THHN Thermoplastic High Heat-resistant Nylon-coated wire is a single conductor wire with PVC insulation and a nylon jacket. THWN thermoplastic heat- and water-resistant wire is essentially the same as THHN and the two are often used interchangeably. THWN is also a single conductor wire with PVC insulation and a nylon jacket. THWN-2 wire is basically a THWN wire with extra heat protection and it can be used in very high heat situations (up to 90°C or 194°F).