Grounding rod philosophy

[Raisedonadeere]

The second grounding rod for my detached garage is only about 12 feet from the grounding rods at my house meter base/distribution box I am connecting to.
I drove the rods into sides of the connecting ditch to the garage. As I was running my ground wire and connecting to the rods I wondered what the rules would be about continuing my ground wire to the ground rods at the meter base. I exposed the end of one of them when trenching to the meter base and my grounding wire will reach. Seems like I could put that extra length to good use

Someone might ask why bother since two rods 6' apart is enough etc., but what if the two complete grounding systems were connected since they are so close to each other?

I even wonder why I could not just have used the house grounding system anyway, but that is another question.

 

[wyliesdiesels]

The second grounding rod for my detached garage is only about 12 feet from the grounding rods at my house meter base/distribution box I am connecting to.
I drove the rods into sides of the connecting ditch to the garage. As I was running my ground wire and connecting to the rods I wondered what the rules would be about continuing my ground wire to the ground rods at the meter base. I exposed the end of one of them when trenching to the meter base and my grounding wire will reach. Seems like I could put that extra length to good use

Someone might ask why bother since two rods 6' apart is enough etc., but what if the two complete grounding systems were connected since they are so close to each other?

I even wonder why I could not just have used the house grounding system anyway, but that is another question.

The 2 GESs are connected to each other via the EGC between the main panel and subpanel. Did run a 4-wire feeder?

the purpose of grounding rods is mainly to have a very low impedance pathway to earth for shunting lightning. Its best to have a system on each building

 

[andyvh1959]

When I prepped the slab site for my detached 24x28 garage, I pounded a 10' ground rod inside the forms so when the concrete was poured the ground rod would project from the floor near the outside wall. But I only did one ground rod. Does the NEC require two ground rods at some distance from each other? The garage is about 20' from the house and the power will come from the meter base on the back of the house.

 

[alfredeneuman]

But I only did one ground rod. Does the NEC require two ground rods at some distance from each other?

1 rod is OK if you can prove it is 25ohms to ground or less, otherwise you have to drive 2.
The test equipment to do this is mega$ to buy or rent. It's much more cost effective to just bite the bullet and drive 2 at the same time (You'll already have the tools out)
They need to be at least 6' apart.

 

[ycgoat]

1 ground rod at the detached garage is correct.

From a theoretical stand point you can not have 2 many ground rods, but you can have them connected wrong and create a ground loop. So do not connect the Garage ground rod to the house ground rod. Ensure a separate ground wire between the main panel and sub panel, and that the neutral in the sub panel is not connected to ground in the sub panel.


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[alfredeneuman]

1 ground rod at the detached garage is correct.

NEC 250.53(A)(2) Supplemental Electrode Required
A single rod, pipe, or plate electrode shall be supplemented by an additional electrode of a type specified in 250.52(A)(2) through (A)(8). The supplemental electrode shall be permitted to be bonded to one of the following:
(1) Rod, pipe, or plate electrode

Exception: If a single rod, pipe, or plate grounding electrode has a
resistance to earth of 25 ohms or less, the supplemental electrode shall
not be required.

 

[andyvh1959]

Thanks guys!! That's what I needed to know. Now, the other aspect. I also located the electrical conduit up through the slab so the SE cable comes up inside the detached garage to the 100 amp main breaker panel I've installed. When I pounded the one ground rod into the dirt, I located it about 12" away from the conduit where both come up through the slab. The SE conduit is the grey ABS product for electrical service, and I know both the conduit and ground rod went straight down and are not in contact to each other. So is there any issue with the ground rod being too close to the SE conduit?

 

[ycgoat]

NEC 250.53(A)(2) Supplemental Electrode Required
A single rod, pipe, or plate electrode shall be supplemented by an additional electrode of a type specified in 250.52(A)(2) through (A)(8). The supplemental electrode shall be permitted to be bonded to one of the following:
(1) Rod, pipe, or plate electrode

Exception: If a single rod, pipe, or plate grounding electrode has a
resistance to earth of 25 ohms or less, the supplemental electrode shall
not be required.

I will have to look it up later but I believe the ground rod is the supplemental ground


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[TRWham]

I will have to look it up later but I believe the ground rod is the supplemental ground


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It might be, but only if another acceptable grounding electrode exists. With the prevalence of plastic water pipe, Ufers and ground rods are the only practical options in most residential construction anymore. My house now has 2 grounds rods because the original copper water pipe was replaced with plastic.

 

[pattenp]

I will have to look it up later but I believe the ground rod is the supplemental ground


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andyvh1959 said he drove only one ground rod at his detached garage. So how do you figure it is the supplemental ground? He did not indicate there is a primary grounding electrode other than the one rod.

 

[pattenp]

….. I also located the electrical conduit up through the slab so the SE cable comes up inside the detached garage to the 100 amp main breaker panel I've installed. ….

I sure hope you did not use actual SE cable in conduit underground because it's not allowed.

 

[ycgoat]

andyvh1959 said he drove only one ground rod at his detached garage. So how do you figure it is the supplemental ground? He did not indicate there is a primary grounding electrode other than the one rod.

I have always put in 1 rod at sheds and garages with feeder ground as Primary Ground.


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[pattenp]

I have always put in 1 rod at sheds and garages with feeder ground as Primary Ground.


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The ground wire in the feeder is the equipment ground and does not fulfill the requirement of the grounding electrode system. Equipment ground is for clearing fault current whereas the grounding electrode system is to divert lighting strikes. Two different purposes with separate requirements.

 

[dave*99]

I have always put in 1 rod at sheds and garages with feeder ground as Primary Ground.


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The feeder ground does not meet the requirement of being an electrode. Look at the code excerpt quoted on this thread for clear proof of that.

The goal of using the electrode(s) Is to establish a low impedance connection to earth nearby the structure. Your feeder ground does not accomplish that.

The code is clear on this. Use 1 local electrode with proof of 25 ohms impedance to earth or use 2 local electrodes to avoid the requirement to prove impedance.

 

[wyliesdiesels]

1 ground rod at the detached garage is correct.

No it is not correct. Please go read the NEC. As has been said, 2 are required, unless one can prove 25ohms or less to earth.

From a theoretical stand point you can not have 2 many ground rods, but you can have them connected wrong and create a ground loop.

This is not true either. No such thing as a ground loop when it comes to the GES. Ground loops have to do with audio systems. This is why audio direct boxes have grnd lifts on them.

So do not connect the Garage ground rod to the house ground rod. Ensure a separate ground wire between the main panel and sub panel, and that the neutral in the sub panel is not connected to ground in the sub panel.

the separate ground wire you mentioned does connect the grounding electrodes at both buildings together.

Thanks guys!! That's what I needed to know. Now, the other aspect. I also located the electrical conduit up through the slab so the SE cable comes up inside the detached garage to the 100 amp main breaker panel I've installed. When I pounded the one ground rod into the dirt, I located it about 12" away from the conduit where both come up through the slab. The SE conduit is the grey ABS product for electrical service, and I know both the conduit and ground rod went straight down and are not in contact to each other. So is there any issue with the ground rod being too close to the SE conduit?

What type of wire did you use? Was it SEU or SER? Does it have an outer sheath or jacket?

If so, then this is the wrong wire and needs to be pulled out. It is not permitted to be used underground and it will rot.

I will have to look it up later but I believe the ground rod is the supplemental ground

How do you make that determination without knowing if the OP has a primary electrode?

I have always put in 1 rod at sheds and garages with feeder ground as Primary Ground.

Apples and oranges. You obviously do not understand the difference between a grounding electrode and an equipment grounding conductor. They serve 2 entirely different functions. you never want to confuse the 2. Please go read the electrical FAQs and the write up on the difference between the 2.

 

[Raisedonadeere]

Interesting discussion but maybe my question was obscured in my giving of the details.

Let me put it this way. House is properly grounded with two rods 6’ apart at the meter base. Garage is properly grounded with two rods 6’ apart. The second rod in the run from the garage is only about 12 feet from the house rods. As I was about to cut the bare copper ground wire from the spool I wondered what if I connected to the house rod?

Would having them connected violate any principles or rules, not that I am concerned, just taking the opportunity to learn from someone who understands the theory of grounding. One rod is not enough, two evidently is good enough but is three better and is there anything about how much territory is covered by the grounding system.

 

[Zeke]

The way I see it (read it), the ground rods at each structure satisfy all requirements unless for some reason the impedance is too high (not likely). If you tie the 4 together, you've done nothing wrong. May not be necessary, but not wrong.

 

[sberry]

I say no to connecting the 2. Makes a separate alternate pathway for fault current rather than the wire with the current carry conductors.

 

[pattenp]

I say no to connecting the 2. Makes a separate alternate pathway for fault current rather than the wire with the current carry conductors.

 

[Raisedonadeere]

I don't understand the alternate pathway problem. What am I missing? Any fault current in the garage panel which is a subpanel is directly to ground, no connection to the common buss. And the ground rods by definition are at ground potential so no current back through any other wires to complete the loop to the fault source. Maybe lightening would be given the alternate path because lightening certainly can raise ground potential along its path.

I wont be connecting my two grounding systems based on knowing that my knowledge of what lightening does certainly doesn't rise above the practices experience has driven us to.

 

[pattenp]

The equipment grounding conductor from panel to panel is for handling fault current. By connecting the two earth grounding systems together you are in essence creating a second equipment ground. I'm not sure if that would create any problems but it's not something I've ever seen done or promoted to be done.

 

[sberry]

They want the grounding conductor to be in or the same pipe cable or raceway. They want the frame of a steel building to be grounded but its not to be used as the grounding conductor, some inverse proportional distance stuff, the best fastest breaker trip is down the service ground wire, 4th wire. Big loop, split route slows it down.

 

[alfredeneuman]

they want the frame of a steel building to be grounded but its not to be used as the grounding conductor

they want the grounding conductor to be in or the same pipe cable or raceway. They want the frame of a steel building to be bonded but its not to be used as the grounding conductor...

fify

 

[nelstomlinson]

I say no to connecting the 2. Makes a separate alternate pathway for fault current rather than the wire with the current carry conductors.

It's just a parallel ground conductor. That's not a bad thing; you want fault current to make a complete circuit so the breaker can trip.

Changing the subject a bit, those silly little ground rods usually aren't much more than a fig leaf. Run a grounding electrode conductor to your well casing. That's a ground.

 

[nelstomlinson]

The equipment grounding conductor from panel to panel is for handling fault current. By connecting the two earth grounding systems together you are in essence creating a second equipment ground. I'm not sure if that would create any problems but it's not something I've ever seen done or promoted to be done.

It's common to run an equipment grounding conductor in the feed for a panel. I prefer to use GEC gauge all the way to the last panel. Nobody has ever complained that the ground was too good.

The pairs of ground rods should be wired to the panels with grounding electrode conductor, and that's what I'd use to connect the two pairs.

You're still bonding ground to neutral in exactly one place, right?

 

[grounded-b]

I have always put in 1 rod at sheds and garages with feeder ground as Primary Ground.

Then you have been doing it wrong... UNLESS you can prove the resistance of that one rod to earth is 25 ohms or less.

The Feeder Ground is an EGC (equipment grounding conductor), NOT a Grounding Electrode.

Steve
master electrician

 

[Raisedonadeere]

It's common to run an equipment grounding conductor in the feed for a panel. ………………………………………………………….
You're still bonding ground to neutral in exactly one place, right?

Yes the only connection between ground and neutral for the house and the detached garage is at the meter pole/distribution box. So I am still trying to fathom why not just connect all the grounding rods since the rods for the two structures are in such close proximity.

 

[sberry]

Because it creates another pathway, doesn't require a fault to take the one true path.

The Feeder Ground is an EGC (equipment grounding conductor)

Dam,,,, way too many letters this morning, I kept trying to come up with the real name,,, ha

 

[checkthisout]

If you were to look at it schematically, it makes absolutely no difference if you were to tie the ground rods at the house and detached structure.

So tying them together would be fine and serve absolutely no purpose.

 

[Raisedonadeere]

Because it creates another pathway, doesn't require a fault to take the one true path.
Dam,,,, way too many letters this morning, I kept trying to come up with the real name,,, ha

Not disagreeing at all. I am trying to figure out the "other pathway"
though.

As I am looking at my garage wiring, the only path a fault has is either hot to neutral or hot to neutral and ground or hot to ground, maybe neutral to ground but how would I ever know about it except the GFI breaker blows and I have to figure out why.

Anyway the ground fault provides a path to ground and the only path the equipment ground has is back the equipment ground to the ground bar in the panel then through the grounding electrode wire to the ground rods. The only way the garage ground fault can see the meter ground rods is through the wire that connects first to the garage ground rods and then to the meter ground rods if I were to connect them.

BTW my house 200 AMP panel is also wired as a subpanel, the only bonding of ground to neutral is at the meter/distribution box. The Ground Electrode Wire bonded to neutral connects to the two ground rods. Maybe that has something to do with this discussion.

 

[AntonLargiader]

If you were to look at it schematically, it makes absolutely no difference if you were to tie the ground rods at the house and detached structure. ..

If you're only going to look at it schematically, you could step back 60 years and do away with grounds entirely, since the neutral is by definition the same potential.

The only point being that a schematic doesn't necessarily illustrate the entire issue.

 

[Raisedonadeere]

If you were to look at it schematically, it makes absolutely no difference if you were to tie the ground rods at the house and detached structure.

So tying them together would be fine and serve absolutely no purpose.

But maybe some purpose in understanding why it is fine but serves no purpose. Some day, if someone asks why did this fella connect the garage ground rods to the house ground rods, some wise one will knowingly respond "because it was there", or "because he could"

 

[SGKent]

If the resistance is 25 ohms or less, could one just drive the rod in, connect an ohm meter probe to the house ground and another to the rod, and if it is 25 ohms or less that would suffice? Why does it require more expensive equipment? Is there any distance requirement on that? For example could one read resistance between a small ground bar stuck into the soil two feet from the shed ground bar to confirm that the resistance is less than 25 ohms? The quote above of the code does not say impedance or resistance nor give a distance what the measurement points are between. Obviously it is "earth," but the planet is pretty good size.

 

[pattenp]

Grounding electrodes are not for clearing fault current. The fault current follows back to the source via the service neutral/ground. The earth grounding electrode system is not the path of least resistance for clearing fault current to trip breaker. This is why there is an equipment grounding conductor in the feed.

 

[rlitman]

If the resistance is 25 ohms or less, could one just drive the rod in, connect an ohm meter probe to the house ground and another to the rod, and if it is 25 ohms or less that would suffice? ...

No.

It requires a ground resistance test.

 

[dave*99]

If the resistance is 25 ohms or less, could one just drive the rod in, connect an ohm meter probe to the house ground and another to the rod, and if it is 25 ohms or less that would suffice? Why does it require more expensive equipment? Is there any distance requirement on that? For example could one read resistance between a small ground bar stuck into the soil two feet from the shed ground bar to confirm that the resistance is less than 25 ohms? The quote above of the code does not say impedance or resistance nor give a distance what the measurement points are between. Obviously it is "earth," but the planet is pretty good size.

The measurement you describe is measuring the resistance between two ground rods. The requirement is impedance between 1 ground rod and earth. So no, they are not the same. The story of ground rod impedance is a detailed one. And the industry reference is available on the Megger website in a book that covers the topic in detail.

http://www.biddlemegger.com/biddle-ug/GettingDownToEarth-MC.pdf

 

[ddawg16]

The second grounding rod for my detached garage is only about 12 feet from the grounding rods at my house meter base/distribution box I am connecting to.
I drove the rods into sides of the connecting ditch to the garage. As I was running my ground wire and connecting to the rods I wondered what the rules would be about continuing my ground wire to the ground rods at the meter base. I exposed the end of one of them when trenching to the meter base and my grounding wire will reach. Seems like I could put that extra length to good use

Someone might ask why bother since two rods 6' apart is enough etc., but what if the two complete grounding systems were connected since they are so close to each other?

I even wonder why I could not just have used the house grounding system anyway, but that is another question.

BTW my house 200 AMP panel is also wired as a subpanel, the only bonding of ground to neutral is at the meter/distribution box. The Ground Electrode Wire bonded to neutral connects to the two ground rods. Maybe that has something to do with this discussion.

I believe the actual ground rod discussion has played out.

But ^^this^^? Connecting to your meter base instead of the garage sub?

As I understand your setup, your meter base is NOT at your house or garage? Or the meter base is at the house, but not part of your load center?

In any case, I would not run a wire from the garage ground rods to the meter base.

What you should have is the ground wire from the rods going to your garage sub-panel EGC. And your feed from the house (or meter base) should be 4 wires, 2 Hots, 1 Neut and a ground. This means your garage ground and house ground are tied together.

Side note....if concrete has not been poured yet, consider an Ufer ground. No ground rods needed

 

[sberry]

Someone might ask why bother since two rods 6' apart is enough etc., but what if the two complete grounding systems were connected since they are so close to each other?

Its going to "work" but,,, in the event of a fault some of the current would go on the equipment grounding conductor and some would take the wire connecting the rods together, some each way. Ideally is to see it go down the equipment grounding conductor that goes with the current carrying conductor, this causes the fastest trip of the breaker, so much faster when the wires are close together, same cable, pipe or raceway. There is technical engineering way over my head but that's the essential reason not to tie these together on top of that maybe not so much so close but want a strike carried to ground and not on the equipment conductor. Parking lot lights,,, essentially a separate structure may have additional rods vs it carrying a strike back to the structure thru the equipment ground wire.
A guy like westom could elaborate on the science, that is relatively basic and what I need to know as an installer.

 

[exranger06]

Not disagreeing at all. I am trying to figure out the "other pathway"
though.

As I am looking at my garage wiring, the only path a fault has is either hot to neutral or hot to neutral and ground or hot to ground, maybe neutral to ground but how would I ever know about it except the GFI breaker blows and I have to figure out why.

Anyway the ground fault provides a path to ground and the only path the equipment ground has is back the equipment ground to the ground bar in the panel then through the grounding electrode wire to the ground rods. The only way the garage ground fault can see the meter ground rods is through the wire that connects first to the garage ground rods and then to the meter ground rods if I were to connect them.

BTW my house 200 AMP panel is also wired as a subpanel, the only bonding of ground to neutral is at the meter/distribution box. The Ground Electrode Wire bonded to neutral connects to the two ground rods. Maybe that has something to do with this discussion.

This is where you're getting it wrong. The ground rods are not there to clear faults. The way faults are cleared is as follows: If a hot leg contacted a neutral, you'd have a short circuit (dead short). Dead short = a LOT of current flowing through the wires. When you have too much current flowing, the circuit breaker trips (that's the whole purpose of a circuit breaker. A 15 amp breaker will trip any time the current goes over 15 amps; that's why it's a "15 amp breaker.") Now the power is off and the fault is cleared. That's it. Notice how I didn't mention the ground rods at all in that explanation. That's because they're not involved at all in clearing faults.

Now if the hot leg contacts a ground wire/equipment ground, the same thing happens because the neutral is bonded to the ground at the main panel. So the hot leg contacting the ground is essentially the same as contacting the neutral, and also creates a dead short. So the fault current would go from the equipment ground to the ground bar in the subpanel, then through the ground wire (EGC) back to the main panel, where it is bonded to the neutral, and the fault current continues on the neutral all the way back to the transformer. Fault current doesn't go to "the ground" (the literal ground you stand on), it goes back to the source (the transformer). Again, the goal is to essentially create a short circuit between the hot leg and the neutral, which both originate at the transformer, not "the ground."

The purpose of the ground rods is to shunt lightning current to "the ground," and to ensure that anything that is electrically grounded is at the same potential as "the ground." If the ground bar in the panel is at a different potential than the literal ground you're standing on, you'd get shocked any time you touched something connected to the ground bar (any metal junction box, conduit, any appliance with a 3-prong cord and a metal enclosure, etc). By connecting the ground bar to the literal ground via the GEC and ground rods, you're ensuring that they're at the same potential. That is the only purpose of the ground rods.

 

[AntonLargiader]

You want a low-impedance pathway for fault clearing; that's the EGC. I don't see how you could inhibit that by creating additional pathways between the sub and the main, which is what you'd be doing with the proposed jumper. That's like saying that the GECs are too good, and are sucking current out of the EGC. It doesn't matter; both panels have the same reference for "ground" and you don't need to get those same exact electrons back to the main if there's a fault.

Its going to "work" but,,, in the event of a fault some of the current would go on the equipment grounding conductor and some would take the wire connecting the rods together, some each way. Ideally is to see it go down the equipment grounding conductor that goes with the current carrying conductor, this causes the fastest trip of the breaker, so much faster when the wires are close together, same cable, pipe or raceway. ..