Detached garage and grounding

[tom baker]

Wayne, nice explanation. Another way arcing can happen is induced voltage thru the air, arcing at loose fittings

 

[Fred B]

Thanks Tom and Wayne for the explanations, so why if it's better to have a seperate GE at the remote building and in fact code, why the need for a 4th grounding conductor (120/240 1ph) is there another reason beside to trip overcurrent device related to the feeder? Is there a minimum maximum distance issue that might explain why RV connection doesn't get it's own GE? A RV would experience the same strike potential as described that says the seperate GE for a remote building is needed.

 

[goldstar]

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.

Would it make a difference if I changed the breaker from a 30A to a 20A ?

 

[goldstar]

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

That's the logic I'm trying to find out.

 

[goldstar]

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

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Thanks for the explanation and graphic Tom. I think 250.4 A(1) (for the most part) answers my question and if that's the only reason I'll leave it at that. However, aside from "that's what the Code says" I still don't see a difference in running a 20A, 120V circuit to a 3-car detached garage, installing a snap switch for a disconnect, then installing (3) receptacles for door openers, 3 luminaires with associated switches, 3 GFCI receptacles and a motion sensor light at the front entrance all on that one circuit and not needing a ground rod. Whereas, if I terminate that 20A circuit into a distribution panel in the first garage, I then need the rod. It just doesn't make sense to me.

Thanks to all for your replies and opinions.

 

[retirede]

However, aside from "that's what the Code says" I still don't see a difference in running a 20A, 120V circuit to a 3-car detached garage, installing a snap switch for a disconnect, then installing (3) receptacles for door openers, 3 luminaires with associated switches, 3 GFCI receptacles and a motion sensor light at the front entrance all on that one circuit and not needing a ground rod. Whereas, if I terminate that 20A circuit into a distribution panel in the first garage, I then need the rod. It just doesn't make sense to me.

That sums up my opinion as well.

 

[tom baker]

We can look at not needed a ground rod in a historical perspective.
The rule was in place long before we had motion sensing luminaires, GFCIs, door openers, etc. A ground rod or rods can be added to a branch circuit.

 

[goldstar]

We can look at not needed a ground rod in a historical perspective.
The rule was in place long before we had motion sensing luminaires, GFCIs, door openers, etc. A ground rod or rods can be added to a branch circuit.

Once again Tom, thanks for your input and opinion on this. If the CMP's in the future were to make a Code change and state that ALL detached structures should have the ground re-established by pounding in a rod (irrespective of whether a branch circuit or a feeder was installed) on the outside chance that lightening may strike the structure or that some other voltage may be induced on the wiring, I'll buy into that theory. In the future I will continue to install wiring in accordance with the Code but you have a long way to go to convince me that installing a rod for a detached garage is required if you terminate any 120V wiring in a breaker panel but not required if you do not. Again, just looking for the logic behind the rule.

 

[petersonra]

What is the point of having a couple extra ground rods out at the detached building anyway. People act like this is somehow protecting the building from a lightning strike, but it can't, and doesn't. it MIGHT protect a person from elevated voltage levels with respect to ground if there was a nearby strike but that seems pretty dubious too.

 

[wwhitney]

What is the point of having a couple extra ground rods out at the detached building anyway. People act like this is somehow protecting the building from a lightning strike, but it can't, and doesn't. it MIGHT protect a person from elevated voltage levels with respect to ground if there was a nearby strike but that seems pretty dubious too.

I reasoned out the hypothesis in post #20. Which was a synthesis on my part, but seems to be consistent with what I've read on the topic. So do you see any issues with that hypothesis, or are you just saying that you're not convince the likelihood is high enough to be worth installing an additional GES?

One conclusion I draw from the hypothesis in post #20 is that it's not so much being "detached" that matters as the overall physical spread over the earth. E.g. a small house with a small outbuilding very close by would be less affected than a large house with a distant outbuilding. Which would also suggest that very large buildings should have multiple spread out GESs, or else a ground ring.

Cheers, Wayne

 

[wwhitney]

Once again Tom, thanks for your input and opinion on this. If the CMP's in the future were to make a Code change and state that ALL detached structures should have the ground re-established by pounding in a rod (irrespective of whether a branch circuit or a feeder was installed) on the outside chance that lightening may strike the structure or that some other voltage may be induced on the wiring, I'll buy into that theory.

I think it's not hard to read the current code as saying (as previously suggested) that a GES would be a good idea in all cases, but we won't force it on you unless there's a feeder involved, as a matter of convenience and practicality.

In the future I will continue to install wiring in accordance with the Code but you have a long way to go to convince me that installing a rod for a detached garage is required if you terminate any 120V wiring in a breaker panel but not required if you do not. Again, just looking for the logic behind the rule.

Certainly that's true where "required" means "required by the NEC" but I assume you mean "required by physics."

BTW, if a 20A MWBC is sufficient for your loads, but you still want to have a breaker panel, you can do that without having to install a GES by simply using oversized breakers in your breaker panel. Then the 20A breaker on the supply to the outbuilding is protecting all the wiring, so everything is a single branch circuit, and the breakers are just acting as switches.

Cheers, Wane

 

[goldstar]

BTW, if a 20A MWBC is sufficient for your loads, but you still want to have a breaker panel, you can do that without having to install a GES by simply using oversized breakers in your breaker panel. Then the 20A breaker on the supply to the outbuilding is protecting all the wiring, so everything is a single branch circuit, and the breakers are just acting as switches.

That sounds logical but based on what we've discussed here so far I don't think that would be an acceptable solution to the ground rod requirement and/or be Code compliant. Just curious to know what others think about this.

 

[jimport]

In one of Mike's videos he shows how multiple rods may make it easier for a surge to enter a building. I believe it might have been with rods for equipment. Also, given the short time transient of a lightning strike how long would someone be exposed to an increased shock risk if no electrode were in place?