The insulation code
Letters describe the insulation compound and its ratings: T for thermoplastic, H for heat resistance, HH for high heat, W for wet location, X for cross-linked polyethylene, R for rubber, F for flexible. THHN, THWN-2, XHHW-2 and RHW-2 are all decodable from those letters.
Voltage and temperature
A voltage figure follows the insulation code — 600 V on building wire and portable cord, 2 kV on tray and some power cable, 5 kV and above on shielded medium voltage. Temperature ratings appear as 60°C, 75°C or 90°C and pair with the wet or dry designation.
Listings and standards
UL 83 for thermoplastic building wire, UL 44 for thermoset, UL 854 for service entrance, UL 1277 for tray cable. Those references identify the product category and are useful when the insulation code alone is ambiguous.
Conductor identification
Sizes appear as AWG or kcmil, with strand count in some legends. Colour-coded conductors, printed numbers on control cable, and tracer stripes on instrument pairs identify individual conductors inside a jacket.