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Outbuilding grounding

Posted By: pauluk

Outbuilding grounding - 09/10/01 09:37 PM

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?
Posted By: sparky

Re: Outbuilding grounding - 09/10/01 10:55 PM

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
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Posted By: electure

Re: Outbuilding grounding - 09/11/01 01:29 AM

Paul,
I believe you've just opened 1 of Pandora's boxes. [Linked Image]
Posted By: pauluk

Re: Outbuilding grounding - 09/13/01 08:24 PM

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?
Posted By: sparky

Re: Outbuilding grounding - 09/13/01 08:39 PM

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]
Posted By: pauluk

Re: Outbuilding grounding - 09/13/01 10:09 PM

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.
Posted By: Anonymous

Re: Outbuilding grounding - 09/13/01 10:22 PM

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

Now that sounds scary! It obviously is not failsafe.
Posted By: pauluk

Re: Outbuilding grounding - 09/13/01 10:42 PM

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.
Posted By: sparky

Re: Outbuilding grounding - 09/14/01 10:38 PM

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..
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Posted By: pauluk

Re: Outbuilding grounding - 09/14/01 11:52 PM

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.
Posted By: sparky

Re: Outbuilding grounding - 09/15/01 12:20 AM

I suppose ( hack...choke...pitouwee...)your right [Linked Image]

There has been equal concern about a pad mount utility x-former feeding (same 3-wire) to a meter. Think of it within reaching distance, hot summer day, sweaty palms [Linked Image]
.....sure makes your isolated nuetral look good!
Posted By: pauluk

Re: Outbuilding grounding - 09/15/01 04:01 PM

Point taken. I woudn't want to be leaning against that service conduit if the neutral went open at the pole.

Quote
Originally posted by sparky:
.....sure makes your isolated nuetral look good!


Swings & roundabouts, I think. It's great with the old town systems with ground via the cable armor, but not so good on rural overhead services where you're then relying on just the local rod for grounding everything.

That's when we start getting involved with a whole-house GFI. My biggest gripe with this arrangement is that a simple ground fault on one branch takes out the entire supply to the building. The older systems have those awful voltage ELCBs I've complained about.

It's after struggling with one of these that PME with bonded N & G looks very attractive!
Posted By: pauluk

Re: Outbuilding grounding - 09/15/01 04:03 PM

Going onto a slightly different angle on my original query, what neutral/grounding arrangement is used on the distribution within a mobile home park?
Posted By: sparky

Re: Outbuilding grounding - 09/18/01 01:10 AM

Hi Paul;
mobile homes are all 4-wire ( isolated neutral) feeders here.

the term 'feeder' meaning after the main disco

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Posted By: pauluk

Re: Outbuilding grounding - 09/18/01 10:16 AM

Similar situation here: The PME system used in some houses is not employed on mobile homes, so it's always a 3-wire cable throughout (only one hot leg, remember).

When you say the main disconnect, do you mean the main at the site service panel or the main disconnect at each mobile home? In other words, is it 3-wire with combined N/G up to the post serving each home then separate N/G, or 4-wire right back to the site's service entrance?
Posted By: sparky

Re: Outbuilding grounding - 09/18/01 11:03 PM

Paul;
the 3 wire ( H,H N&G ) would be up to the first disco as fed by the utility, 4 wire afterwards, regardless of how many disco's in one power line.
Posted By: pauluk

Re: Outbuilding grounding - 09/18/01 11:25 PM

Gotcha. Pretty much the same N/G isolation as we have on mobile home parks then.

From the earlier discussion about outbuildings, I assume that you also drive a separate ground rod at each home paralleled up to the feeder ground wire.
Do you also bond the chassis/body to the ground busbar?

Come to think of it, I know that aluminum siding is popular on regular homes there. Is that normally bonded as well?
Posted By: sparky

Re: Outbuilding grounding - 09/19/01 12:33 AM

Paul;
normally G-rods would be @ the disco only. A mobile home disco is required within 30'(but not on) a mobile home here. That's where we bond the GEC and the rods.

there has been some discussion on this that get's rather flamed, but that's usually the norm here.....

this is a very good example of theory, terminology and NEC intent not having a harmonious existence in 250-32 .

the was, the last cycle (99') an ad-hoc committee that turned 250 inside out. i think thier goal was to clean it up, there is now a cross reference guide in the back for all the changes. i guess they tried....but it still reads hard
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Posted By: sparky

Re: Outbuilding grounding - 09/19/01 12:35 AM

oops,,
forgot your other Q,

yes the chasis is bonded to the MH panels G-bar, this in turn bonds al other connecting metal
Posted By: pauluk

Re: Outbuilding grounding - 09/19/01 10:40 AM

Thanks Sparky.

Our IEE Regs. always seem a bit vague to me when it comes to mobile homes (or "caravans" in British terminology).

The section on caravans specifies that where twin & earth (Romex) is run along chassis members there must be an insulating material between them. They also specify that the main earth terminal at the intake be insulated from the chassis, which tends to suggest that they would like the body completely separated from the grounding system.

This seems to be totally at odds with the regs. in the general section on earthing which state that all extraneous metalwork must be bonded to the main earth bar.

The confusing thing about the caravan section is that when it comes to a bath or shower compartment, they switch to "bond everythng" mode, which means that the whole body is likely to be grounded indirectly via this bonding anyway.

See, it's not just the NEC which can be vague...
Posted By: sparky

Re: Outbuilding grounding - 09/19/01 09:00 PM

LOL,
perhaps the electrical industry as a whole has a 'retentive' nature concerning grounding !
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