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#94626 - 08/04/05 08:16 AM Grounding of Detached Garage  
pauluk  Offline
Joined: Aug 2001
Posts: 7,520
Norfolk, England
A thread on the British IEE Forum is talking about grounding arrangements to a detached garage. As some of you are aware, we do things rather differently over here!

One poster is comparing British practice with the American NEC, but as far as I can tell he's got his facts rather distorted.

So, can one of our NEC-experts confirm that I have the right interpretation here please?

As I understand it, you have two ways to feed a detached building and be code compliant:

#1. Run a 3-wire feeder, bond the neutral to case/ground at the garage panel, and install a ground rod.

#2. Run a 4-wire feeder, install the garage like any other sub-panel with neutral and ground separate, but a grounding electrode is still needed (obviously to the ground bar only, not the neutral).

Correct? Or am I off-base somewhere?

Edited to add:

The poster is also claiming 30mA GFI protection for the feeder, which is definitely not required, is it?

Receptacles in the garage would need normal 6mA GFI protection under 210.8 (with exceptions, e.g. inaccessible for fixed appliance).

[This message has been edited by pauluk (edited 08-04-2005).]

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#94627 - 08/04/05 08:25 AM Re: Grounding of Detached Garage  
harold endean  Offline
Joined: Feb 2002
Posts: 2,233
Boonton, NJ

I believe that you are correct. You can wire with either a 3 wire or a 4 wire feeder. With the 3 wire feeder the neutrals and the grounds go together on the same bar. In either case I believe the ground rod is required to go from a ground rod ( or concrete encased electrode to the grounding bar. The reason I was told for the ground rod is for lightning. It will help to disapate the lightning from going throughout the building.

#94628 - 08/04/05 08:29 AM Re: Grounding of Detached Garage  
BobH  Offline
Joined: Oct 2004
Posts: 135
Newfane, N.Y USA
Ground rod required either way. With a three wire there can't be any other metallic paths between garage and house. The gfci is the typical 6ma required for receptacles.

#94629 - 08/04/05 09:38 AM Re: Grounding of Detached Garage  
winnie  Offline
Joined: Sep 2003
Posts: 649
boston, ma
Not for a _feeder_ to a subpanel, but for a single branch circuit (including a multiwire circuit) to a detached garage, you can use GFCI protection at the source side of the feeder in order to reduce burial depth requirements. A direct buried cable in this application normally requires 24" burial depth, but if GFCI protected only requires 12" burial depth. 300.5 does not specify the trip rating of the GFCI, and it might make sense to use a 30mA GFCI breaker combined with 6mA GFCI receptacles for coordination.

Of course, GFCI protection is not viable if the neutral is bonded to the ground at the garage.

Okay, not directly applicable to the current thread, since the current thread describes a subpanel [Linked Image]


#94630 - 08/04/05 06:14 PM Re: Grounding of Detached Garage  
Roger  Offline
Joined: May 2002
Posts: 1,716
Paul, you are correct. The article and section would be 250.32(B)(1)and(2).

As already pointed out, a second metallic path (conduit, water piping, phone conductors, etc...) will eliminate 250.32(B)(2) as an option.

This feeder is not required to have GFP protection. I think he has heard of the feeder as described in 215.10 which is required to be GFP protected and is confusing it as a general requirement.

A particular point of interest would be the article 100 definitions of GFCI (Ground-Fault Circuit Interrupter) verses GFP (Ground-Fault Protection of Equipment) the latter being the 30ma and higher protection.


#94631 - 08/04/05 09:05 PM Re: Grounding of Detached Garage  
gfretwell  Offline

Joined: Jul 2004
Posts: 9,060
I would tend to dissagree about the phone (LAN or whatever) being a parallel path if it was not grounded/bonded at the garage end.

Does anyone really use metal plumbing these days?
I was the last guy I ever saw using copper and people told me I was screwing up.

Greg Fretwell

#94632 - 08/05/05 06:34 AM Re: Grounding of Detached Garage  
Roger  Offline
Joined: May 2002
Posts: 1,716
I would tend to dissagree about the phone (LAN or whatever) being a parallel path if it was not grounded/bonded at the garage end.
and that is specifically spelled out in 250.32(B)(2) "where there are no continuous metallic paths bonded to the grounding system in both buildings or structures involved".


#94633 - 08/05/05 06:37 AM Re: Grounding of Detached Garage  
pauluk  Offline
Joined: Aug 2001
Posts: 7,520
Norfolk, England
Thanks everyone! [Linked Image]

I was pretty sure I'd got it right, but just wanted to check. I've set the record straight. The thread is here, for anyone interested:

Exporting PME, Detached Garage

GFCI (Ground-Fault Circuit Interrupter) verses GFP (Ground-Fault Protection of Equipment)

That's quite possibly a distinction lost on most people over here, since the most sensitive RCD (Residual Current Device) generally used here is 30mA. More sensitive types are employed only in specialized applications.

Does anyone really use metal plumbing these days?

Plastic (e.g. polyethylene) piping is now almost universally installed here for main service lines and underground sub-feeds. Our electrical code was amended way back in 1966 to stop the use of the water line as the sole grounding electrode due to the increasing use of plastic by the utility companies.

When it comes to the plumbing within a building though, copper is still very widely used, with either soldered or compression couplings.

#94634 - 08/05/05 12:38 PM Re: Grounding of Detached Garage  
gfretwell  Offline

Joined: Jul 2004
Posts: 9,060
Here in Florida we have had enough law suits over pin holed copper pipe to where builders are not willing to use it.
There is still no consensus as to whether it is a pipe problem, a water problem or an electrical problem. Suits are still going on.

There is also a very significant labor/material saving so copper just is not being used.

As my favorite builder's plumber says "anyone can paste pipe".

Greg Fretwell

#94635 - 08/05/05 06:16 PM Re: Grounding of Detached Garage  
renosteinke  Offline
Cat Servant
Joined: Jan 2005
Posts: 5,316
Blue Collar Country
Paul, don't feel bad. On this very issue, the IAEI (Inspectors' trade group) asked three of its' experts, and got four different answers! The NEC, in part due to poor grammar, is still confused -even self-contradicting- despite some major improvements to section 250 in the last edition.

My "solution" to the mess is to refer back to the purposes of the different part of the system.

The ground rod, and entire grounding system, is there primarily for lightning protection. So, a detached building gets a ground rod- with this rod ultimately bonded to the grounding system of the main building as well.
Exception: if the garage is separated by only a very narrow "breezeway," has only one or two circuits serving it, and is considerably shorter than everything else around- I might overlook this part. The "ground wire" would still get connected to the grounding system of whatever feeds the garage.
Indeed, the ground wires of the two buildings are connected, no matter what.

Now, for the "neutral." This is separated from the "ground" at the main panel (service supply).....and forever stays separate.
You most definitely do not want to connect the "neutral" of the garage to the ground network in the garage....or you may create a voltage gradient (stray voltage) between the two ground rods. Murphy's law, and all that.

So, a detached garage that's getting a panel of its' own will have four wires going to it....two "hots", one "neutral," and a "ground." It will also have a ground rod (or other approved electrode).

If the building is getting only one circuit- due to code language, read this as "one or two hots sharing a neutral"- then code allows you to forgo the ground rod, and only have the grond wire connecting the two buildings.

That's how I read it, and apply it. I know there are different opinions, and the exact circumstances may change things, but I look at it this way:
- Ground rod for lightning
- Ground wire for clearing faults
- Neutral and ground NEVER touch after the main.

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