evidently isn't objectionable because you can do those things.

250.92(B)(1)
250.64(D)(1)&(2)

And before you say that 250.92(B)(1) doesn't preclude following 250.6(B)(3), let me just say that...
1) I've never seen anyone in the field care.
2) In SF, where I got started, only hard metallic conduit is permitted for service conductors so there's really no way around it.
 
You must log in or register to reply here.
Top