Doug asked:
What is the purpose of PVC sleeves in concrete? Is there any reference material discussing ground wire installation methods in concrete?
Our Answer:
The PVC sleeve is a requirement under NEC 250.64(B) for mechanical stress relief of equipment grounding conductors. There is an excellent illustration in the NEC Handbook at 250.52(A)(3) Exhibit 250.23.
We do not recommend PVC sleeves as there are a number of vendors who sell 2 and 4 hole brass bonding plates specifically designed to provide a rebar/concrete interface connection point. A grounding electrode conductor that is broken entering a concrete slab, whether protected by a PVC sleeve or not, will require you to bust the concrete slab up to repair it. A brass plate with bolt holes in it makes for a quick and simple repair. It also allows for future testing as a simple wrench can remove the connections.
Corrosion is not the reason for the PVC sleeve, however it would be advisable to test the concrete prior to pouring to ensure that the pH is slightly on the caustic side. Acids corrode copper, caustics protect.
Note that concrete has a very high level of water content, even when cured as relative humidity levels between 60 to 95% RH are common. Because concrete is so porous and tends to absorb water, vapor barriers are commonly installed between the slab and the earth, so as to prevent water seepage in to a building. In fact, your moisture content in your slab will vary with the seasons. Humidity monitors are sometimes installed directly into concrete slabs for monitoring reasons. This is why concrete will explode if you put a blow torch to it or pass a large electrical current through it, the inherent water in the concrete will turn in to steam, thereby exploding the concrete.
The Engineering Experts at E&S Grounding Solutions
