Test of ohms for ufer ground

[crtemp]

I recently wired a house that has a ufer ground. It has 1 piece of rebar sticking out of the foundation by the electrical panel and another sticking up about 10 feet away. That is all the inspectors require here. If they don't see it before the concrete is poured the two ends sticking up is good enough so they could do a continuity test between the two and verify that its tied into the grid that way. Anyway after I wired the house the homeowner showed up and claimed he did a ohms test on the ufer ground and it came back at 800 ohms. I'm not sure what he did to test it but I wouldn't think that could be correct.

I have access to a Fluke 1520 megohmmeter I'm just not sure how to go about testing it myself.

I don't see anything in the code that says that the ufer ground has to be under a certain amount of ohms. Is this true? I only see where it talks about resistance of rod, pipe, and plate electrodes in 250.56 (2008 NEC)

 

[GoldDigger]

Nothing in the code requires either a measurement or a specific value because of experience with a properly installed Ufer.
Now if the rebar is coated everywhere but where it pops out or there is plastic under the entire slab including footings, I could see a potential for high resistance.
If you do not have a clamp type ground electrode tester AND a known good ground such as utility neutral or rod array to connect it too, you will have to use the three point method with a dedicated tester or use an ordinary meter and do the math yourself.
There are good articles on both, possibly on the Fluke website.
I do not think the Fluke megohmmeter will help.

Tapatalk!

 

[infinity]

Since you have a CEE the NEC doesn't care what its resistance is. Even if it really is 800 ohms as the HO stated it's still code compliant. I would love to hear how the HO actually measured the resistance.

 

[crtemp]

Since you have a CEE the NEC doesn't care what its resistance is. Even if it really is 800 ohms as the HO stated it's still code compliant. I would love to hear how the HO actually measured the resistance.

That's what I was thinking too. The homeowner says he is a retired state electrical inspector from California so I just wanted to make sure I was reading the code correctly before I email him and tell him that it doesn't matter what the ohms are.

Thank you for the replies

 

[GoldDigger]

Since you have a CEE the NEC doesn't care what its resistance is. Even if it really is 800 ohms as the HO stated it's still code compliant. I would love to hear how the HO actually measured the resistance.

But, if the resistance really is anywhere near 800 ohms, I would feel pretty strongly that unless the slab was poured on solid rock in the desert, there is something wrong with the Ufer. Something that should have been noticed if inspected before the pour.
The stub to stub continuity check does not replace that inspection.
And if the Ufer is not compliant it is an NEC violation even if the resistance reads low. A low resistance read is just circumstantial evidence that it is compliant.

Tapatalk!

 

[acrwc10]

I recently wired a house that has a ufer ground. It has 1 piece of rebar sticking out of the foundation by the electrical panel and another sticking up about 10 feet away. That is all the inspectors require here. If they don't see it before the concrete is poured the two ends sticking up is good enough so they could do a continuity test between the two and verify that its tied into the grid that way. Anyway after I wired the house the homeowner showed up and claimed he did a ohms test on the ufer ground and it came back at 800 ohms. I'm not sure what he did to test it but I wouldn't think that could be correct.

I have access to a Fluke 1520 megohmmeter I'm just not sure how to go about testing it myself.

I don't see anything in the code that says that the ufer ground has to be under a certain amount of ohms. Is this true? I only see where it talks about resistance of rod, pipe, and plate electrodes in 250.56 (2008 NEC)

It is possible that the foundation contractor just stuck two pieces of rebar in the wet concrete and they don't tie to the foundation steel. Without a pre-pour inspection its just a guess.

 

[crtemp]

It is possible that the foundation contractor just stuck two pieces of rebar in the wet concrete and they don't tie to the foundation steel. Without a pre-pour inspection its just a guess.

I doubt it but anything is possible I guess. Even if the ground did have a resistance of 800 ohms would that be terrible?

 

[gadfly56]

It is possible that the foundation contractor just stuck two pieces of rebar in the wet concrete and they don't tie to the foundation steel. Without a pre-pour inspection its just a guess.

I was thinking along those lines. I wonder if the HO just put the meter between the two pieces of rebar?

 

[acrwc10]

Pop in 2 ground rods and make him happy.

 

[crtemp]

Pop in 2 ground rods and make him happy.

Washington does not accept ground rods anymore. Only ground rings if no ufer ground.

 

[acrwc10]

Washington does not accept ground rods anymore. Only ground rings if no ufer ground.

Sweet!

 

[GoldDigger]

If Ufer really measures 800 ohms, which is not a given, the GC may be on the hook to fix it.
But if the inspector approved the Ufer, you can just drive some rods anyway as supplemental grounds and the owner will be happier and maybe even, by some stretch of the term, safer.

Tapatalk!

 

[gadfly56]

Washington does not accept ground rods anymore. Only ground rings if no ufer ground.

That's interesting. What was the thinking behind that?

 

[GoldDigger]

Typically gives a far lower earth electrode resistance than you can get with 10' rods.
Now the thinking about why that should be necessary is another issue.

Tapatalk!

 

[crtemp]

That's interesting. What was the thinking behind that?

Not sure

052 Grounding electrodes.
(2) Except for mobile/manufactured homes, a concrete encased grounding electrode must be installed and used at each new building or structure that is built upon a permanent concrete foundation. If the concrete encased grounding electrode is not available for connections, a ground ring must be installed per NEC 250. The electrode must comply, with NEC 250.52 (A)(3). Inspection of the electrode, may be accomplished by the following methods:
(a) At the time of inspection of other work on the project, providing the concrete encased electrode is accessible for a visual inspection;
(b) At the time of the service inspection providing the installer has provided a method so the inspector can verify the continuity of the electrode conductor along its entire length (e.g., attaching a length of copper wire to one end of the electrode that reaches the location of the grounding electrode conductor that will enable the inspector to measure the resistance with a standard resistance tester). The concrete encased electrode does not have to be accessible for a visual inspection; or
(c) Other method when prior approval, on a job site basis, is given by the inspector.
If a special inspection trip is required to inspect a grounding electrode conductor, a trip fee will be charged for that inspection in addition to the normal permit fee.

 

[dereckbc]

Well one thing often over looked today is most concrete foundations have VAPOR BARRIER installed between the ground and concrete with 6 mil thick plastic sheathing is used. That pretty much renders the CCE to useless status as a ground electrode.

 

[GoldDigger]

Which is why the code states that such a vapor barrier prevents the rebar connection from being a Ufer ground and why the code specifies foundation or footing concrete which is less likely to have a vapor barrier.
If the stub ups of rebar the OP mentioned were in the slab they could well not make a good connection to the rebar in the footings.
Yet another reason for a pre-pour inspection.
If the whole mass of concrete is over a vapor barrier, it looks like WA will require a more expensive (especially after the fact) ground ring!
The only ground connection for a mass of concrete over plastic may well be water or drain pipes that go through the vapor barrier. And those also may not conduct well to the rebar.


Tapatalk!

 

[ceb58]

Since you have a CEE the NEC doesn't care what its resistance is. Even if it really is 800 ohms as the HO stated it's still code compliant. I would love to hear how the HO actually measured the resistance.

That's the main point. If it was inspected and passed and you got paid then all is good with the world.

 

[GoldDigger]

Does that same philosophy apply to any known NEC violation that you were not responsible for that passed inspection?
An opposing attorney will dispute that, I am sure.
You can reasonably deduce from a valid 800 ohm measurement that there is something badly wrong with the alleged Ufer.
But the first step is still to find out how that resistance was measured.

Tapatalk!

 

[ceb58]

Does that same philosophy apply to any known NEC violation that you were not responsible for that passed inspection?
An opposing attorney will dispute that, I am sure.
You can reasonably deduce from a valid 800 ohm measurement that there is something badly wrong with the alleged Ufer.
But the first step is still to find out how that resistance was measured.

Tapatalk!

What violation? As said there is no mention of required resistance for a CCE. Is it a valid measurement? We don't know. Break out the equipment and lets do a FOP test.