Hi Brian, All, "THHN" is a designation for the type of insulation on the wire, where "T" means 'very common, dry only'. "HH" means 'much higher loaded current tempratures may be used, up to 90 deg. C'. "N" means 'the jacket is extrudid nylon or thermoplastic polyester and is very resistant to gas and oil and is vercommercial buildings. Happy Coiling, Tony Greer