What you mentioned is UL 62, flexible cords, and different kinds of flexible cords have different requirements. For the strands, you can reference table 3 of UL62:
10AWG:64mm
12AWG:51mm
14AWG:44mm
16AWG:38mm
18AWG:32mm
But I still suggest you that you must know how to use the standard by yourself, it's much better...