Detached garage and grounding

[goldstar]

I apologize for bringing this topic up again, I know it's been beat to death. I'm interested in the logic behind the Code section. I researched and found this older thread :

detached garage one circuit; grd rod, disconnect required ?

For a residential detached garage with only one branch circuit (20 amp, single pole), is a ground rod or disconnect required ? Thank you.

forums.mikeholt.com

So, as I understand it, if you run a single 20 (or 30) amp, 120V circuit to a detached garage you don't need a supplemental ground rod to re-establish the ground. You can also use one snap switch as a disconnect means. However, if you terminate that single circuit in a 4-circuit main lug breaker panel, that single circuit becomes a feeder and now you'll have to install a ground rod.

If someone can explain the logic behind that I would appreciate it

 

[thomasbl7]

You terminated into a distribution panel. It's no longer a branch circuit, it's a feeder.

 

[retirede]

You terminated into a distribution panel. It's no longer a branch circuit, it's a feeder.

I think he means the technical logic, not the code logic. What is it about a feeder that makes it need a rod while a branch circuit is fine without one?

I don’t have a really good answer.

 

[LarryFine]

What is it about a feeder that makes it need a rod while a branch circuit is fine without one?

The presence of the OCPD(s) in the building.

 

[don_resqcapt19]

I apologize for bringing this topic up again, I know it's been beat to death. I'm interested in the logic behind the Code section. I researched and found this older thread :

detached garage one circuit; grd rod, disconnect required ?

For a residential detached garage with only one branch circuit (20 amp, single pole), is a ground rod or disconnect required ? Thank you.

forums.mikeholt.com

So, as I understand it, if you run a single 20 (or 30) amp, 120V circuit to a detached garage you don't need a supplemental ground rod to re-establish the ground. You can also use one snap switch as a disconnect means. However, if you terminate that single circuit in a 4-circuit main lug breaker panel, that single circuit becomes a feeder and now you'll have to install a ground rod.

If someone can explain the logic behind that I would appreciate it

You can't run a 30 amp circuit because the code requires a 20 amp garage receptacle circuit an installing the 20 amp OCPD at the garage makes the 30 amp circuit a feeder.

 

[Little Bill]

Leaving out the 30A circuit, why would a single 20A or 20A MWBC not need a grounding system and a feeder would?

 

[winnie]

I don't believe that there is strict technical logic behind this. IMHO _if_ a feeder to a detached garage requires a new set of grounding electrodes, then the same exact physics will apply to a single 20A circuit. (Not going down the rabbit hole of if grounding electrodes are needed at all....)

I believe that the logic applied is pragmatic. Having a light and receptacle in a garage is likely safer than not. So you don't want to make the requirements for such a basic installation to difficult. If someone is installing a 100A feeder to a garage, adding grounding electrodes is a small increase in cost. But for just a single circuit? Pounding the ground rod might be more expensive than the wire needed.

If you want to know the real reason you will need to dig through the CMP discussions on the issue.

Jon

 

[wwhitney]

(Not going down the rabbit hole of if grounding electrodes are needed at all....)

But that's the interesting question. Absent overhead high voltage lines that may fall on the feeder or outbuilding, what fault scenario is there for which having a GES at the outbuilding will reduce problems, compared with relying on the GES at the service?

Cheers, Wayne

 

[winnie]

But that's the interesting question. Absent overhead high voltage lines that may fall on the feeder or outbuilding, what fault scenario is there for which having a GES at the outbuilding will reduce problems, compared with relying on the GES at the service?

Cheers, Wayne

Agreed that this is an interesting question. However the answer to this question is the _same_ weather you have a feeder going to the outbuilding or a single branch circuit. That is why I figured discussing if grounding electrodes should be required in either case was a rabbit hole with respect to the present OP question.

-Jon

 

[LarryFine]

The presence of the OCPD(s) in the building.

Here's what I mean:

When a circuit is run to an outbuilding, the circuit originates in the panel where the OCPD is.

With a feeder and panel, the circuit(s) originate in the outbuilding, so a new GES is needed.

I suppose the logic is to have a good earth reference for the source, just like a service does.

Thus, a GES for an outbuilding serve the same (debatable) purpose it serves for a service.

 

[retirede]

Here's what I mean:

When a circuit is run to an outbuilding, the circuit originates in the panel where the OCPD is.

With a feeder and panel, the circuit(s) originate in the outbuilding, so a new GES is needed.

I suppose the logic is to have a good earth reference for the source, just like a service does.

Thus, a GES for an outbuilding serve the same (debatable) purpose it serves for a service.

That makes perfect sense.

But my train of thought is that there is nothing magical about a feeder that would make the building more safe by adding a GES when it’s perfectly safe to have a branch circuit without a GES.

I think your inclusion of the word “debatable” is the key to this discussion, but would drive the discussion into Winnie’s rabbit hole.

 

[kec]

So the questions are referring to a detached [Garage] Then this also applies to, let's say a 10x10 shed?

 

[infinity]

So the questions are referring to a detached [Garage] Then this also applies to, let's say a 10x10 shed?

It applies to any separate structure.

 

[Fred B]

So the questions are referring to a detached [Garage] Then this also applies to, let's say a 10x10 shed?

It applies to any separate structure.

Around here a 10x10 shed is considered a portable non perminant structure that is hauled in on a trailer. Some are hooked up like a RV with a L14-30 or 50. Does that need a ground rod?

 

[LarryFine]

So the questions are referring to a detached [Garage] Then this also applies to, let's say a 10x10 shed?

Even a panel on a post.

 

[tom baker]

If someone can explain the logic behind that I would appreciate it

Based on what I have read from papers at IEEE, many grounding classes from Mike Holt and being an electrical instructor for 20+ years, I will share some possible reasons.
There has been a requirement for many years, perhaps since 1920 to have a grounding electrode at at separate building. There possibly was research done by IEEE or its predecessors on grounding and lightning, and some of the documentation may be lost.
Originally electrical systems were not grounded (connected to earth). There was a long debate over grounding, as a grounded system presented a shock hazard. Eventually the debate was settled and we ended up with a grounded conductor.
In Section 250.4 General Requirements for Grounding and Bonding,
A 1 limit the voltage imposed by lightning, contact with higher voltage lines, and stablize the voltage to earth
A 2 Connected to earth to limit the voltage to ground

1 is obvious, 2 not so much. In 2, its a nearby lightning strike that will induce voltage on metal parts

IMO
For separate buildings, once you install electricity, you accept the risk of electrical equipment being damaged by lightning or induced voltages
For a single or MWBC, there won't be a lot of electrical equipment to be damaged
Once you have feeder, either 40 amps or 400 amps, there is more electrical being installed and greater risk. The CMP drew a line in the sand and for a feeder a GES is required.

In 2002 the NEC changed and said, run a 4th wire to a separate building, don't use the neutral for a EGC, unless there is no parallel paths
Then in 2008? the NEC said, hey, always run 4 wires.
Where I got confused (and many others) is that if I ran 4 wires, I didn't need a ground rod. I was confusing grounding and bonding. The NEC always required ground rods, or a GES, at a separate building, and let us use the white wire for grounding.

The rules in 250.4 (performance) are often over looked but they lay out all the prescriptive requirements (do it this way, look at 250.8 for a great example of how to do it)

250. A 2 was important when we ran GRC in buildings to minimize fire from loose joints from induced voltages

 

[tom baker]

Even a panel on a post.

See article 100 Definition of Structure. Think Field of Dreams

 

[LarryFine]

Some are hooked up like a RV with a L14-30 or 50. Does that need a ground rod?

The building won't, but the box will if it contains OCP.

 

[tom baker]

Around here a 10x10 shed is considered a portable non perminant structure that is hauled in on a trailer. Some are hooked up like a RV with a L14-30 or 50. Does that need a ground rod?

How I answer a question like this is to determine what definition applies. Then we can find the appropriate rules. This little building, does it meet the definition of a moble home? Its it permitted and inspected?

 

[wwhitney]

There has been a requirement for many years, perhaps since 1920 to have a grounding electrode at at separate building.

OK, so based on the graphic you posted, the best hypothesis I can come up with for why to have an additional GES at the outbuilding, rather than rely on the GES at the building with the service, is the following. I would appreciate any comments or other reasons for the additional GES, as I'm not sure if this is correct:

Say you have a nearby by but not direct lightning strike. That could induce a voltage gradient that travels through the earth across the ground on which both buildings stand. [Is this physically plausible for lightning?] I'm imagining a single traveling spike, a wave like you'd get if you took a rope and jerked it up and then down once.

Say the voltage wave hits the ground under the building with the service first. The bonded metal components in the building are locally earthed. The earth and those metal components rise and fall in potential together, so no (or low) potential difference between bonded metal close to earth results.

Now say the voltage wave hits the outbuilding with no GES, and the potential at the building with the service has fallen back towards normal. The bonded metal components in the outbuilding are connected to earth potential near the service, while local earth at the outbuilding is at a different potential. You could get the "arcing" referred to in the graphic.

If instead the outbuilding feeder EGC and bonded metal components are re-earthed at the outbuilding via a second GES, then those components should instead rise and fall in potential along with local earth, so no (or low) potential difference results locally. The two ends of the feeder EGC will be at different potentials, so current will flow on the feeder EGC, but that's better than having arcing.

Thoughts?

Cheers, Wayne