Is it safe to use a 220v extension cord for my shop heater?

[Bill Howatt]

FWIW, here is how much heat is generated by running 20A down the various common sizes of 2 conductor cord (half of shown value for each single conductor):
#12 1.3W Per foot for 2 conductor cable
#14 2.0W
#16 3.2W
#18 5.1W
Only the #12 cable is the legal size for a 20A load so the others are effectively overloaded.
If the load is dropped to a more typical 15A, then the #16 wire value drops to 1.8W per foot for 2 conductor cable
I'm not going to make much of a call on fire risk but I doubt 15A through a #16 cord would heat it up that much such that it would cause ignition. It's not much more than the dissipation of the legal #12 at 20A. However, insulating it such as covering it with carpet would make the heating from the power dissipation worse but I'm going more for the damaged (broken) conductors when under a rug. Also, how well the wire is bonded to the plug and receptacle pins, strain relief and pin construction for a good connection is something I'd be more concerned about on cheapie cords.