Thermoplastic high-heat-resistant nylon (THHN) and thermoplastic high-heat and water-resistant nylon (THWN) building wire is a coated wire typically run through conduit or cable trays. This general purpose wire supplies an electrical current in a range of indoor wiring projects, including control circuits and appliances. It has a durable nylon jacket that resists abrasion, protects against shock, and shields the thermoplastic insulation and metal conductor from damage. THWN building wire is moisture-resistant and withstands higher temperatures than THHN building wire.

Thermoplastic flexible nylon (TFFN) and thermoplastic fixture nylon (TFN) wire is a coated wire typically run through raceways specifically designed for fixture wiring. It's primarily used in commercial applications to power hardwired light fixtures in ceilings and walls. It's more flexible than THHN building wire but less resistant to heat, and it is only available in limited gauges (18 and 16 AWG). TFN wire is a single solid wire that's not as flexible as TFFN and is used in applications where flexibility isn't a concern.