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#78342 - 09/10/01 05:37 PM Outbuilding grounding  
pauluk  Offline
Member
Joined: Aug 2001
Posts: 7,520
Norfolk, England
What are the NEC requirements for grounding of a separate building?

I'm thinking of something like a detached garage or workshop. Presumably the sub-panel in the said building will have isolated neutral & ground busbars, but do you rely on the feeder cable ground or do you install a separate ground rod for that sub-panel as well?


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#78343 - 09/10/01 06:55 PM Re: Outbuilding grounding  
sparky  Offline
Member
Joined: Oct 2000
Posts: 5,311
Paul;
that would be NEC 250-32,
#1-
we can either run a separate N & G wire, and run G-rods off the G-bar ( N would be isolated, or 'floating')

OR......

#2-
we can run one conductor serving the purpose of both N & G, and run G-rods off the common bar.

this is the basic jist of the article, it is debated regularly here
[Linked Image]


#78344 - 09/10/01 09:29 PM Re: Outbuilding grounding  
electure  Offline


Member
Joined: Dec 2000
Posts: 4,260
Fullerton, CA USA
Paul,
I believe you've just opened 1 of Pandora's boxes. [Linked Image]


#78345 - 09/13/01 04:24 PM Re: Outbuilding grounding  
pauluk  Offline
Member
Joined: Aug 2001
Posts: 7,520
Norfolk, England
Quote
Originally posted by sparky:

#1-
we can either run a separate N & G wire, and run G-rods off the G-bar ( N would be isolated, or 'floating')
OR......
#2-
we can run one conductor serving the purpose of both N & G, and run G-rods off the common bar.


Correct me if I'm wrong, but I believe the NEC specifies that a sub-panel has to have separate neutral & ground. Do I take it that a sub-panel in a separate building is treated as special case and is exempt from this requirement?

In this case (#2), I can see that a parallel ground rod would certainly be desirable.

In case #1, is the second rod an NEC requirement, or just common practice?


#78346 - 09/13/01 04:39 PM Re: Outbuilding grounding  
sparky  Offline
Member
Joined: Oct 2000
Posts: 5,311
Correct me if I'm wrong, but I believe the NEC specifies that a sub-panel has to have separate neutral & ground. Do I take it that a sub-panel in a separate building is treated as special case and is exempt from this requirement?

yes, that is correct...

In this case (#2), I can see that a parallel ground rod would certainly be desirable.

Yes, i agree, think of it as a new service.

In case #1, is the second rod an NEC requirement, or just common practice?

The language reads hard here, but most agree that G-rods are required.

G-rods, and other grounding methods supposively exist to , as the NEC puts it, address 'voltage gradient' and 'lightning protection'. We, as field electricians in this forum could quite likley post many conflicting scenarios to this end....

electure's short version says it best [Linked Image]


#78347 - 09/13/01 06:09 PM Re: Outbuilding grounding  
pauluk  Offline
Member
Joined: Aug 2001
Posts: 7,520
Norfolk, England
Just one of many points open to lively debate, eh?

Case #2 with common N/Gnd conductor and multiple rods is starting to look very much like our PME (Protective Multiple Earthing) distribution system here - Except that it's within the customer's premises instead of the utility's system.

Because we have neutral & ground separation at the main panel, obviously this extends to a sub-panel in an outbuilding. Sometimes an extra ground rod is used, but it's not specified as a requirement by the IEE.

In fact, in an older installation which uses a voltage-operated ELCB (if you can cast your mind back a couple of weeks!), and extra rod must NOT be used, or it could render the breaker inoperative.


#78348 - 09/13/01 06:22 PM Re: Outbuilding grounding  
Anonymous
Unregistered

>an extra rod must NOT be used, or it could render the breaker inoperative.

Now that sounds scary! It obviously is not failsafe.


#78349 - 09/13/01 06:42 PM Re: Outbuilding grounding  
pauluk  Offline
Member
Joined: Aug 2001
Posts: 7,520
Norfolk, England
Quote
Originally posted by Dspark:
>[b]an extra rod must NOT be used, or it could render the breaker inoperative.
Now that sounds scary! It obviously is not failsafe.[/B]


It can actually get a lot more complicated that just not installing an extra rod, because any parallel ground path could desensitize the ELCB.

If someone were to run a copper pipe underground to bring water to the garage, and a water heater was installed, the ground connection via the pipework could cause problems.

That's one of the reasons why this type of ELCB was/is so problematical and they are no longer made.

Fortunately, I've been able to "educate" the local plumber I work with regularly so that he can identify these units and advise the customer to get it checked or changed if he's run new pipes.


#78350 - 09/14/01 06:38 PM Re: Outbuilding grounding  
sparky  Offline
Member
Joined: Oct 2000
Posts: 5,311
parallel ground path

ah ha! key word Paul!

we cannot run #2 ( a 3-wire, hot , hot and dual usage N & G) with any mettallic pipe interconecting the mother & outbuilding here .
very perseptive of you Paul

however, the utility will commonly run the same 3-wire configuration to 5 homes off the same X-former with city water lines to all.
crap!, i'm foamin' at the mouth again..
[Linked Image]


#78351 - 09/14/01 07:52 PM Re: Outbuilding grounding  
pauluk  Offline
Member
Joined: Aug 2001
Posts: 7,520
Norfolk, England
Quote
Originally posted by sparky:
[b]parallel ground path
ah ha! key word Paul!
we cannot run #2 ( a 3-wire, hot , hot and dual usage N & G) with any mettallic pipe interconecting the mother & outbuilding here . [/B]


I can see how that makes sense; you don't want the part of the neutral current flowing through the pipework.

Quote

however, the utility will commonly run the same 3-wire configuration to 5 homes off the same X-former with city water lines to all.


I hadn't considered that before, but I certainly see your point. Typical bureaucracy I suppose: "Do as I say, not as I do."

There are a few special cases here where PME is not allowed to be used. One such place is in a petrol (gas) station where a broken neutral could result in the load current flowing through fuel lines.


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