why to not bond subpanels

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jcoale

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i know how to do it but can someone explain why subpanels are not bonded with the neutral. explain the safety issue with not separating the grounds from the neutral. which number in art. 250 tells you
 
jcoale said:
i know how to do it but can someone explain why subpanels are not bonded with the neutral. explain the safety issue with not separating the grounds from the neutral. which number in art. 250 tells you
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Once you have installed wiring and equipment PAST the first point of disconnect (the service disconnect), the equipment grounding conductor (EGC) and the grounded conductor (neutral) are separated, as the current flowing back towards the source (usually the utility transformer supplying the house/building) will take all of the available paths BACK TO THE UTILITY TRANSFORMER. The grounded conductor past the service disconnect will usually have some kind of current flowing on it, back towards the utility transformer. If the neutral and the EGC are bonded together at the subpanel or other places past the service disconnect, then there will be current flow on them back towards the utility transformer. If that were so, then the effective ground fault current path (including metallic paths such as raceway and enclosures, they are also bonded), could be bare metallic parts, would all have current on them. This could potentially cause shock or fire hazards...I tell people FIRE AND DEATH!!!
 
I would also look at 250.32(B)(2) which actually allows the EGC and the grounded conductor to be connected together at a subpanel. There are certain restrictions however.
 
infinity said:
I would also look at 250.32(B)(2) which actually allows the EGC and the grounded conductor to be connected together at a subpanel. There are certain restrictions however.
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??? what am I missing? 250.32(B)(2) states where an egc is not run with the supply to the building. If you run an EGC you must separate it from the grounded conductor. If you run an EGC you would connect the EGC to the grounding electrode but not the grounded conductor. Yes????
 
Dennis Alwon said:
??? what am I missing? 250.32(B)(2) states where an egc is not run with the supply to the building.
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Exactly, the EGC is not run to the separate building and all of the EGC's and grounded conductors are terminated together at the separate building. I was just pointing out that sometimes subpanels can have their EGC's and grounded conductors connected together and not separate as mentioned in the OP.
 
I got you. I thought you were saying that a 4 wire feeder to a separate building would allow the egc and Grounded conductor to be connected together. Sorry for the misread.
 
I have a MS Powerpoint presentation (*.*ppt) that describes the problem of parallel ground paths in detail ~ but not overly technical, has LOTS of graphics, easy to understand .
It was sent to me by another forum member.



Here's the deal:

If ANYONE else would like a copy of it send me a PM with your email address in it.

The presentation is 738kb
FYI:
The presentation is a Microsoft Power Point file.
If you do not have Power Point (like me), you can download a Power Point viewer from Microsoft....or I can send the viewer under a seperate email
The Power Point viewer is 2,837KB

Did I mention both are free ~ just for asking :grin:


(This is a re-post from a thread on 11/11/06 Grounding Presentation)
(This is a re-post from a thread on 10/06/06 back to back bonding )


Offer is ALWAYS open...just ask.
 
georgestolz said:
Hey T, I think it's the same one available for immediate download here. :)
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Oh yes it is!

I didn't know you had that tucked away in your FAQ :)

Do you also have the HiVoltageShock.pps hidden away someplace?
 
The actual reason is so you have an equipotential ground within the structure.

Everyone else has the right explanation of how that is achieved, ie by avoiding neutral voltage drop being reflected across the grounds at different locations.
 
Celtic,

Please send me a copy of the power point presentation.

----------
Edit to remove e-mail address. If you wish to contact this member via email, please communicate via PM.

ssadvis, please note that both powerpoint presentations are available for immediate download by clicking on the links provided above. - George
 
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why to not bond subpanels

250.6 prohibits any current to flow over the grounding conductors or grounding paths.
Electrons leaving the power supply always return to the power supply, and will take any path available.
The reason the code prohibits the re-bonding the grounded (neutral) conductor past the service main bonding jumper or system bonding jumper, is that it would places the equipment grounding system (conductor/s) in parallel with the grounded (neutral) conductor energizing the grounding system. Electrical current will take all paths available, except more current will flow on the paths with the lowest resistance.
250.342(B)(2) does permit the grounded (neutral) conductor to be bonded to the electrode system of the second building or structure, if there are no continuous metallic paths bonded to the grounding system in each building or structure involved. The metallic paths could metal piping system, the shield of any communication wire, or metal raceways. If you can not ensure that there are no metallic paths bonded between the two building or structures, you have to install an equipment grounding conductor and bound it the grounding electrode system of the second building or structure. The grounded (neutral) conductor will be isolate at the second build or structure; this will insure not parallel paths.
 
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