25 ohms
- Thread starter domnic
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skeshesh
Senior Member
- Location
- Los Angeles, Ca
Is THERE ANYWHERE in [the] USA where one can measure 25 Ohms or lower on a 8' ground rod driven in earth?
(sorry could not resist)
I wonder, what is the difference if you find someone who has achieved that rating? Bottom line is, if you are a proper contractor, you should have the means to measure the rod's resistance really easily. Once you do, if you do not achieve the criteria, then you have to drive another rod to do so. Alternatively you can just drive more than one in any case.
I'm not trying to discredit your question as a fun fact to know, but I doubt it has any practical outcome on any installation you may do or encounter.
(sorry could not resist)
I wonder, what is the difference if you find someone who has achieved that rating? Bottom line is, if you are a proper contractor, you should have the means to measure the rod's resistance really easily. Once you do, if you do not achieve the criteria, then you have to drive another rod to do so. Alternatively you can just drive more than one in any case.
I'm not trying to discredit your question as a fun fact to know, but I doubt it has any practical outcome on any installation you may do or encounter.
Last edited:
broadgage
Senior Member
- Location
- London, England
I have certainly achieved less than 25 ohms here in the uk from a single ground rod.
Readily achieved in damp or marshy land, but unlikely to be achieved in dryer conditions.
Readily achieved in damp or marshy land, but unlikely to be achieved in dryer conditions.
- Location
- New Jersey
- Occupation
- Journeyman Electrician
I would bet that most contractors do not have the equipment to measure the resistance of one rod nor do they care to.
brian john
Senior Member
- Location
- Leesburg, VA
Many places in Northern VA, and the Wash DC suburbs of Washington DC.
- Location
- Windsor, CO NEC: 2017
- Occupation
- Service Manager
I have never even seen a FOP resistance tester in the flesh, much less used one. :roll:
kingpb
Senior Member
- Location
- SE USA as far as you can go
- Occupation
- Engineer, Registered
Frankly, unless you have a soils report where resistivity measurements were done, and an engineer to do the grounding design, I would assume you will not achieve 25 ohms or less with one rod, and make sure you put 2 rods and associated hardware and labor in your bid, and be done with it.
zog
Senior Member
- Location
- Charlotte, NC
I posted a series of maps showing soil resistivity a while back but can't seem to find that thread. But in swampy areas or areas where the soil is very fertile you may be able to acheive 25 ohms. But as others mentioned, 99% of EC's do not have the equipment to test it anyways so it is just easier to drive a 2nd rod.
gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
101208-1118 EST
In my yard and at the shop I have less than 25 ohms. From memory in the 10 to 20 ohm range. This is a clay loam soil and generally there is moderate moisture present below a few inches.
In our part of the country we have green foliage as compared to somewhere like Colorado.
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In my yard and at the shop I have less than 25 ohms. From memory in the 10 to 20 ohm range. This is a clay loam soil and generally there is moderate moisture present below a few inches.
In our part of the country we have green foliage as compared to somewhere like Colorado.
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zog
Senior Member
- Location
- Charlotte, NC
Cavie
Senior Member
- Location
- SW Florida
In Florida it is easy to get 25 ohm on one rod in the summer rainy season. I do FDOT roadway and signal work. You will drive as many rods as needed to acheive 25 ohms on lighting and 15 on many signal projects. I have a clamp on ground rod tester. looks just like a standard amp probe only bigger. I am in a fight with an inspector on the east coast that is telling me I need 3 POINT TESTER and I must check every one of the 196 rods on the project one at time. The directions for my tester tells me to hook up a complete grounding system and check it once at the service point. Eliminating the need for individule testing. The inspectors boss says I can follow the directions on my tester. The inspector and his boss are discusing the matter. I let you know the outcome.
- Location
- Massachusetts
The clamp tester is almost junk IMO.
Unless the test is done perfectly it is mealiness.
Zog has posted many times about the clamp meters short comings.
Unless the test is done perfectly it is mealiness.
Zog has posted many times about the clamp meters short comings.
Cavie
Senior Member
- Location
- SW Florida
Shusssh. Don't let the inspector hear you say that. I believe that one 20' copper ground at each pole is enough. They are all connected together with a #6. All 196 of them
I agree with you Cavie and to go one step further, (I know it won't help in your battle) it wouldn't matter if they measured in the hundreds or even thousands of ohm's as far as performing their intended job.
Roger
And in most cases, especially residential and small commercial projects, that will suffice but, on projects that have specific GES performance specs the tests must be done, and if the permitting process of the particular area includes plan review it can become part of the inpectors requirement for the final.
BTW, we just drive two rods too if we are not required to test
Roger
gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
101208-1749 EST
ZOG:
One test was done between So. Industrial and the Ann Arbor Railroad tracks midway between Stadium and Eisenhower. The tracks follow what was once a creek bed.
I used a source and sink, each was an 8 ft rod, with a spacing of about 40 ft. There is virtually no ground path current here, millivolts, maybe 100 MV. Thus, a negligible error from ground path 60 Hz currents, or any other frequency. Excitation was 60 Hz from a Stancor 28 V 2 A transformer. Then used Fluke 27 to essentially do a fall of potential test. No frequency filtering on the input to the Fluke.
I do not know where to find the experimental data at the present time. This test was a number of years ago.
If I ran this test in Dearborn where there is some very sandy soil I would never get this low a reading. But it also means you do not get water in your basement.
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ZOG:
One test was done between So. Industrial and the Ann Arbor Railroad tracks midway between Stadium and Eisenhower. The tracks follow what was once a creek bed.
I used a source and sink, each was an 8 ft rod, with a spacing of about 40 ft. There is virtually no ground path current here, millivolts, maybe 100 MV. Thus, a negligible error from ground path 60 Hz currents, or any other frequency. Excitation was 60 Hz from a Stancor 28 V 2 A transformer. Then used Fluke 27 to essentially do a fall of potential test. No frequency filtering on the input to the Fluke.
I do not know where to find the experimental data at the present time. This test was a number of years ago.
If I ran this test in Dearborn where there is some very sandy soil I would never get this low a reading. But it also means you do not get water in your basement.
.
brian john
Senior Member
- Location
- Leesburg, VA
And I make money doing this, SO TEST AWAY PLEASE!!!!
glene77is
Senior Member
- Location
- Memphis, TN
Gar,
In Memphis, TN, I have asked the code enforcement office about GEC requirements in situations where the residential plumbing is all plastic.
They say, "If the plumbing is plastic, then there is nothing to attach to", and
the inspectors do not require a second ground rod (as required by NEC).
So, we have just let the inspectors determine what is sufficient.
When in doubt, ask the AHJ.
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