Installed in soil
Where installed in contact with soil, steel RMC and IMC do not generally require supplementary corrosion protection unless:
- Soil resistivity is less than 2000 ohm-centimeters or
- Local experience has confirmed that the soil is extremely corrosive. The authority having jurisdiction has the authority to determine the need for additional protection.
NOTE: Soils producing severe corrosive effects have low electrical resistivity, expressed in ohm-centimeters. Local electric utilities commonly measure the resistivity of soils. The authority having jurisdiction has the authority to determine the necessity for additional protection.
EMT in direct contact with the soil generally requires supplementary corrosion protection. However, local experience in some areas of the country has shown this to be unnecessary.
Transition from concrete to soil
Where steel RMC, IMC and EMT emerge from concrete into soil, it is recommended that protection be provided a minimum of 4 inches on each side of the point where the raceway emerges. In areas such as coastal regions, use the same method of protection for EMT emerging from concrete into salt air to lengthen the service life. Examples of protection include paint, tape and shrink-tubing.
Installed in concrete slab
Where installed in a concrete slab below grade, determine if EMT requires supplementary protection for that location. RMC and IMC do not require supplementary corrosion protection in this application.
Supplementary protection methods
Where supplementary corrosion protection is required for the conduit or EMT, the authority having jurisdiction must preapprove the method selected.
Following are typical methods of providing supplementary corrosion protection:
- A factory-applied coating which is additional to the primary coating for conduit or tubing.
- A coating of bitumen.
- Paints approved for the purpose. Zinc-rich paints or acrylic, urethane or weather-stable epoxy-based resins are frequently used. Oil-based or alkyd paints should not be used. Surface preparation is important for proper adherence. For best results, the conduit / EMT should be washed, rinsed and dried. It should not be abraded, scratched or blasted, since these processes could compromise the protective zinc layer. A compatible paint primer or two coats of paint add protection.
- Tape wraps approved for the application. Wraps must overlap and cover the entire surface of the conduit / EMT and all associated fittings. Shrink wraps are available that will protect the conduit and fittings without requiring a heat source.
- Couplings and fittings can also be shrink-wrapped.