Bonding at detached Sub-panel with no ground.

D

Dmo57

Member
Location
Illinois
Occupation
Electrician
Theoretical Situation:

Older detached garage, fed by 3 conductor. No grounding conductor, no grounding path to main house. Subpanel at garage has grounding and grounded bonded

NEC 250.32 (B)(1) exception 1 allows older panels with no grounding from supply to use grounded as ground fault path.

If a new panel was installed with 2 grounding rods at garage would bonding at subpanel be kept? What are the dangers of separating grounding and grounded in the new panel if ground rods are installed?

Can it the existing supply be made to work within code?
 
It's low risk, but everyone will still tell you not to do it. The risk is that something happens to the neutral and then you have a surface potential on everything and between things. The ground rods won't cancel it out sufficiently once there is even a small load.

Also, during a fault, the ground potential can be elevated. That is especially bad if it is sustained for some reason.

The better thing to do would be to convert it to a single leg 120 volt panel (L, N, G), if that covers that quantity of circuits you need. Or, a little more complicated, but it gets the job done: switch the uninsulated conductor for G (I'm assuming it's already landed in the correct place) and derive a new 240 to 120-0-120 transformer there.
 
Welcome to the forum.

Dmo57 said:
If a new panel was installed with 2 grounding rods at garage would bonding at subpanel be kept?
Click to expand...
Yes; the reason hasn't changed.

Dmo57 said:
What are the dangers of separating grounding and grounded in the new panel if ground rods are installed?
Click to expand...
No bond means a fault could energize everything expected to be grounded, including grounds.

Dmo57 said:
Can it the existing supply be made to work within code?
Click to expand...
Only if it was compliant when installed, and the exception conditions haven't changed.
 
LarryFine said:
Welcome to the forum.


Yes; the reason hasn't changed.


No bond means a fault could energize everything expected to be grounded, including grounds.


Only if it was compliant when installed, and the exception conditions haven't changed.
Click to expand...
Thanks!

It's been drilled too deep in my head to never bond sub-panels.
 
hokadof188 said:
It's low risk, but everyone will still tell you not to do it. The risk is that something happens to the neutral and then you have a surface potential on everything and between things. The ground rods won't cancel it out sufficiently once there is even a small load.

Also, during a fault, the ground potential can be elevated. That is especially bad if it is sustained for some reason.

The better thing to do would be to convert it to a single leg 120 volt panel (L, N, G), if that covers that quantity of circuits you need. Or, a little more complicated, but it gets the job done: switch the uninsulated conductor for G (I'm assuming it's already landed in the correct place) and derive a new 240 to 120-0-120 transformer there.
Click to expand...
Thanks for the info. I've done the single leg 120 conversion as a work around for no ground. I'm a newer electrician so integration with older systems that are "wrong" according to what I learned gets confusing, especially when I have no experience to reference as to why it was done that way.

I read about this situation in another forum, and honestly I probably would have opted to go with the fail safe of just running a grounding conductor out to the sub.
 
Dmo57 said:
It's been drilled too deep in my head to never bond sub-panels.
Click to expand...
You must be young. Up until 2008, it was legal to wire a panel in a detached structure like a service (i.e. no separate grounding conductor). It is critical that the remote panel grounds be bonded to the neutral when the feeder has no EGC.
 
suemarkp said:
Up until 2008, it was legal to wire a panel in a detached structure like a service (i.e. no separate grounding conductor).
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Even then, there could not be another metallic pathway such as a water supply to the detached structure.

This was to minimize the risk of a neutral-current-voltage-drop-caused difference between the electrical neutral/ground and the water-pipe ground reference.
 
suemarkp said:
You must be young. Up until 2008, it was legal to wire a panel in a detached structure like a service (i.e. no separate grounding conductor). It is critical that the remote panel grounds be bonded to the neutral when the feeder has no EGC.
Click to expand...
Yes, for us newer guys it's kinda excruciating having to integrate with things that are seemingly "wrong" to what we've been taught.

Critical flaw in training, we don't really learn about the old and why things are the way they are.
 
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