250-32(b)

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A little input please. I have a existing metal prefab building with 6 interior panels mechanically attached to the metal of the building, all of the panels DO NOT have a EGC installed with the feeder and fed from the same source. All of the EGC's of the branch circuits are connected to a bar in each panel which is connected to a electrode only. So with the install as it is there is not a effective ground fault current path back to the MDP that is located outside. I believe that if the neutrals in the six panels are bonded in each of these panels it poses a risk of neutral currents flowing across and between the metal parts of the building.
 

raider1

Senior Member
Staff member
Location
Logan, Utah
Welcome to the forum.:)

I have moved this post into it's own thread.

First are the 6 panels fed from another building?

Also what is the wiring method used for the feeder IE are the feeders run in a metallic raceway that can qualify as an equipment grounding conductor?

Chris
 

Dennis Alwon

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Occupation
Electrical Contractor
A little input please. I have a existing metal prefab building with 6 interior panels mechanically attached to the metal of the building, all of the panels DO NOT have a EGC installed with the feeder and fed from the same source. All of the EGC's of the branch circuits are connected to a bar in each panel which is connected to a electrode only. So with the install as it is there is not a effective ground fault current path back to the MDP that is located outside. I believe that if the neutrals in the six panels are bonded in each of these panels it poses a risk of neutral currents flowing across and between the metal parts of the building.
Your bonding of the neutral should go no further than the MDP. The six so called "sub panels" do not need an equipment ground if the conduit is metallic. In this case, the conduit is the egc.
 
250-32(b)

The feeders to the six panels and fed with a non metallic cable and the MDP is at the exterior of the building but not in contact with it. There is no raceway for the feeders, each cable has three phase condutors and a neutral. I believe that the only way to safely correct this is to install a EGC from the exterior MDP to each of the 6 interior sub panels. I believe that if you were to bond the neutral of each panel then this would possibly allow neutral current to flow across the metal of the building creating a parallel path to another panel in the building (250-32 (B) EX (2). If the building was not metal then I would be able to safely bond each neutral but NOT if the building is a metal prefab. The reason for my question is that I have a person that disagrees with me.

Thanks for the input
 

RUWired

Senior Member
Location
Pa.
A little input please. I have a existing metal prefab building with 6 interior panels mechanically attached to the metal of the building, all of the panels DO NOT have a EGC installed with the feeder and fed from the same source. .
If the sub panel have the neutrals bonded in the sub panels, this was the norm up untill a few code cycles back. For existing wiring it is still allowed providing you meet the rules in 250.32(B) exception. Also for seperately derived systems this method is allowed providing you meet the exceptions in 250.30(A)1 exception 2.

Untill the contractors get up to speed with the 2008 code and the inspectors start enforcing, you'll see this a lot.

Rick
 
I may be a bit confused or may have not provided idepth detail.
The situation is as follows:

I have a steel framed metal prefabbed building, the MDP to this building is outside of the building and has no direct contact with the structure.
There are 6 SDP panels scattered throughout the building and are all fed via a 3 Phase multi-conductor cable assembly witch have no E.G.C.
The panels are all physically mounted to the structural steel of the building and each of the sub panels have a grounding electrode conductor that is used for the bonding of the building and connected to the rod.

pulling a separate E.G.C from the MDP is not feasible quite possibly impossible.

If we bond at each sub panel would we not be creating a parallel path for neutral currents?

since there are continuous metallic paths this violates art. 250.32 (b) exp. 2

and 250.136 (a) does not permit structural framing members to be used as the E.G.C

What would the correct fix for this be??

also where can i find this in the code book.
 
My colleague believes though it is a clear violation of 250.136(a) if it is bonded to the MDP the use of the structural steel as the E.G.C would be the most logical solution.

though we cannot find an exception to allow this type of a fix.

Is there such an exception?
 

chris kennedy

Senior Member
Location
Miami Fla.
As Rick pointed out...

250.32(B)Exception:
For existing premises wiring systems only, the grounded conductor run with the supply to the building or structure shall be permitted to be connected to the building or structure disconnecting means and to the grounding electrode(s) and shall be used for grounding or bonding of equipment, structures, or frames required to be grounded or bonded where all the requirements of (1), (2), and (3) are met:

(1) An equipment grounding conductor is not run with the supply to the building or structure.
(2) There are no continuous metallic paths bonded to the grounding system in each building or structure involved.
(3) Ground-fault protection of equipment has not been installed on the supply side of the feeder(s).
Where the grounded conductor is used for grounding in accordance with the provision of this exception, the size of the grounded conductor shall not be smaller than the larger of either of the following:

(1) That required by 220.61
(2) That required by 250.122
 
Sir,

I apologize in advance if i dont quite understand or am in some way confused

But since all of the sub panels are mechanically attached to the structural steel of the structure would that not violate item (2) There are no continuous metallic paths bonded to the grounding system in each building or structure involved.
 

augie47

Moderator
Staff member
Location
Tennessee
If all of your subpanels were located in seperate buildings under the '05 Code you would have no problems (if there was no metallic connection between the buildings)
The situation you have does not meet Code.
The structural steel can not serve as an equipmemt grounding conductor.
 
First I would to thank all of you who have posted on this topic.

I am Iraq Tiger's Colleague and we have been having this discussion for several days now.

This is a unique situation that i have not encountered before.

I know that it is clearly a violation however using the building steel as EGC seems the only logical solution.

Since the panels are all in the same building and there are multiple metallic paths connected and ruining a new EGC from the MDP is not an option
what would the fix be?

1. bond at every sub panel ( potentially creating a parallel path)
2. Bond the structural steel to the MDP which can be done and since the panels are attached to the steel call it good? (Clearly a violation)


Thank you in advance..
 

chris kennedy

Senior Member
Location
Miami Fla.
The panels are all physically mounted to the structural steel of the building and each of the sub panels have a grounding electrode conductor that is used for the bonding of the building and connected to the rod.
Is this connected to the grounded conductor bus or the EGC bar?


If we bond at each sub panel would we not be creating a parallel path for neutral currents?

since there are continuous metallic paths this violates art. 250.32 (b) exp. 2
How? #2 of the exception is not addressing paths in one building, but a parallel path back to MDP, in your case.
 

augie47

Moderator
Staff member
Location
Tennessee
Assuming you are in Iraq and have enough problems to fight, from a realistic standpoint, if MDP had been simply a service point, and each of your panels was in a "seperate occupancy" and you bonded each your panels to an electrode and building steel the installation could be viewed as acceptable (multiple occupancy building).
Since bringing it into compliance might not be a possibility, I would bond MDP to the electrode system and builing steel. You would at least have a fault path back to MDP in case of a fault in the panel.
 
Again Thank you to everyone for your help.

I believe our questions have been answered.

yes, we are in Iraq and this is exactly what we have done

"Since bringing it into compliance might not be a possibility, I would bond MDP to the electrode system and building steel. You would at least have a fault path back to MDP in case of a fault in the panel."

We call it "Iraq Good"

However as we both want to know the proper method of repair if we were trying to make it code compliant.

it appears as if we were both misinterpreting the code.

You all have been of great help and this was a great and enlightening discussion.
 

RUWired

Senior Member
Location
Pa.
With all the parallel paths via the grounded neutral from all the subs back to the MDP, you only have one easy choice to fix your problem. Based on your assesment that the wiring cannot be up graded, you will have to put in 3-wire primary to 4-wire secondary transformers and make the feeder neutral from the outdoor MDP an equipment ground. In this method you'll create a new neutral conductor and a new equipment ground conductor.This is assuming that you need three phase panel boards. If you don't, changing out the panels to single phase is another solution.

Rick
 
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