Bonding Multiple services

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sceepe

Senior Member
I Have a building with two services (230.2 (C)) Both service disconnects fed from one POCO transformer. At both disconnects a main bonding jumper has been installed. Contractor has installed a 4/0 bonding jumper between the Equipment grounding conductor bus in each service disconnect. Questions:

1. Does this meet code?

2. Will neutral current flow in the 4/0 between each service disconnect?

3. How is this any different than bonding both service disconnects to the same building steel. Won't you have the same issue with neutral current flowing in the building steel between disconnects??

Thanks in advance
 

karl riley

Senior Member
I will be interested to hear the replies. In an identical situation there was neutral flowing on the bonding jumper since one of the service neutrals evidently had lower impedance than the other. This resulted in a lot of net current on each service cable.

Karl
 

George Stolz

Moderator
Staff member
Location
Windsor, CO NEC: 2017
Occupation
Service Manager
1. If it is replacing the GEC, then no, it's not. 250.24(A)(1). A GEC shall be run from each service disconnect. If it is not replacing the required GEC, then I would say still "no", based on 250.6(A).

2. Yes.

3(a) It's not.
3(b) Yes, you would.

One way to mitigate the effects might be to make both service neutral conductors the same length from the POCO transformer. It is not required, and might be very difficult to accomplish.

Perhaps this can be addressed best at the planning stage, by putting a specification into the plans? For this circumstance, that doesn't seem like a very handy suggestion, as the work is already complete. I wonder if anybody has ever had this spec'ed to them?

Disclaimer: I have no practical experience on this topic.
 

malachi constant

Senior Member
Location
Minneapolis
i am in the planning stages of a multiple service building and ran across this in a thread search. i'll keep researching code and going through the archives here, but before i forget i have a question/suggestion on this one.

One way to mitigate the effects might be to make both service neutral conductors the same length from the POCO transformer. It is not required, and might be very difficult to accomplish.

what about increasing the size of the shorter neutral conductor such that the volume is approximately equal? unfortunately this would not prevent objectionable current, only minimize it.
 
malachi constant said:
what about increasing the size of the shorter neutral conductor such that the volume is approximately equal? unfortunately this would not prevent objectionable current, only minimize it.

Actually it would even further lower the resitance of that wire... might make it even worse. Length is the biggest factor here because you need the phase to reach both places as close as possible to the same time.
 
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George Stolz

Moderator
Staff member
Location
Windsor, CO NEC: 2017
Occupation
Service Manager
malachi constant said:
what about increasing the size of the shorter neutral conductor such that the volume is approximately equal? unfortunately this would not prevent objectionable current, only minimize it.
I think it would help, but the other way around. The longer neutral would be upsized, to decrease the resistance and make them more equal.

Given that there are houses interconnected (by virtue of a metallic water pipe between services) I don't know how big an issue this really is. You might do the math to see what size each service neutral would be, and post that, and see what kind of comments you get. I'd be curious to see what you come up with.

Another key question that should be brought up: are the multiple services from the same transformer, or different transformers?
 
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