Current carrying capacity is also partly a function of the number of strands in each conductor. So more strands, the more current carrying capacity, this is because electrons only flow on the outside of the conductor, thus is a function of surface area.
Thanks for that Jim. I have just checked most of the equipment in the house and while many have UL markings on them, none have UL marked cords except for an old treadmill motor. The others have various other testing laboratory markings on them. Maybe it is that in Australia we have different voltage/plug configurations to that of the US but if the plug/cord is not UL stamped, would that not make the UL sticker on the appliance void. Just thinking .The UL (Underwriter's Laboratory) tests equipment to determine if it's safe to use as manufactured. Cheap equipment without the UL sticker may not meet all safety requirements and codes. The 12 gauge cord may be OK on the heater because of its length and the insulation type used in the cord. Check an electrical code book wire chart and there are different current ratings for number of conductors, and temperature ratings of the insulation. In the US #12 copper is rated at 20 amps for type TW, 60C to 30 amps at 90C for THHN insulation (usually limited to a 20 amp breaker).
Does it have a UL sticker? That could answer some of your questions. The insulation type in the cord will determine the current carrying capacity of the cord.