Ground rods

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hhsting

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I have single ground rod which at the present has more than 25 ohms resistance. So added an additional ground rod but together ground rods have resistance more than 25 ohms.


So where in NEC 2014 it says to add additional ground rod to multiple ground rods?
 
I don't have the code reference but IIRC, the choice is: put in one ground rod with a resistance of 25 ohms or less; or put in two ground rods at least six feet apart and no resistance measuring is required. You never have to put in more than two rods.

I have never attempted to measure ground rod resistance. I just always put in two rods.
 
AFAIK, a secondary rod is not required if the resistance of the electrode is less than 25ohms. So, rather than checking for resistance I just install the 2nd electrode.

Edit to add. Same as Coppersmith said.
 
Here is the section

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
(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.
Click to expand...
 
Dennis Alwon said:
Here is the section
Click to expand...

Dennis that section says if single rod is installed then an additional electrode be installed could be ground rod.

Does that section mean that if I ready have two ground rods and together resistance is More than 25 ohms then I need third ground rod or however many ground rods to meet 25 ohms or less resistance?
 
With the exception that says if one rod is installed and you have 25 ohms that is all that is needed. If you don't have 25 ohms with one rod just add another and forget about the 25 ohms.
 
Dennis Alwon said:
With the exception that says if one rod is installed and you have 25 ohms that is all that is needed. If you don't have 25 ohms with one rod just add another and forget about the 25 ohms.
Click to expand...

So even if the first of two rods is 60 ohms, you still only have to install a second rod? That would mean that the combination of the two rods (assuming identical soil condition) is 30 Ohms, unless there is something I'm missing. Somehow 30 Ohms is OK for 2 rods, but not OK for 1 rod.
 
Carultch said:
So even if the first of two rods is 60 ohms, you still only have to install a second rod? That would mean that the combination of the two rods (assuming identical soil condition) is 30 Ohms, unless there is something I'm missing. Somehow 30 Ohms is OK for 2 rods, but not OK for 1 rod.
Click to expand...
It wouldn't matter if the combined resistance is 1000 ohms or greater, once the second rod is installed you have complied with the NEC's rule.

Roger
 
jim dungar said:
Your are correct. The NEC never requires more than two electrodes.
Click to expand...

What is the physical basis for the "two and done" rule we are discussing? What is it about 2x 60-ohm rods that intuitively would combine to 30 Ohms that makes it OK, despite 1 rod at 30 Ohms not being good enough? Is it that exceeding 50 Ohms on one rod is so rare in the first place, that it isn't worth the effort to have a rule that covers that possibility? Does the mathematics behind the theory of ground rod resistance, work in a manner such that the conductance does more than simply add?
 
Carultch said:
What is the physical basis for the "two and done" rule we are discussing?
Click to expand...

There is no basics in physics, for this rule. Quit trying to find one.

Somebody, somewhere, sometime evidently thought 25 ohms was a magic number, but it also has no more basis in physics than any other number.
 
Carultch said:
So even if the first of two rods is 60 ohms, you still only have to install a second rod? That would mean that the combination of the two rods (assuming identical soil condition) is 30 Ohms, unless there is something I'm missing. Somehow 30 Ohms is OK for 2 rods, but not OK for 1 rod.
Click to expand...


I did a test on two rod and got 89 ohms. According to you I would have 44.5 on each rod... Not so I got 98 on one rod.
 
Dennis Alwon said:
I did a test on two rod and got 89 ohms. According to you I would have 44.5 on each rod... Not so I got 98 on one rod.
Click to expand...

Other way around. According to the way resistances in general combine in parallel, given 89 ohms on two rods combined, I would expect 178 ohms on each rod individually. I can understand getting less than twice the conductance out of two rods in parallel as you measured, due to interference in the current path between the two rods, but I cannot understand getting more than twice the conductance out of two rods in parallel.
 
Following picture explains the theory of adding ground rods in parallel. More than 2 does not accomplish much. This picture is from AEMC, they had an excellent publication on why ground rods are copper coated, size of ground rods, depth of ground rods.
So its drive two and go home.
 

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tom baker said:
Following picture explains the theory of adding ground rods in parallel. More than 2 does not accomplish much. This picture is from AEMC, they had an excellent publication on why ground rods are copper coated, size of ground rods, depth of ground rods.
So its drive two and go home.
Click to expand...

Your chart shows resistance reduce to 30% with 4 rods while 2 rods resistance was 60% about half. How more than 2 does not accomplish much?
 
tom baker said:
Following picture explains the theory of adding ground rods in parallel. More than 2 does not accomplish much. This picture is from AEMC, they had an excellent publication on why ground rods are copper coated, size of ground rods, depth of ground rods.
So its drive two and go home.
Click to expand...

To put it in plain speak:
“If one isn’t good enough, use two. If two isn’t good enough, just stop because 3,4,5....etc. ain’t gonna be good enough either.
 
jim dungar said:
There is no basics in physics, for this rule. Quit trying to find one.

Somebody, somewhere, sometime evidently thought 25 ohms was a magic number, but it also has no more basis in physics than any other number.
Click to expand...
My guess is this requirement with 2 ground rods was worked out many years ago. 25 ohms comes from the telegraph industry, Mike Holt had some information on it.
 
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