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).]

Paul,

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.

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.

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

-Jon

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.

Roger

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.

OT
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.

Glen, 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".

Roger

Thanks everyone!

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


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.

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.

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".

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.

The only problem with that analysis is a second building with no bonded metalic paths can become another virtual "main" in respect to bonding the neutral.

There is certainly a lot to consider over this issue.

Looking at the case of a detached garage fed via 3-wire cable with bonded neutral and ground, is it really much different than two houses fed from the same transformer, each with a bond a the main service?

In fact if the two homes are fed via metallic water pipes from a common system, it could be considered worse, since with metallic pipes to the detached garage you would need the isolated 4-wire feeder.

Paul, you are correct, but this is outside of the NEC. (POCO's are under the NESC)

I guess the NEC is trying to keep this potential problem at bay on their side.

Roger

Pun intended, Roger?

As a related issue, have you actually looked at the unbalanced current going into the ground from a pole pig?
I get about 4a on the one in front of my house measuring the 8ga solid coming down the pole. My GEC is contributing a couple amps of that.

Paul, actually as much as I want to say yes, I didn't think about it to after I hit the "Submitt Reply" button.

Roger

That's 'cuz you're not supposed to use copper for conduit!

Aw, c'mon now..... You can use it for both surely?

After you've pulled wires through there's still plenty of room in the pipe for the water to flow.......

I would love to find a source of appropriately rated RMC in _brass_, with associated fittings. Not for any current job, but because I have an image of polished brass pipe used for an elegant exposed installation.

-Jon

Greg,

One house that had problem with pin hole leaks had a problem with the grounding of the house. A plumber friend of mine called me up and told me that one house kept having pin hole leaks. I said, I am an electrician not a plumber. He said, he was running out of ideas and maybe I could come up with something. Long story short, all I could find was a plastic water filter in the middle of the house. It broke the copper water pipe and didn't have a jumper over it. I ask the plumber where the leaks were, front of house or back, he said back only. Put a jumper across the filter and never had a leak since.

Across the river from me in Cape Coral this went on for years. The copper pipe association blamed the RO water plant, the water company blamed the electricians and the electricians said it was the wrong grade of pipe. This paper/rock/scissors thing swapped around a bit, but as far as I know they never really decided on one bad guy.
I think it was a little of all three. I'm sure there were grounding issues in this sandbox we live in.

Reno,
Now I've got four fingers in front of me here....
How does that work?.

{Edited for a spelling error}

[This message has been edited by Trumpy (edited 08-12-2005).]

One of the "experts" is obviously a polititian who can speak out of both sides of his mouth at the same time.