Measuring for 25 Ohms or Less - 250.56

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DSamson

DSamson

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San Francisco, CA
I do residential work, including service upgrades. Most of the places I do upgrades, the service is more than 30 years old. They are using a metal water pipe as the sole grounding electrode.

When I do the upgrade, I always bring the grounding electrode system up to current code. This ususally means driving two ground rods 6 ft. apart. My body hurts for days.

My first Question:

Is there an inexpensive way to measure for resistance to earth? I called Fluke, most of waht they sell is in the over $1,000 range.

I'd like to find a way of only driving one ground rod, but I've got to be able to demostrate less than 25 ohms reistance to ground.

And a second question:

I can't supplement the one driven rod with the underground water service. I'm allowed to supplement a single electrode with one listed in 250.52(A)(2) - (7), but NOT 250.52(A)(1). Correct?

Just looking for a way to save my back, one ground rod is gentler on the body than two.

Thanks,

David
 
It's not just the cost of the equipment that is a problem. The test must be performed and documented in such a way that the AHJ will accept the test results. Some AsHJ might want something unexpected and unusual, such as a Professional Engineer signing and sealing the test results. Or the AHJ might want to watch you perform the test. Also, what if the test results are not below 25 ohms?
 
In addition to what has been mentioned, there are two types of meters one requires a bit of expierence and takes more time that driving an additional electrode, the second meter (clamp-on) will give error readings if there are neutral ground issues.

Lastly if you want lower readings spread those rods apart a bit further.
 
My suggestion is to find a better way to drive the ground rods, such as a hammer drill attachment or an apprentice. I have an Ideal 61-920 ground resistance clamp-on tester and find it to be very unreliable. A measurment taken a week ago isn't the same I would get today.

And as stated by Charlie, there is really no convienent procedure for getting AHJ approval for a ground resistance measurment. It will likely casue you greater heartburn ontop of the aching back.
 
In my jurisdiction, ground resistance measurements are not accepted. If ground rods are used, you must drive two. The reasoning is that a resistance of under 25 ohms today is no guarantee that it will be the same tomorrow, and also avoids having the AHJ verify the test.
 
Thanks All!

Thanks All!

Original poster here - just want to say thanks. That was all very helpful information. This forum is such a great resource.

I guess I'll stick with driving two rods and get the roto-hammer attachment.

Thanks.
 
In my jurisdiction, ground resistance measurements are not accepted. If ground rods are used, you must drive two. The reasoning is that a resistance of under 25 ohms today is no guarantee that it will be the same tomorrow, and also avoids having the AHJ verify the test
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I CANNOT BELIEVE I READ THIS.

I'll hold my comment on the stupidity of this idea as I do not want to be censored. Wait on second thought and 2 rods resolves this issue?


Why test anything it might change after testing, that cable you megger might deteriorate and have lower reading in 20 years.................
 
brian john said:
I CANNOT BELIEVE I READ THIS. ...
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I can.

Where I live the AHJ is the utility. They say one ground rod is good enough. Several years ago I ask one of the engineers. Answer was, "Nominal ground rod resistance in this area is 300-1000 ohms, going up to 3000 ohms in the winter. We have no evidence that 25 ohms is any better than 3000 ohms. What is the point of driving a second rod dropping the ground resistance from 300 ohms to 150 ohms?"

Drive across town, a whopping 10 miles, to an industrial facility and the state inspector is the AHJ. They drive two rods.

Story related to me (Not me I wasn't there, and did not involve any AHJ I know): Work crew out in the middle of nowhere (maybe 300 miles North) has a small gen (100kw) connected to a temp power panel. Middle of summer and the ground is frozen about 1' down. They have driven two ground rods. Inspector shows up and puts his clamp-on ground tester on the GEC between the panel and the first rod. Reads, frowns, clamps meter back on and reads again. Frowns again, says, 'meter must not be working.'

So what's not to believe:rolleyes:

carl
 
coulter said:
Where I live the AHJ is the utility. They say one ground rod is good enough. Several years ago I ask one of the engineers. Answer was, "Nominal ground rod resistance in this area is 300-1000 ohms, going up to 3000 ohms in the winter. We have no evidence that 25 ohms is any better than 3000 ohms. What is the point of driving a second rod dropping the ground resistance from 300 ohms to 150 ohms?"
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This I believe and they are probalby right.
 
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I think it makes good sense to just require two rods. Ground resistance changes with temperature, moisture content of the soil, etc. I am not sure that there is any real magic about a ground rod being at 25-ohms or less. At 25-ohms it will not do much for clearing faults, and at resistance much higher than that it would still be effective in the event of a high voltage event. But the code says what it says and 25-ohms remains the target. Since ground resistance is variable, and test procedures are less than precise (that whole three electrode fall-of-potential method reminds me of witchcraft!) it makes sense to just assume that the resistance is higher than 25-ohms and require the second rod and then go on with life. It is usually faster, cheaper, and easier to drive the second rod than to perform the test, especially if the AHJ is required to witness the test. With the advent of Ufer grounds the whole ground rod issue may be a thing of the past anyway.
 
I once had a job where driving or burying a rod was not an option, the building was on solid rock. They had a footing poured on the rock with an exposed rebar so I hooked on to it. When the inspector questioned this I asked him if he would approve the installation if I could proove 25 ohms or less & he said OK. I disconnected the GEC @ both ends and took a #12 wire from a CB to the rebar, turned on the CB and measured the current. Using Ohm's Law I calc'd the resistance at 9 ohms. He passed the installation. Hey, sometimes a little BS can work when all else fails.
 
haskindm said:
I think it makes good sense to just require two rods.
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I think it may be a good practice, but I would not agree that it makes sense for an AHJ to require what the code itself does not require.
 
haskindm said:
I think it makes good sense to just require two rods. ....
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I don't think it makes any sense at all. My preference would be for this code panel to take a lightning stroke (pun intended), wake up and do their job on this particular section.

I'm trying to be real specific here, I believe the code panels in general do a pretty good job.

carl
 
#12 Today, 09:32 AM
haskindm
Senior Member Join Date: May 2003
Location: Maryland
Posts: 799



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I think it makes good sense to just require two rods.
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If 2 are good then lets mandate 3, that's gotta be better. Better yet, lets get the separation distance in line with what appears to be proven. I have tested systems with multiple rods at 6' separation and the AHJ and electrician argued the NEC mandated this distance. Which it does as a minimum seperation.
 
Why are 2 ground rods better than one, when we may not even know what the resistance of the 2 rods may be.
Lets say the first rod is 150 Ohms, and adding a second rod (in perfect conditions) yields 75 Ohms. What is the real difference? For the general type of installations we install does it really make a difference?

I say one ground rod is sufficient, and I do not know where someone came up with the idea that 2 would really make any difference at all.

BTW: In our area, the resistance generally measured is much greater than 150 OHMs.
 
I am not saying that two rods are better than one. All I am saying is that the way 250.56 is written, the only way to be sure that you have complied is to install two rods. 250.56 does not care what the resistance is on the two rods, just that a second rod is installed if the resistance on one rod is greater than 25-ohms. No testing required, no guessing as to whether the resistance is lower or higher than normal on the day the test is performed, no argument regarding the method used for testing the resistance, no question about whether the AHJ should witness the test; just drive two rods and go on with life. I have not seen many installations where one rod would get anywhere close to the suggested 25-ohms resistance. Either we need to re-write 250.56 or the simple thing is to just install two rods. If you want to jump through all of the hoops in order to just install one ground rod, by all means do so. Whatever is acceptable to the AHJ in your area. For my money, I'll just slap two rods in the ground and move on.
 
Driving Ground Rod

Driving Ground Rod

Yes indeed my friend. Must get hammerdrill with vibra bit. I just rest my arm on top as it slices through the ground like a hot knife in butter. My father tells me of the old days of sledgehammers. The oldtimers are tough you know whats!
 
We had a job few years back 250 10" rods, in hard pack rocky soil, the techs would start the hammer drill walk away to do something eles and return in 5 or 10 minutes to finish off the install and set the next rod. This device is more than worth it weight in rods.
 
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