OK, guys, here's the results of today's experiment.
I used an (nominal rating) 1800 watt space heater as my load. This heater does have temperature settings; I set it on the maximum. Since the day was rather nice (65 degrees F), the heater would not operate at full bore.
For a cord, I wound about 80 ft. of #12 SJEOW (about 2.3mm dia for stranded) around a cable reel.
The reel was 4" diameter at the smallest, and about 5" long. The core was cardboard tube, while the ends were plastic. I had 4 or 5 layers of cable when done.
I placed a thermocouple in the middle of the length, atop the first layer...approximating the location of the char in these pics.
After an hour of running, the thermocouple registered 110 degrees F; I weasured a draw of 11.4 amps. This draw did not vary for the few minutes I measured it.
I did note that one of the molded-on plugs of the cord supplying this set-up was noticeably warm to the touch. The test spool did not seem warm; I did not uncoil the cord.
I would conclude that overheating on a coil is NOT a concern. I would like to repeat this test again some time, with a load closer to 20 amps, and under conditions that would eliminate any cycling that the heater may do; I won't see those conditions until the holigay season.
