What Is Building Wire?

Allied Wire and Cable's building wires include THHN, TFFN, and TFN wires, as regulated by the National Electrical Code (NEC). TFFN wires are for branch circuits in buildings and commercial premises, while TFFN and TFN are fixture wires for luminaires and control panels. These wires must be installed in a conduit, flexible tubing, or raceway except if they're used for machine tool or appliance wiring. Building and fixture wires conform to various UL styles, and many have dual Machine Tool Wiring (MTW) certification. AWC building wires use solid or stranded copper conductors, PVC insulation, and a nylon jacket.

Types of Building Wire

THHN Building Wire

THHN wire is a relatively stiff single conductor wire with solid or stranded copper conductors, PVC insulation, and a nylon jacket. It's a tough, abrasion-resistant wire suitable for use in conduits and raceways. THHN wire has a voltage rating of 600 volts. You can use THHN wire for branch circuits in building and commercial wiring installations. The wires have a maximum temperature rating of 90°Celsius in dry and damp conditions. Available sizes are from 14 American Wire Gauge up to 1000 MCM, with current ratings between 15 and 615 amps. The letters THHN have the following meaning:

  • T: Thermoplastic insulation
  • HH: High heat resistance and a temperature rating of 90°C
  • N: Nylon jacket

TFFN Fixture Wire

Fixture wire is a single-conductor, flexible stranded copper wire with PVC insulation and a nylon jacket. You can use this wire for wiring luminaires to branch circuits and for internal wiring inside luminaires and other enclosed equipment where the wires aren't subject to movement, bending, or flexing. The NEC requires that TFFN wire is enclosed, and unlike THHN wire, you can't use TFFN wire for branch circuit wiring. The wire's voltage rating is 600 volts, and it has a maximum temperature of 90°C. The NEC permits only two gauge sizes: 18AWG and 16AWG. The letters TFFN stand for:

  • T: Thermoplastic insulation
  • FF: Flexible fixture
  • N: Nylon jacket

TFN Fixture Wire

TFN fixture wire is a less flexible version of TFFN wire, with a solid copper conductor. The NEC also permits versions with seven strands. The letters TFN stand for:

  • T: Thermoplastic insulation
  • F: Fixture
  • N: Nylon jacket

Other Building Wire Applications

Most AWC THHN wires carry dual ratings. These include machine tool wire, UL1063 microphone wire, and certain types of UL758 appliance wire. Our TFFN and TFN wires have dual UL style ratings including UL1316, UL1408, and UL1452. These wires also conform with the Canadian Standards Association Appliance Wiring Material (AWM) 1 A/B standard and TWEN certifications.  

Building Wire General Specifications

  • Conductors: Solid or stranded annealed copper wire
  • Insulation: Flame-retardant extruded PVC
  • Jacket: Abrasion-resistant nylon
  • Size THHN: 14 AWG to 1000 MCM
  • Sizes TFFN and TFN: 18 AWG and 16 AWG

Building Wire Certifications


THHN wire certifications include:

  • MTW
  • UL83
  • UL1063
  • UL758
  • RoHS (on request)


TFFN and TFN wire certifications include:

  • TFFN
  • TFN
  • MTW
  • UL1316
  • UL1408
  • UL1452
  • CSA AWM 1 A/B
Contact us for help and technical advice on selecting the right building wire.