TFFN Wire

TFFN (Thermoplastic Flexible Fixture Nylon) wire can be used as a fixture wire, a machine tool wire and an appliance wire. This wire is limited in sizing to 18 AWG and 16 AWG exclusively. The wire is designed with a stranded conductor that makes it extremely flexible. The flexibility of the wire limits its use to extremely large raceways. If the conductor is solid, which is called TFN, it is not flexible. This variation of the wire is only used in fixtures that do not require flexibility. TFFN wire does not perform well when exposed to water or moisture, and therefore cannot be used in outdoor conduit. It is immune to oil and gas and can be used in dry places up to 90 degrees Celsius.
TFFN vs THHN: The main difference between TFFN and THHN is that TFFN is a fixture wire. This implies that the wire is flexible and can be used in lighting fixtures, limited power circuits, control circuits and Class 1 fire alarm circuits. Fixture wire is prohibited by the National Electrical Code (NEC) for use in overall branch circuits due to the limited capacities of such wire in branch circuits.
Products
Thermoplastic, High Heat resistant, Nylon jacket insulation (THHN) wire is used in conduit or in cable trays due to its rigidity and tough insulation characteristics. THHN can be stranded (several small strands of wire grouped together), allowing for more flexibility and less chance of breakage, or solid (one wire), which is more durable and rigid. THHN wire is suitable for use in dry locations with temperatures that do not exceed 90 degrees Celsius.
Cross-linked polyethylene, High Heat-resistant, Water-resistant (XHHW) wire is very similar to THHN. The difference is that XHHW wire is overall a more durable wire that can withstand higher temperatures and is water resistant. XHHW wire can also be found in both stranded and solid wire options.
Machine Tool Wire (MTW) wire is much more flexible than THHN wire with insulation that is not as durable, making it a poor choice to run through conduit. The flexibility makes it ideal for winding through tight areas, such as wire ducting in an industrial control cabinet. MTW wire is almost exclusively found in stranded wire, which allows for much of the flexibility and reduces the chances of the wire breaking. MTW wire is intended for use in dry locations that do not exceed 90 degrees Celsius or wet locations that do not to exceed 60 degrees Celsius.