When the chemical composition of the soil is unknown (in the absence of soil testing), it is the user’s choice to stipulate tinning or bare. In most cases, bare copper is sufficient, although we recommend exceeding the rather minimal gauge requirements of the NEC. 4/0 AWG or larger is commonly found in industrial environments.

When soils are highly acidic, or alkaline, or suspected of being so, backfill material such as bentonite can greatly extend the life of bare copper. Further, bentonite retains water, thus can greatly increase contact area with the soil, thus reducing grounding resistance. Carbon-based backfills often contain other contaminants, such as sulfur and other elements which are harmful to copper and should be avoided.

Outdoor air can act as an electrolyte because it contains a variety of components which can cause corrosion of any metal. Varying moisture levels, salt and other contaminants are just a few. In overhead, outdoor construction, copper runoff can cause staining of pavement materials. It’s one reason why older overhead telephone wiring (before plastic coverings) was usually tinned. In addition, copper runoff can be very corrosive to galvanized steel support structures, even when not in direct contact. In such cases, tinning of the conductors is recommended to prevent such conditions.

“Precipitation run-off from copper and copper alloys can attack galvanized parts (BS 6651:1999 and IEC 61024-1-2, section 5.2); therefore, bare copper conductors or copper bus bars shall not be installed above galvanized steel, such as a tower, unless the steel is protected against the precipitation run-off “ (IEC 61024-1-2, section 5.2, quoted in Motorola R56.)