Our school bus fleet at work all have heaters on them, we plug in about 150 buses everyday that it's below 20* outside. We have found over the years extension cord quality plays a big role in how the heaters work and how long the cords last. Some cords have only two small contact points in the female end, the better cords have full length contact of the receiver cord prongs. The standard 14g or 12g cords from the home center use the small contacts, while the electrical supply house had the better full contact cords. Once we switched to full contact cords, we haven't had the need to replace them after two years of service, the Home depot cords were lasting one season or less. We had a couple of the cheap cords catch fire, the side of the bus from the socket up was black from the plastic fire of the cords! Use only the clear end cords so you can see the connectors, and use the heaviest cord you can. We use nothing but 12 gauge, granted our heaters for the diesel engines are much larger, 1400w I believe, but its best to be on the safe side when it comes to outlets, cords and heaters. Double check you breaker box, and outlet before next winter.
I started cutting cords apart to figure out why some burnt up and some didn't. Once you see how the cheap ones work, you will never use them again!
