It’s not bad to plug anything into an extension cord, it’s bad to plug anything that draws more power than the extension cord is capable of safely handling. But rather than explaining gauges over length and telling people that heater isn’t safe to plug into a 50 foot, 20 gauge cord, we just say don’t use cords.
The cords that can handle the power over decent lengths tend to be expensive, so a lot of people don’t have them around, and they’ll probably use the same thin cord that was fine for their desk lamp.
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