iwire, the 20 amp receptacle should be polarized for 20 plugs and therefore the 15 amp 16 gage extension cord couldnt be plugged into it, so then it wouldnt be carrying the 16 amps or so that the 20 overcurrent would allow. By the way, I am here in Canada, so maybe there is some difference between our 20 amp receptacles and the US version. The use of 20 amp overcurrent on the 15 amp rated cord seems to go back to the screw in fuse days when homeowner, frustrated with the 15 amp fuse always blowing, installs a 30 amp screw in fuse to fix the problem.
