The wire insulation rating is just as important. For example, a wire from an NM-B cable should never be used to replace power wring inside a unit even though it might be the same material (copper) and gauge. The wiring inside most equipment is rated as machine tool wiring (MTW). MTW insulation is a thermoplastic that is rated for up to 600 volts and is moisture, heat, and oil resistant. The insulation on NM-B cable is not moisture, heat, or oil resistant.
The wire insulation rating can be found printed or embossed on the wire. The figure below shows the marking.