Oil-filled heater cord gets hot

It's an older DeLonghi. We have several, but this is the only one that is going bad like this. I replaced the plug housing with a modular one from Ace, but the cord itself heats up. Do I need a better plug, or do I need to open the appliance housing itself (if possible) and replace the entire cord? Other than the heat, the cord does not seem brittle or frayed, nor was the cord in a location subjected to movement or abuse.
Along the same lines, one that was taken to work had its plug replaced by the building maintenance with one claimed to be "heavy duty", but I'm not sure what is meant by that -- and it's a grounded plug, while the cord is just plain polarized (meaning the ground plug is useless). Replace with a proper two-wire plug?
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Dan Hartung wrote:

Where along the cord does it heat up. I ask because a bad connection at the plug end will cause the cord to heat at that end. If the cord heats along the entire length, then the cord is too small and should be replace (or the appliance has changed as is using a lot more current). If the cord heats at the plug end, the plug may not be bad, the outlet it is plugged into maybe worn and need replacement. There is no reason to remove a grounded plug as long as it is properly wired to the two strand cord (i.e., the ground should not have any wire attached to it).
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George E. Cawthon wrote:

Thanks (as well to other posters), that points to my plug replacement as the problem. I understand that space heaters have special "heater cord" as opposed to the old trick of taking a standard 3' extension cord and cutting off the end. Is there a special "heater plug" I should get? Or is this just too risky? I've replaced plugs on lamps, vacuums, etc. without this type of problem. Can you get "heater cord" with a prefab plug (i.e. @ Ace, Borg?

OK, good to know.
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While it is not unusual for a cord to get warm, it should not get hot. If it is the cord getting hot, then the cord, not just the plug needs to be replaced. If it is the original cord and if the cord was properly sized to begin with and if it is not damaged, then you should be fine.
However did you notice all those "if"s. Maybe it is time for a new heater. It is not always easy or safe to replace a cord, unless you know what you are doing. Maybe you could get your building maintenance to fix this one as well.
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Joseph E. Meehan

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try to measure the power used by the heater, see if the heater is somehow going bad and drawing too much (although I can't imagine how exactly a heater might start drawing more power)
if the heater is still good you need to replace the cord with a wire capable of withstanding the current draw of the heater.
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--
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Appliance cords are governed by U.L, not the NEC. As such, It's quite common to see undersized appliance cords. 18 amp air conditioners with a #14awg cord. 16 amp hairdryers witha #16 awg cord. Likewise, it's also not uncommon to see percolators, toasters, portable heaters and the like with cords 1 or 2 awg sizes smaller than the appliance calls for under NEC rules.
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Fundamental fact is that plug-in electrical space heaters are dangerous. One place where I worked, the town fire marshall would not permit them, period.
The danger is that they develop hot-spots in their wiring under sustained use, like at plug contacts. Metal oxidizes, contact becomes more resistive, and temps continue upwards. Sometime when nobody's looking, smoke becomes visible, then flames.
You really want to use properly oversized wiring, and screwed-down, large-contact-area connections. Or not use them.
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