In wire classification, what does the T indicate?

• A. Thermoplastic insulation
• B. Thermal rating
• C. Temperature-of-operation
• D. Type of conductor

Answer: (A)

In wire designations, the letters describe the insulation and construction. The T specifically flags Thermoplastic insulation, meaning the insulation material is thermoplastic rather than thermoset. This matters because thermoplastic insulations soften with heat and can be remolded, which is different from other insulation types. In a designation like THHN, the T tells you the insulation is thermoplastic, while the other letters indicate additional properties (for example, heat resistance and nylon coating). Temperature rating or operating temperature is provided separately (as a numeric rating), and the conductor material (copper or aluminum) is indicated elsewhere, not by the T.