I've 'tested' three listed extension cords, one at a time, by plugging a 4 year old 1500W fanned heater ($10 at Home Depot) into a 20 amp house circuit (no other appreciable load). The cords:
- orange 20+ year old 9 foot 14/3 round
- grey 20+ year old 8 foot 14/3 flat
- yellow unknown-age (but modern) 25 ft 12/3 flat
All cords have molded plugs and are unmodified. None are damaged or look worn. All are single-taps, that is, none are triple-taps. Likely three different manufactures, none known.
I suspect code probably requires such a heater be plugged directly into the wall a outlet. I don't know. Regardless, this is how I tested them.
Anyway, used one at a time, all three female ends get warm. The middle of the cords stay 'room temperature' 60F. The female ends get warm to the touch, noticeable, but no where near 'hot', just pleasantly mildly warm. The 12/3 flat cord has very thick insulation, and the warmth travels from the plug further up the cord - say 12" - than the either of the two 14/3 cords.
The plugs on the 14/3 cords feel warmer than the adjacent cord. The plug on the 12/3 cord feels the same, or possibly a bit cooler. I attribute the differences to differences in heat-conduction of 12 ga vs 14 ga wire.
I imagine all 9 female terminals (molded within rubber plug) are soldered to the conductors with no increase in resistance at the connection.
I suspect this means that there is a huge 'contact' resistance between female terminals and the male terminals in the heater's end.
How can I further investigate and hopefully eliminate the 'problem' if there is one. Maybe cord amperage/gauge/length tables are for peak amps, not sustained amps.