I do agree that either way you wire them resistance comes out to two grs. The issue is that you are trying to achieve less than 25 ohms of resistance on your gec. In order to achieve this you need to wire them in series with each other whether it be one continuous conductor or one conductor and a bonding jumper in between the two. Either way it should be panel rod rod.
Ground Rod Installation
- Thread starter jeff48356
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zog
Senior Member
- Location
- Charlotte, NC
OK now you are obviously just messing with us.
- Location
- Massachusetts
Of course it can, that 6 can carry tens of thousands of amps for a short time.
- Location
- New Jersey
- Occupation
- Journeyman Electrician
You certainly can wire the two rods that way but according to the NEC it's not required.
renosteinke
Senior Member
- Location
- NE Arkansas
Am I the only one here with access to the "American Electricians' Handbook?"
On the topic of ground rods, they have an interesting series of graphs. Before I discuss them, though, I want to bring up the mathematical concept of 'limits.'
A 'limit' is the value a curve approaches, but never quite reaches. For example, the series 1 + 1/2 + 1/4 + 1/8 .... on to infinity, approaches a total of 2, but never quite gets there. "2" would be the 'limit.'
Looking at the graphs relating to ground rods, two things jump out:
First, that making the rod longer results in little improvement after eight or ten feet. You're 'approaching the limit.' It's certainly clear that even an eight foot rod is NOT twice as good as a four foot rod.
Next, regarding a second rod .... the effectiveness of that second rod doesn't get much better beyond six ft. While having six ft. between rods is LOTS better than five ft., increasing the distance even further (say, to 7 ft.) results in little improvement. Six ft. is just about at the limit;' that is, performance is as good as it's going to get. Having the rods farther apart does no harm, but results in little improvement.
Looking at all this data in a 'big picture' sort of way, it's a bit easier to understand why the Ufer ground, the system of turning the entire foundation slab into a GEC, is such an effective method of grounding.
Regarding the comment made about 'there's a rod at the pole- why bother with the rod at the house:' You've revived some of the muddled thinking that led to Article 250 becoming such a witch's brew of conflicting concepts. While there will be SOME current flow between the rods, the dirt under our feet is not a very good conductor. That's why were use that nice, fat wire (the neutral) as our return path. It is the neutral back to the PoCo transformer that lets all the breakers and fuses work. The ground rod, because of the poor conductivity of the dirt between the rods at the pole and the house, is pretty much irrellevant in clearing faults.
The only electricity that benefits from having a good path into Mother Earth is electricity that comes from Mother Earth. This means lightning and static electricity- and not the stuff the PoCo sells us. Thus, there's value in every lightning 'target' having its' own ground rod.
On the topic of ground rods, they have an interesting series of graphs. Before I discuss them, though, I want to bring up the mathematical concept of 'limits.'
A 'limit' is the value a curve approaches, but never quite reaches. For example, the series 1 + 1/2 + 1/4 + 1/8 .... on to infinity, approaches a total of 2, but never quite gets there. "2" would be the 'limit.'
Looking at the graphs relating to ground rods, two things jump out:
First, that making the rod longer results in little improvement after eight or ten feet. You're 'approaching the limit.' It's certainly clear that even an eight foot rod is NOT twice as good as a four foot rod.
Next, regarding a second rod .... the effectiveness of that second rod doesn't get much better beyond six ft. While having six ft. between rods is LOTS better than five ft., increasing the distance even further (say, to 7 ft.) results in little improvement. Six ft. is just about at the limit;' that is, performance is as good as it's going to get. Having the rods farther apart does no harm, but results in little improvement.
Looking at all this data in a 'big picture' sort of way, it's a bit easier to understand why the Ufer ground, the system of turning the entire foundation slab into a GEC, is such an effective method of grounding.
Regarding the comment made about 'there's a rod at the pole- why bother with the rod at the house:' You've revived some of the muddled thinking that led to Article 250 becoming such a witch's brew of conflicting concepts. While there will be SOME current flow between the rods, the dirt under our feet is not a very good conductor. That's why were use that nice, fat wire (the neutral) as our return path. It is the neutral back to the PoCo transformer that lets all the breakers and fuses work. The ground rod, because of the poor conductivity of the dirt between the rods at the pole and the house, is pretty much irrellevant in clearing faults.
The only electricity that benefits from having a good path into Mother Earth is electricity that comes from Mother Earth. This means lightning and static electricity- and not the stuff the PoCo sells us. Thus, there's value in every lightning 'target' having its' own ground rod.
- Location
- New Jersey
- Occupation
- Journeyman Electrician
Funny you should say that because the NEC thinks differently. I've read that spacing the rods at a distance equal to twice their length is even better than the code required minimum of 6'.
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
How does that meet the definition of series? To be series current would have to travel from the service equipment to the first rod, through the first rod, through the bonding jumper between the rods then through the second rod, and then into the earth, if some current flows to earth in the first rod, or even in the bare GEC or bonding jumper then it is not a series circuit.
As you are describing it is is just as parallel as running a separate wire to each rod. Current travels through the GEC and at some point at or near the first rod it splits and has parallel paths to each rod.
The electrodes are electrically parallel to each other as well as parallel to other electrodes such as a CEE or metal water pipe. They may all be connected to same GEC or may have separate bonding jumpers connecting them to the GEC.
By installing grounding electrodes you are not trying to "earth" the electrodes, you are trying to "earth" a service conductor, which we will call the "grounded" service conductor, can be a neutral, can be some other conductor in some cases.
Add: on separately derived systems we are earthing the system grounded conductor instead of a grounded service conductor, but we do also bond it to the service grounded conductor at some point.
Last edited:
petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- engineer
of course it can. but does that actually serve any useful purpose when dealing with the average lightning strike? that was what the comment was directed toward.
I am not convinced that the claim that the ground rod on a typical residential service provides some measure of protection from lightning strikes is actually true.
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
You have to specify what protection from lightning strikes means. A grounding electrode gives the energy in the strike a place to go, does not gurantee that all of the enerrgy will follow that path, or even gurantee any of the energy will follow that path. Even a lightning protection system is not a 100% protection but gives enough protection to be fairly effective. I think the grounding electrode system is more important for bringing non current carrying metal objects to same potential as earth than it is for lightning protection. Lightning protection just happens to be something it does to some degree.
stockexpert
Member
NFPA 780 - Lightning protection
NFPA 780 - Lightning protection
Per NFPA 780, ground rods must be "..seperated a distance equal to the driven depth of the rods..." A little more exacting than NFPA 70 - (the NEC)....
NFPA 780 - Lightning protection
Per NFPA 780, ground rods must be "..seperated a distance equal to the driven depth of the rods..." A little more exacting than NFPA 70 - (the NEC)....
renosteinke
Senior Member
- Location
- NE Arkansas
"The NEC thinks different ..."
Not really.
First, to clarify: I was simply relaying the information presented in the American Electricians' Handbook. This was not my opinion; I was simply describing the graphs published in that book, graphs derived from actual tests.
The AEH does not say it's bad to place the rods farther than six feet apart. What the graphs make plain is that making the rods more than six feet apart results in very little improvement. The graphs also make clear that as the spacing is reduced from that six ft. span, the improvement greatly reduced. Ultimately, if the rods are close enough to each other, the improvement is so little as to be no improvement at all.
Is seven ft. spacing better than six? Yes, it is. Is eight better than seven? Absolutely. The trouble is, you can carry that argument on into infinity. Yet, the improvement is not a straight-line relationship. After some point, you've 'captured' just about all the improvement you can get; the graphs place that point at somewhere near the six foot mark. It gets back to that concept of 'limits.'
Someone writes another code and wants a different distance? Good for them. Yet, one cannot ignore / edit reality to fit their own 'feelings.' The graphs are real. Likewise, I have no issue with anyone's 'rule of thumb;' using the rod to measure off the spacing is certainly convenient. All I did was direct everyone to the data upon which these rules muist be judged.
Note that I also mentioned the data relating to the length of the ground rod. I am sure that there are folks here who have banged in multi-section rods to depth of fifty ft. That happened a lot in the earlier times, in an attempt to obtain that magical 25-ohm reading. Had the folks who wrote those procedures but looked to the AEH data, they would have seen the futility of using rods longer than 8-10 ft.
Discussion of the 25-ohm limit is beyond the scope of this thread; but, I suspect, it will pass away as grounding is better understood. Article 250 still has plenty of references to now-discredited theories and confused goals. The best thing I can say about the 25-ohm rule is that is is very useful in demonstrating that the ground rod is completely irrellevent to the operation of your overcurrent devices.
Not really.
First, to clarify: I was simply relaying the information presented in the American Electricians' Handbook. This was not my opinion; I was simply describing the graphs published in that book, graphs derived from actual tests.
The AEH does not say it's bad to place the rods farther than six feet apart. What the graphs make plain is that making the rods more than six feet apart results in very little improvement. The graphs also make clear that as the spacing is reduced from that six ft. span, the improvement greatly reduced. Ultimately, if the rods are close enough to each other, the improvement is so little as to be no improvement at all.
Is seven ft. spacing better than six? Yes, it is. Is eight better than seven? Absolutely. The trouble is, you can carry that argument on into infinity. Yet, the improvement is not a straight-line relationship. After some point, you've 'captured' just about all the improvement you can get; the graphs place that point at somewhere near the six foot mark. It gets back to that concept of 'limits.'
Someone writes another code and wants a different distance? Good for them. Yet, one cannot ignore / edit reality to fit their own 'feelings.' The graphs are real. Likewise, I have no issue with anyone's 'rule of thumb;' using the rod to measure off the spacing is certainly convenient. All I did was direct everyone to the data upon which these rules muist be judged.
Note that I also mentioned the data relating to the length of the ground rod. I am sure that there are folks here who have banged in multi-section rods to depth of fifty ft. That happened a lot in the earlier times, in an attempt to obtain that magical 25-ohm reading. Had the folks who wrote those procedures but looked to the AEH data, they would have seen the futility of using rods longer than 8-10 ft.
Discussion of the 25-ohm limit is beyond the scope of this thread; but, I suspect, it will pass away as grounding is better understood. Article 250 still has plenty of references to now-discredited theories and confused goals. The best thing I can say about the 25-ohm rule is that is is very useful in demonstrating that the ground rod is completely irrellevent to the operation of your overcurrent devices.
stockexpert
Member
Grounds rods longer than 8'
Grounds rods longer than 8'
Well,
Here in Florida, the standard distance of "one" rod is 20' down..that is, two-10' rods welded togeter (end-to-end) to obtain a 20' driven depth..it must be the sandy soils here that require it..but, it's the best way down in these parts!
jimmy
Grounds rods longer than 8'
Well,
Here in Florida, the standard distance of "one" rod is 20' down..that is, two-10' rods welded togeter (end-to-end) to obtain a 20' driven depth..it must be the sandy soils here that require it..but, it's the best way down in these parts!
jimmy
- Location
- New Jersey
- Occupation
- Journeyman Electrician
Is that for lightning or an electrode covered under Article 250 of the NEC?
Where can we find this formal requirement?
BTW, the times I have driven deep rods (30') we never welded them we used threaded rods and couplings.
Roger
- Location
- Windsor, CO NEC: 2017
- Occupation
- Service Manager
I've been watching Mike's Grounding vs. Bonding DVD. If you have it, fast forward about 1 hour into the first disc. Our grounding is to dissipate induced voltage from nearby lightning to reduce equipment failures - not to deal with a direct strike.
There is a connection between the lines and the neutral - the loads and the transformer.
Lightning induces energy onto all phases, and all phases are connected to earth in a grounded system.
No, you're not. You are reading too much into this - it is simple: if a ground rod is 25 ohms of resistance, all by itself (no GEC connection yet), from itself to earth, you need a second ground rod. Period.
It makes absolutely no difference. You can connect the rods together, or connect separate GECs from the service. 250.64(F).
petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- engineer
Finally - we have a winner!
The size of the wire probably matters very little in this role.
However, being as in most residences the transformer is out on a pole, and the neutral is earthed there, what benefit is gained by earthing it again 10 or 20 feet away?
T
T.M.Haja Sahib
Guest
The reason the neutral is earthed second time is that the neutral should be earthed at multiple points along its route from transformer to the service point to reduce the potential rise at the service equipment enclosures and possible rise at the other equipment enclosures inside the residence,should there be a break in the neutral upstream to the service to the residence.
- Location
- Windsor, CO NEC: 2017
- Occupation
- Service Manager
Again, due credit to the DVD - it is a localized induction. The grounding a few hundred feet away does nothing for the premises. Think of applying a torch to the end of a copper pipe - there will be a cherry red spot in one place, and the temperature is lower down the pipe.
The difference of potential between a conductor from remote earth and close earth is destructive to sensitive equipment - which makes auxilliary electrodes a bad idea.
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
Take any typical service, assuming no grounded metal paths to other structures such as a metal water pipe, and disconnect the neutral under load at any point ahead of the service equipment. Now stand barefooted on a damp concrete floor and touch anything bonded to the service ground and tell me there is no rise in potential, if you survive.
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