Why 2-Ground rods???

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That is all the code requires unless a single rod measures 25 ohms or less and in that case a single rod is all that is needed


250.56 Resistance of Rod, Pipe, and Plate Electrodes.
A single electrode consisting of a rod, pipe, or plate that does not have a resistance to ground of 25 ohms or less shall be augmented by one additional electrode of any of the types specified by 250.52(A)(4) through (A)(8). Where multiple rod, pipe, or plate electrodes are installed to meet the requirements of this section, they shall not be less than 1.8 m (6 ft) apart.
FPN: The paralleling efficiency of rods longer than 2.5 m (8 ft) is improved by spacing greater than 1.8 m (6 ft).
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Roger
 
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250.53(A)(2):

(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
(2) Grounding electrode conductor
(3) Grounded service-entrance conductor
(4) Nonflexible grounded service raceway
(5) Any grounded service enclosure


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.
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The exception says if you can prove your first rod has a resistance of 25 ohms or less then you don't need to supplement it. Many people do not have the equipment or knowledge to accurately test it, or figure it is less cost or easier to just drive the second rod and be done with it. This rule applies to pipe and plate electrodes also, but ground rods is what is most commonly used.
 
That was the confusing part (the use of the 2nd).I guess the use of just one rod in residential settings in this area of the world is adequate for the 25 ohm requirement or this was never paid much attention to.I have never seen a 2nd rod in use.This area being with in 50 miles of the coast may play a signifcant part.

thanks dick
 
I think the answer to your question of "why 2" is that the code writers could not justify making us put in 3, or for that matter making us continue to add more and more rods until we get a reading below 25 ohms.

By the way, there are two ground rods at my house.
 
dicklaxt said:
That was the confusing part (the use of the 2nd).I guess the use of just one rod in residential settings in this area of the world is adequate for the 25 ohm requirement or this was never paid much attention to.I have never seen a 2nd rod in use.This area being with in 50 miles of the coast may play a signifcant part.

thanks dick
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I think it was the way it was worded. It used to be worded in a way that the installer could say to the inspector "prove to me that the rod is more than 25 ohms and I will install a second one". Most inspectors do not have equipment to test this and let it go. That is why people generally drove 1 rod and called it done.

What Roger quoted is what the wording was prior to 2011, what I quoted is the 2011 wording. It now says you must use a supplemental electrode unless you can prove the first one is 25 ohms or less.
 
They ought to just make it a rule that 2 grouind rods are required period, and forget the 25 ohm rule.
That rule got beat to death anyway.
If you drive 1 you always wonder is it actually 25 ohms or less? and if you drive 2 you wonder why you had to if 25 ohms or less was acceptable, so its a no win situation.

They need to take out the variable which is not a constant.
 
jap said:
They ought to just make it a rule that 2 grouind rods are required period, and forget the 25 ohm rule.
That rule got beat to death anyway.
If you drive 1 you always wonder is it actually 25 ohms or less? and if you drive 2 you wonder why you had to if 25 ohms or less was acceptable, so its a no win situation.

They need to take out the variable which is not a constant.
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The changes in wording in 2011 now make it so that you must use 2 rods unless you want to go through the effort to prove the first one is low enough resistance to stand alone.
 
kwired said:
The changes in wording in 2011 now make it so that you must use 2 rods unless you want to go through the effort to prove the first one is low enough resistance to stand alone.
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okay that would be the answer,,,,,,,,,2011 said

as a side note most all General Specificatiions from industrial plant engineering gurus stated that the grid systems would be interconnected driven rods to 25 ohms,,,,,this was normally met with rods 75 to 100 feet apart for the plant proper usually coastal,,,,,,,,,,,,,substations were different.
 
dicklaxt said:
okay that would be the answer,,,,,,,,,2011 said

as a side note most all General Specificatiions from industrial plant engineering gurus stated that the grid systems would be interconnected driven rods to 25 ohms,,,,,this was normally met with rods 75 to 100 feet apart for the plant proper usually coastal,,,,,,,,,,,,,substations were different.
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The engineering gurus can specify whatever they want and for their own reasons.

The NEC minimum is 25 or less for a single rod, if that is not met drive a second rod. If still not 25 or less NEC does not make you drive any more, but if engineers want 25 or less it will likely mean keep adding rods or making other modifications until 25 or less is met.
 
kwired said:
The changes in wording in 2011 now make it so that you must use 2 rods unless you want to go through the effort to prove the first one is low enough resistance to stand alone.
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"You Must, Unless" is still a variable.

The resistance to earth changes daily, that's why I think they should do away with the 25 ohms or less rule but that JMHO.
 
kwired said:
The engineering gurus can specify whatever they want and for their own reasons.

The NEC minimum is 25 or less for a single rod, if that is not met drive a second rod. If still not 25 or less NEC does not make you drive any more, but if engineers want 25 or less it will likely mean keep adding rods or making other modifications until 25 or less is met.
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Thats exactly what was done,about every 4th rod was a Ground Well(accessable rod,used for testing) where all others were driven rods not accessible with out a shovel and some times a big concrete busting hammer.

dick
 
jap said:
"You Must, Unless" is still a variable.

The resistance to earth changes daily, that's why I think they should do away with the 25 ohms or less rule but that JMHO.
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I think a little too much emphasis is put on grounding electrodes, but they are in the NEC and I must do what it says as a minimum.

Lightning strike, we have not only the GES at a structure we also have bonds between that and every structure connected to the utility system. The rest of the grid will have lower impedance to earth than a single electrode, because there are so many electrodes. If the strike is direct you have local damages no matter what you have for electrodes.
 
kwired said:
I think a little too much emphasis is put on grounding electrodes, but they are in the NEC and I must do what it says as a minimum.
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Don't get me wrong. I agree with installing grounding electrodes. I just think there should be 2 required and the clause about the "Unless 1 will do it" should be removed.
 
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IMO the requirement should be one rod regardless of anything else or any other GE's being present.

Roger
 
POCO

POCO

Power company here requires 2 rods. It doesn't matter if you have a Ufer. It probably doesn't matter if you build your house in an abandoned substation with a grounding grid coming it at less than an Ohm. Just show me to two rods, or no juice.
 
Its based on the law of diminishing returns. Adding a second ground rod reduces the resistance by 60%, 3 gets a down to 40%, 3 is 30%.
Some one a long time ago determined that 2 was enough. This was all worked out probably by 1920. 6 feet apart is because closer than that the concentric shells of the earth overlap and it becomes one ground rod.
But 25 ohms? No one knows. Mike Holt emailed me once he thought he had the reason. But I don't think he ever found out for sure.
So....
Drive two and go home
Or
Use a concrete encased electrode.
 
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