Ground Electrode 3

[dougjl]

I have a site where a concrete foundation will be being poured for a new building. I had planned to connect to the re-bar with a bonding jumper in order to use the concrete as a grounding electrode. I have since found out that the foundation will be poured on a vapor barrier. My understanding of that is that it will no longer be a ground, which makes sense.

I plan to drive some ground rods and create a ground electrode for the building since the foundation is insulated from ground.

My question is there any value to tying a bonding jumper to the re-bar even though is is not a ground? Is there anything that can be done to ground the foundation? Is there value in tying the re-bar to the ground rods?

Thanks again,
Doug

 

[dpc]

I would tie to the rebar. It will avoid the need to do a lot of explaining to the electrical inspector. This will also put the concrete floor at ground potential. And the vapor barrier might not be as great an electrical insulator as you might think.

The ground rod is needed regardless of the ufer ground. NEC article 250 covers how to bond everything together at the service.

 

[MiketheEngineer]

Check IBC 2009 - I think you need something like a 20' copper rod at bottom of foundation??!!

Can't remember but you can look it up

 

[dougjl]

If I am not mistaken, article 250 says a ground rod (supplemental grounding) is only "required" if there is a connection to incoming water pipe. In my case I have been told to expect that the incoming main will be plastic. I am putting the ground rods in regardless since it will be my only "real" tie to earth.

I like the idea of tying to the re-bar and bringing the foundation to ground potential.

Yes 20' is the length that is required. However it can be re-bar or cable as long as it is encased in the concrete.

Thanks for the input.

 

[7anoter4]

The polyethylene foil resistivity is elevated and a sheet of 2 mm thickness may present some megohms resistance.
Our practice is to build- from the beginning- a layer of embedded rebar beneath the actual foundation and to connect this with the upper foundation rebar using a rebar of 1/2" dia through the vapor barrier. In any case you have to connect the foundation grounding to the nearby Grounding System. Note: the new NEC does not limit the minimum rebar diameter as for heavy short-circuit current 1/2" is not enough.IEEE-80 art.14.6 indicates how to calculate the required rebar size.

 

[dougjl]

How big of a layer of embedded rebar are you talking? Just a 20' rod in a casing of at least 2" of concrete around it or more? I had considered this but wasn't sure how much rebar was enough for it to be effective.

Thanks,
Doug

 

[MiketheEngineer]

DO NOT tie it to the water pipes. I have talked to many workers in our water district. They say you cannot imagine the "shocks" they get while working on water pipes.

 

[dougjl]

Mike,
It is not an option as NEC 250.50 requires that a metal underground water pipe be bonded to and used as a part of the grounding electrode system.

 

[7anoter4]

2" around will be fair. But considering a possible small deviation 2.5" will be better.

 

[MiketheEngineer]

Dougjl -

While I am not positive - I believe that rule of tying to water pipes may be changing - at least in this part of the world!

 

[dpc]

The 2011 NEC 250.52 has an Informational Note indicating that concrete installed using a vapor barrier is not considered to be in direct contact with the earth. So this would not be considered, by itself, a grounding electrode meeting the requirements of the NEC to most inspectors.

You could install a separate concrete-encased grounding electrode that does meet NEC requirements, or drive a ground rod, or both. You can have as many grounding electrodes as you want.

You haven't mentioned the type or size of building, or the size of the electrical service. This could have an impact on grounding system design. The NEC is a minimum requirement, not necessarily the best engineering practice.

We always install a ground rod, no matter how good the ufer ground might be. It's cheap and will satisfy even the most brain-dead inspector. The NEC rules have changed over the years. Water-pipe grounds are still required for metal water pipes, but these are rapidly becoming extinct. I would never trust one as the sole grounding electrode even if metal pipe is used.

 

[dpc]

Just to clarify/correct my last statement - the NEC does not allow a water pipe to be used as the only grounding electrode. A supplemental ground must be used in addition to the water pipe. So I was going on about nothing - as usual.

Article 250.52 covers all of the acceptable grounding electrodes.

 

[7anoter4]

You are right dpc: NEC-up to the last edition-did not refer to the foundation insulation override and even in the last edition it is mentioned only as an information note.
The foundation grounding equalizes the potential throughout in a building reducing the touch potential and for short-circuit current to ground another path has to be created.
And as usual your posts are very interesting and very correct elaborated.

 

[dougjl]

The new building will be 95,000 sq. ft. steel building. It will be serviced initially by one 2500 KVA 480V transformer. A second transformer may be added when more load is added to the production area.

 

[7anoter4]

If it is a steel building then there are embedded columns [without vapor barrier] so you could use them as embedded electrodes as per NEC art.250.52:
(A)Electrodes Permitted for Grounding.
(2)Metal Frame Electrode. The metal frame of a building/structure can serve as a grounding electrode when it meets at least one of the following conditions:
(1) At least one structural metal member is in direct contact with the earth for 10 ft or more, with or without concrete encasement.

 

[dougjl]

There will be concrete footings that the frame will be bolted to. These footings will not have a vapor barrier beneath them, but the footings will only be a couple of feet deep. Since the steel will be bolted to the footing I am not seeing how this would be in contact with earth for 10' or more.

 

[dpc]

Reinforced concrete is a good grounding conductor when it is in direct contact with soil and stays moist (and doesn't freeze). You will definitely want to tie into these footings regardless of any NEC issues. Even a two-foot deep footing will be beneficial to the grounding system. Do a web search for "Ufer grounding" for more technical info.

 

[dougjl]

Thank all for your input. It has been very helpful.

 

[Reg87]

There! Its earthed.

 

[waross]

LOL

Bill
--------------------
"Why not the best?"
Jimmy Carter