The most common type of insulation used for simple parts store wire is a THHN (Thermoplastic High Heat Resistant Nylon), a PVC-covered wire mostly used in the industrial and housing markets.

I'm terminating No. 14 THHN stranded wire to a circuit breaker. One of the electricians on the job said terminating stranded wire to a breaker was a Code violation and that the NEC requires a fork ...