Supplementary Equipment Grounding Conductors and Corrosion
Question: Does a supplementary equipment grounding conductor (EGC) solve corrosion concerns in metal raceways?
Answer: The belief that a supplementary wire type equipment grounding conductor will solve corrosion concerns is unfounded. However, the National Electrical Code (NEC) and Underwriters Laboratories are quite clear on how to deal with corrosive atmospheres. None of the requirements or recommendations suggests a supplementary wire type equipment grounding conductor (EGC) as a means to solve corrosion.
The NEC and the UL product IQ information clearly require supplementary corrosion protection where the atmosphere (soil) has severely corrosive effects. “In the absence of specific local experience, soils producing severe corrosive effects are generally characterized by low resistivity (less than 2000 ohm centimeters).” Corrosive effects are very easy to determine in all local jurisdictions. Generally, the corrosive conditions will vary throughout most states, but they do exist in jurisdictions nationwide. For example, in Maricopa County, AZ, the corrosive conditions described above are an issue, and supplementary protection on all metal in contact with the earth is required. Contractors do a very effective job dealing with this in the greater Phoenix area at minimal additional cost.
This is a design issue. To determine if corrosion is an issue in your jurisdiction, you can check with local sources such as the local utilities: electric, gas and water. Generally, the general contractor will have this information long before the electrical installation begins. Soil conditions will vary some locales, or they may be similar in a whole jurisdiction. It is up to the Authority Having Jurisdiction (AHJ) to determine this and to enforce section 300.6 (NEC) “Protection against Corrosion and Deterioration”.