THHN - 10 Ga 600v Hook Up Wire BrownSold in sets of 500 THHN (Thermoplastic High Heat resistant Nylon coated) wire is designed with a PVC (polyvinyl chloride) insulated nylon sheath. Most THHN building wire comes with the dual rating THHN THWN, meaning it is usable in both wet and dry conditions, as well as both indoor and outdoor use. THHN carries a UL rating of 90 degrees Celsius in dry environments and 75 degrees Celsius in wet environments. The conductor can either be copper or