THHN or THWN nylon PVC wire B series are used for fixedly laying in engine devices of A.C .Rated voltage 450/750v or lower; R series are used for household Detailed Product Description THHN (THWN) 1) PVC Insulated Nylon Sheathed Cable 2) Standard:UL83 3 Voltage:450/750V 4) Ranging: 24AWG to 500MCM.