Virtually all electric heaters that I've seen say to never use them with an extension cord and this advice seems to be echoed by fire safety organizations and such.
However, I'm curious about exactly what the danger is.
I'm aware of why you shouldn't use a flimsy little 18 gauge extension cord or hundreds of feet of cord (too much resistance, leading to the cord heating up and possibly starting a fire.) However, what's the problem with using say, 20 feet of 12 gauge extension cord? Or is that really even a problem?
It seems odd to me that 100 feet of 12 gauge wire from the panel to an outlet is fine, but an extra 10-20 feet of 10 or 12 gauge wire in an extension cord is problematic.
Is the "never use an extension cord" just overly cautious advice given to prevent people who don't know the difference between 12 gauge wire and 18 gauge wire from hooking up a heater through a wire not meant to handle that much current? Or is there an actual reason why adding maybe another 10-20% to the total wire length from the panel using the same wire gauge as what's actually in the wall is a problem?
For what it's worth, it's probably obvious from my use of units, but I'm in the USA and curious particularly with regard to the 120 V / 20 A circuits that are normal for power outlets in homes here.