Bare Copper Through Concrete

[Dobber1978]

Does anyone has information on running bare copper cable through concrete? This came up in a job I am working on and my worries are that either the cable or concrete will corode creating a weak point in my ground system or concrete pad,

Thanks
dob

 

[Dobber1978]

This bare copper cable that I am passing through the concrete is going to be used as a ground conductor

Dob

 

[Electic]

Commonly refered to as a "eufer ground". I am not familiar with possible chemical reaction between concrete and bare copper, but others have done it.

 

[itsmoked]

It is required here in my town.

The uefer grounds are in all new contruction. You tie the service drop ground to the rebar inside the concrete. This makes a very good ground because the concrete conducts pretty well but is also in intimate contact with thousands or square feet of earth due to the intimate rough interface between the concrete and the dirt.

But I am not sure exactly how this is done physically.. Sorry.

 

[stevenal]

We sleeve the copper with PVC conduit where ever it passes through concrete.

 

[dpc]

It's done all the time for Ufer grounds as well as connections to embedded ground grids, etc. Reinforced concrete is reasonably compatible with bare copper. If anything, you might get some galvanic action that reduces the iron in the rebar, but that is generally not a problem unless you have a constant dc potential on the ground grid wrt to the remote earth. The corrosion engineers I've worked with don't seem to get too worked up about putting bare copper directly in concrete.

The copper should not corrode. Moist concrete is a reasonable conductor - a lot better than you might expect. The biggest problem is probably physical damage to the ground conductor during construction.

Erico and others make flush grounding pads that you can install to allow a bolted connection to the ground pad. But these are expensive and a pain in the neck to form and keep in place when pouring the slab or deck. Normally, the bare copper is brought up above the finish grade and a five foot pigtail is coiled up for future connection to building column, equipment, or whatever you are trying to hit.

 

[rbulsara]

It is ufer grounding, named after a German engineer/scientist, I beleive. This is described in NEC as well as in IEEE green book. It is the best grounding electrode you can get in new construction.

When used for ufer grounding the bare copper conductor or the steel rebar has to be in direct contact with concrete. Passing thru a pvc sleeve defeats the purpose.

Ideally it should lie horizontally, within 2 to 4 inches from the bottom of the footing. One end is brought out of the footing and connected to the base of the steel columns. Now you have a nicely grounded steel structure, which should be preferred grounding electrode for the building systems.

If a horizontal 20' long rebar (1/2 inch dia. min.) already exists, a copper conductor is exthermically welded to it and a lead brought out as indicated above.

A mass of concrerete is considered better condcutor than just earth.

 

[davidbeach]

You can get some corrosion of the copper at the copper/concrete boundary. Take the approach of stevenal, of a PVC sleeve around the copper conductor at the point it enters the concrete, maybe a couple of inches long, and fill the space between the copper and the PVC with RTV and the concrete/copper boundary is no longer exposed and the corrosion stops/slows drastically.

 

[stevenal]

Dobber1978 said nothing about Ufer grounding. My response was to his question about passing the ground wire through concrete, not about bonding to rebar within. Sleeving the copper keeps the alkaline concrete away from the copper. We've seen bad corrosion when not sleeved. For a Ufer, it's probably better not to expose the steel rebar.

 

[RRaghunath]

There is discussion in IEEE80 on allowing the earth currents to pass through the concrete rebars. The code cautions that the bond between rebar and cement weakens due to drying effect of small earth currents that seem to be flowing in the circuits all the time. The rusting and swelling of rebars is another issue. Galvanic corrosion at the contact points between steel and copper also cannot be ignored.

I am not an expert in earthing systems but am very apprehensive of passing the copper through concrete or connecting the earth grid to the rebars in any way due to the above concerns.

I also would like to know more about Ufer grounding and specifically how the above concerns are addressed.

 

[jghrist]

Grounding pads mentioned by dpc may be desirable if you want to keep water from penetrating the concrete.

 

[dpc]

I agree that if there is going to be corrosion, it will be at the boundary where the conductor leaves the concrete. The short PVC sleeves are probably a good way to prevent this, but I have to say that I don't recall ever actually seeing it done. I was thinking mainly of indoor facilities. If I had outdoor substations in a coastal environment, the PVC sleeves might be worth the trouble. I don't think a short PVC sleeve would have much detrimental effect on a ufer ground as long as there was still sufficient bare copper in contact with the concrete below grade.

But bare copper coming out of concrete, even outdoors, is a very common practice, at least in the US. Maybe not the best practice, but common.

rraganuth - the Ufer ground is actually required by the NEC. If you have concerns about the copper to rebar connection, you might want to consult with local corrosion engineers. The copper will have more problems in the soil than in the concrete in most locations.