Concrete-Encased Electrodes 250.52(A)(3)

[eprice]

I'd like to throw out some questions for discussion and opinions. There are three methods that I see in my area for providing a concrete-encased electrode. The two most common are:

1)A length of bare #4 copper, usually solid, but occasionally stranded is placed in the footing so that at least 20' runs parallel with the footing rebar. This is usually loosely draped around a rebar and is sometimes clamped to a rebar, sometimes not. The other end is long enough to come out the top of the foundation. Usually it is long enough to reach unspliced to the disconnect enclosure. In this approach, I believe the #4 copper wire is an NEC legal concrete-encased electrode and the contact with and/or clamping to the rebar is just added value.

2) a length of #4 copper conductor is clamped to the rebar in the footing and run out the top of the foundation. Often this conductor is long enough to run unspliced to the disconnect. In this approach there is no particular length of the #4 copper than runs along the length of the footing. In this approach, I believe the #4 copper is a GEC and the footing rebar is the NEC recognized concrete-encased electrode.

3) A third, less common method is to bend a 20' length of footing rebar so that one end comes out of the top of the foundation. The lower leg of the bent rebar is tied to the rest of the footing rebar with the usual wire ties.

My questions are:

1) Does anyone see anything about any of these approaches that would be a violation of the NEC?

2) Any opinions as to which method provides the best electrode over time? Does anyone have any data to support an opinion one way or another? I wonder about the effect of moisture and contact with concrete on the copper wire over time.

3) Is the green copper oxidization film a conductor or is it an insulator. If approach #1 above is used, will oxidization cause the electrode to become less effective over time?

 

[dereckbc]

Re: Concrete-Encased Electrodes 250.52(A)(3)

I am happy you brought this topic up. I have been waiting for it for some time now. The "Concrete Encased Electrode" requirements in the NEC are not a Ufer ground as specified by Herbert Ufer.

Originally posted by eprice:
1) Does anyone see anything about any of these approaches that would be a violation of the NEC?

2) Any opinions as to which method provides the best electrode over time? Does anyone have any data to support an opinion one way or another? I wonder about the effect of moisture and contact with concrete on the copper wire over time.

3) Is the green copper oxidization film a conductor or is it an insulator. If approach #1 above is used, will oxidization cause the electrode to become less effective over time?

1. This method most closely matches the method required by the NEC. However I have made the mistake of using this method in a few sites that had high lightning risk exposure. As a result three sites were struck, and there was significant damage to the concrete foundation. The reason for the damage was the moisture in the concrete vaporized and caused the concrete to crack and split. I do not condemn this method as long as the copper wire is bonded to the re-bar, and all the re-bar is bonded together. The only other problem I can think of using this method is the copper will react with the PH in the concrete and eventually deteriorate over time.

2 & 3. I think I already covered that in 1.

A method I sometimes use in communications facilities is a Ufer ground system. It is a very inexpensive method if used during initial construction, and provides very low impedance up to 1 Mhz at the point of attachment. It is very simple. Specify that all re-bar be bonded by either double tie-wires, compression connections, or best of all a spot weld. Stud a section of re-bar out of the foundation near the service equipment as to facilitate a GEC connection via exothermic weld.

To keep an inspector happy you may have to supplement with a # 4 AWG with 20 feet of conductor in one of the footers to comply with the NEC. But in all the designs I have installed, I consulted with the inspectors and had that requirement waived.

In extreme cases such as facilities with towers, D.O.D and/or other communications facilities, you can also stub out the re-bar every twenty feet just below grade and weld ground rods along with copper wire radials extending outward from the building. But that is another topic.

[ December 16, 2003, 02:29 PM: Message edited by: dereckbc ]

 

[tom baker]

Re: Concrete-Encased Electrodes 250.52(A)(3)

The orginal Ufer ground was 20 ft of 4 AWG bare solid wire. I have copy of the IEEE article by Ufer on his proposed grounding method.

 

[pierre]

Re: Concrete-Encased Electrodes 250.52(A)(3)

Dereck
I cannot quote the building code, but I believe it is not permitted to stub the rebar out of the concrete footing at all.

Pierre

 

[bennie]

Re: Concrete-Encased Electrodes 250.52(A)(3)

In Oregon State, the rebar is required to be stubbed a minimum of 12 inches above the floor plate nearest the panel location.

The building inspector places a tag on the rebar during the footing inspection. Some paint it green with spray paint.

 

[roger]

Re: Concrete-Encased Electrodes 250.52(A)(3)

Bennie, a green Grounding Electrode, surely not.

I have actually had engineers require this rebar stub up in their specs. It is not a local code here though.

Roger

[ December 16, 2003, 07:34 PM: Message edited by: roger ]

 

[ryan_618]

Re: Concrete-Encased Electrodes 250.52(A)(3)

Originally posted by pierre:
Dereck
I cannot quote the building code, but I believe it is not permitted to stub the rebar out of the concrete footing at all.

Pierre

Unless it's a New York thing it is perfectly OK by the building code(s)

 

[stamcon]

Re: Concrete-Encased Electrodes 250.52(A)(3)

Pierre, "rebar" used as for concrete reinforcing must be encased with concrete, a minimum distance from the outside surfaces of the concrete that is exposed to earth or weather. Under the UBC, there is no restriction against stubbing up through the top(provided the top surface of the concrete is not exposed to earth or weather).

steve

[ December 16, 2003, 10:14 PM: Message edited by: stamcon ]

 

[eprice]

Re: Concrete-Encased Electrodes 250.52(A)(3)

I agree with stamcon on the concrete cover for the rebar. the IRC and the IBC refer to ACI 318 for concrete. ACI 318 section 7.7.1 requires 1 1/2" concrete cover for #4 rebar in formed footings and foundations that will be exposed to soil or weather. 3" is required if the footing or foundation is cast against earth without a form. The same requirements can be found in the UBC I would have a problem with a bunch of rebar protruding out of the footing and exposed to soil or weather.

 

[a.wayne3@verizon.net]

Re: Concrete-Encased Electrodes 250.52(A)(3)

Eprice,Thw AHJ here has addendums that require just that A 20 ft piece of rebar tied into the footer that stubs at the service location.They cannot pour a slab without the uffer inspection.
Sometimes during grading it gets ripprd out and they want a 30in deep ditch 20 ft. long with bare #2 in it and 2 ground rods.We did about 1250 uffer grounded homes this year.On a final they require that the uffer is cut flush with grade

 

[eprice]

Re: Concrete-Encased Electrodes 250.52(A)(3)

Wayne,

I'm not sure whether you were responding to my first post in this thread, or my second. Are you saying that they are "requiring" the Ufer to exit the foundation at or below grade, in contact with soil? My understanding is that one of the reasons for the concrete cover requirements in the building code is to protect the rebar from corrosion. I would be interested to know how long a Ufer in direct contact with soil can be expected to last. I know it will depend on soil conditions.