ASTM Standard Building Wire

TW/THW wire are a solid or stranded, soft annealed copper conductor insulated with Polyvinyl chloride (PVC).

TW wire stands for a thermoplastic, water-resistant wire.

THW wire is also thermoplastic, water-resistant wire, but is heat resistant, denoted by the H in the name.

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).

XHHW wire stands for "XLPE (cross-linked polyethylene) High Heat-resistant Water-resistant." XHHW cable is a designation for a specific insulation material, temperature rating, and condition of use (suitable for wet locations) for electrical wire and cable.