25 ohm ground rod

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travis301

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As we know it we are supposed to have 25 ohms or less on thru our ground rod. If 1 rod doesnt do it,we add another rod. My question is,how many ground rods do you have to add? Do you stop at 2 or is there a requirement to place more than 2 to achieve this 25 ohm or less rule?
Also,is it legal to install a ground rod horizontal,such as in a trench,or are they required to be vertical.
 
The rule says that if you don't get 25, you must add a second rod. There is no continuation of that rule to say that you must go back and see if you have 25 yet. So once you have the second rod, you are done.

There are times that a trench is allowed. But you can't do it just because you want to. See 250.53(G).
 
I was distracted, and didn't hit the "submit reply button" fast enough. I wonder if I can still sue Pierre for being a copy cat. :D :D
 
travis301 said:
I wondered because some of the soil types out there,you may never get the 25 ohms.
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I live in the desert southwest, and 10 driven ground rods wouldn't get 25 ohms. Some of our local inspectors still believe in a "good ground", but we don't get that with a rod or two.

Jim T
 
In Washington (State and I get tired of saying that) we can't lay horizontally unless we show-by leaving a stuck partially driven ground rod.
(When you get much east of Idaho if you say Washington the assumption is DC)
I once drove a rod with an hilti hammer that took most of an hour. I marked the rod, hung a threader on the hammer and let it go. If I couldn't drive it I would have to get the backhoe on the job and lay horizontally
 
tom baker said:
I once drove a rod with an hilti hammer that took most of an hour. I marked the rod, hung a threader on the hammer and let it go. If I couldn't drive it I would have to get the backhoe on the job and lay horizontally
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A backhoe is more than is really needed. A 12" x 12" x 1/4" steel plate at 30" deep meets the requirements of 250.52(A)(5) and 250.53(H). Dig a little hole with one of those two-handled post-hole diggers, make a little mud in the bottom with some salt in it, drop in the electrode with the GEC connected. After the inspector sees it, add a little more salty mud on the top and fill it in.
 
I've got a mineral spring behind my shop and the ground is always wet. There is also a lot of iron in the water.
When I installed the electric, I drove a 8' long, 5/8" diameter ground rod vertically into the ground.
This should be one of the best locations available for a ground rod.
The resistence of the rod measures 30 Ohms.
I was suprised that it was so high.
It would not trip a 15A breaker.
If I added another rod, it still would not trip the breaker.
Thanks to this forum, I've dispelled a (incorrect) long held belief of mine.
A ground rod performs a few vital functions.
Causing a 120V (or 240V) overcurrent device to open a (ground fault) is not one of them.
Just my opinion
steve
 
I recall a story one time from a course instructor where he said that as part of an experiment, they had driven some crazy number of ground rods on a high school football field, something along the lines of 50 or 100 rods, equally spaced in a grid pattern, 1 or 2 feet apart from each other and they could not acheive the 25 ohms to ground! He went on further to say that the best ground rod set up was one that was encased in a salt water brine which easily met the 25 ohm requirement. I wonder what the reasoning is behind the 2 ground rod requirement when in most cases the second rod a mere 6 feet away will not make a difference from the single rod installation.
 
Assuming we can get get it down to 25 ohms.Just what good will it really do to the system.It can not clear a fault.And lightning strikes will still be a problem.There are millions of home without this rod and no problems.
 
Racer: While the high number of driven rods should lower the resistance driving side by side negated some of the benefits of so many rods.

There is probably no REAL magic number, the NEC panel made a determination of 25 ohms based on some reasoning, be it past industry standards or nationwide averages. BUT no matter what the number, any attempt you make to lower the ground resistance can be beneficial during lighting strikes.

Having said that way too many engineers and electricians fret about improving the grounding electrode system when there are ground issues in a facility, when the best way to resolve the ground issues is correcting wiring errors.
 
Jim W in Tampa said:
Assuming we can get get it down to 25 ohms.Just what good will it really do to the system.It can not clear a fault.And lightning strikes will still be a problem.There are millions of home without this rod and no problems.
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The lower the ohms the less chance of surface arcing.
 
racerdave3 said:
I wonder what the reasoning is behind the 2 ground rod requirement when in most cases the second rod a mere 6 feet away will not make a difference from the single rod installation.
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Two ground rods will reduce the resistance 605
Three is just a bit more
Its based on the law of diminishing returns. Someone long ago determined two ground rods are alls thats needed to meet the minimum intent of the NEC
I'd like to see the code proposals on that.
Also the effectiveness of multiple rods can be greatly increased by installing twice the length apart. Closer than 6 ft and the shells of earth overlap and its basically one ground rod.
Regarding the plate electrode, studies by the IAEI show it as having the worst electrode resistance.
And most of the time you can't have one plate, you need two. But its something to consider if you can't drive an ground rod. I wonder what the availability of a plate electrode is??
 
tom baker said:
Regarding the plate electrode, studies by the IAEI show it as having the worst electrode resistance.
And most of the time you can't have one plate, you need two. But its something to consider if you can't drive an ground rod. I wonder what the availability of a plate electrode is??
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It really doesn't matter whether the plate electrode is effective or not; it just matters that the code accepts them the same as a rod. You can buy a piece of steel at a steel supply or even at a junk yard.

It is easier to supplement the conductivity of the plate with a little electrolyte like alum or salt.

A 12" x 12" x 1/4" steel plate exposes 2 square feet of surface and a 5/8" x 8 ft ground rod has only 1.3 square ft. If you use stainless or copper you need only 1/16" thick.
 
I am reading a book that states, "the resistance of a cylinder of spring water 1-yard long is equal to that of the resistance of a copper conductor of the same diameter 7X the distance of the earth to the moon." I don't know how this fits into this discussion; however I thought it was an interesting comment.

What does fit into this discussion is the book itself. It's really interesting to read about this subject matter dating from the late 1800's. There are over 500 pages of good articles and author commentary. Here is a link to an online copy.

http://books.google.com/books?id=gb...hLH0&sig=wJMTChNIb5D8ynS3Ys_rgG0gMgM#PPP21,M1
 
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