Professionals in the construction industry use electrical wire that is rated THWN for the alternating current systems of industrial, commercial and residential buildings. The THWN rating indicates that the wire is thermoplastic, heat-resistant, water-resistant and nylon-covered.
Wire with a THWN rating is resistant to solvents, oil and gasoline. It can be used in both wet and dry locations. Therefore, it is the type of wire that is most often used in underground applications. The conductor can be stranded or solid, and it can be made from either copper or aluminum.
Some manufacturers produce wire with a THWN and a THHN rating because the manufacturing process is almost identical. The THHN rating identifies the product as being high-heat-resistant but not water-resistant. Wires with the dual rating can be used in dry locations with a temperature that is less than 194 degrees Fahrenheit and in wet locations with a temperature that is less than 167 degrees Fahrenheit.