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I don't know if my question belongs to this community but I'll start here.

Long story short, my grandparents were using a small personal heater in their room and used an extension cord to plug it in. Apparently, the heater melted the extension cord receptacle because I can't unplug the heater from the extension cord and I can see the receptacle looks slightly warped. From what I can tell, only the heater and a lamp with LED bulbs were plugged into this cord. I didn't check the power draw of the lamp but I assume with LED bulbs, it wouldn't draw too much power. If it helps, I was able to unplug the lamp from the slightly warped receptacle.

Reading the instructions on the heater, it said to plug it straight into the wall and not thru an extension cord (oops, I guess grandpa missed that). In any case, I was looking at the cord's specs. It said 13A, 125V, 1625W. The heater uses 750W on low and 1500W on high and mentions 120VAC but doesn't mention amps. Curious that their cord couldn't handle this load without melting even though the listed wattage of the cord is higher than the heater's usage.

I think grandparents got lucky but at the same time, there is no outlet close to where they want to place the heater so it looks like I'll need to get a cord for them.

I went to the hardware store to get another heater and to see what cords might be able to handle the heat/wattage. The funny thing is I found a cord that said "Heavy Duty". Its specs are 15A, 125V, 1875W. I found another cord that said "Air Conditioner Cord", it too has the same specs of 15A, 125V, 1875W.

My question is if I buy the Heavy Duty or AC cords, what makes them more capable of handling the personal heater's power draw than the normal extension cord? Looking at the specs, the only difference is 150W and 2A. What makes the AC cord able to handle the obvious huge power draw from an AC unit without melting? What makes the Heavy Duty cord "Heavy Duty" and handle a large amount of power without melting?

I assume if the AC cord can handle the power draw of an AC unit, it should definitely be able to handle the draw of the personal heater but I wanted to know how the 3 cords (normal, heavy duty, AC) were designed and manufactured such that I have peace of mind my grandparents aren't going to burn their house down or melt another cord by using the Heavy Duty or AC cords.

UPDATE: Thanks for all the replies thus far. I've updated some of my language/terms since it seems like I used the wrong words when describing my grandparent's situation but nonetheless, you guys understood what I was trying to say :)

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    Check the grade of cable - and the quality of plugs and sockets - cheap extensions are cheap for an obvious reason. I have several really good extensions and they are not cheap, for big loads cheap is not good as you have found out.
    – Solar Mike
    Apr 8 at 10:08
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    If the plug melted, the problem was the plug connection, not the cord itself. It was probably loose, which is very dangerous. Advice below on selecting an extension cord is good, but you may also need to replace the heater, if its plug is old/shoddy/loose/melted. I also suggest running it on "low" if at all possible, especially on an extension cord, especially if someone is not watching it. That cuts the power in half, but it cuts the heating of wires / plugs by a factor of FOUR. Apr 8 at 16:33
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    @GlennWillen strictly speaking, you can cut the bad plug off and put in a new one, but if OP knew how to do this, they wouldn't even ask this question to begin with.
    – Nelson
    Apr 8 at 17:15
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    Is there any chance you can get a qualified electrician to fit a cord of the required length directly to the new heater so that there is no need for an extension cord? Apr 8 at 17:21
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    Also consider that the cord may have melted due to the heat from the heater, not the electrical load. Placing the cord or connector too close to the unit (or directly in it's air stream) can subject it to a tremendous amount of heat in a short period.
    – bta
    Apr 9 at 1:03

3 Answers 3

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Words do not mean that much.

Extension cords come with different gauges. The one used on the heater was probably 16 gauge wire.

You want at least 14 gauge or even better 12 gauge wire for heaters.

You want an extension that is made for more power/watts than you need, not one close to the extensions limit.

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  • Rule of thumb is do not exceed 75% of the rated amperage of a wire.
    – Questor
    Apr 9 at 20:46
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Firstly, make sure the extension cord is not coiled up. This will make it behave like a coil and melt/catch fire under heavy load. Try sizing new cord so it must be fully unwrapped to reach.

Now, the extension cord might have been rated for the load, but it is likely the heater plug was either old or loose and making a poor contact. That made the contact point between plug and extension cord heat up and melt. It is "normal" place of failure. Do check the wall socket the cord was plugged in for damage too! If you are keeping the old heater, replace the plug with one rated for the load. Make sure it fits tight into the extension cord.

It is hard to compare cords without seeing them, but if manufacturer does not lie in the electrical specifications, all three types you list are the same. The heavy duty will likely have tougher shell on the cable, but same wires inside. Look at the labels if manufacturer gives the sizing of wires, and pick the cord with thicker ones. This does not necessarily mean the one with thicker cable (it can have more rubber on it) - look at the labels. If the price difference is negligible, just get the AC one. But look at the manufacturer first - a reputable brand standard cord trumps super-heavy-duty noname junk.

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    Good answer! +1 Tightness of the plug-in connections is essential. If one has to push or pull reasonably hard to plug/unplug, then barring manufacturing defects it will probably be a good connection. Apr 8 at 11:57
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    One way to test a plugged-in connection is to feel the temperature of the plug and socket while the device is running for a few minutes. Cool to lukewarm is good. Rather warmer to hot is bad. Heat means power dissipation, which is bad in a plug & socket. Apr 8 at 12:01
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    +1 to “get the shortest cord possible”. Apr 8 at 13:03
  • @AloysiusDefenestrate the heat caused by the cable resistance is spread over the length of the cable. So with longer cable the heat will be spread over the larger length (unless it is packed / coiled) and in similar condition it should achieve a similar temperature like a shorter cable. Apr 8 at 21:04
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    @pabouk-Ukrainestaystrong The resistive heating is per meter, so longer cable equals more heat total. AloysiusDefenestrate The "keep cable short" is just to make sure the user has to uncoil it before plugging in the heater vs having a couple meter "tail" and a 8m roll on the floor.
    – Thomas
    Apr 9 at 8:04
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US electrical rules are a bit of a mess at times. For permanent wiring, the NEC normally sets the rules. There things are quite clear for wire size:

  • 15A circuit - 14 AWG or larger copper wire
  • 20A circuit - 12 AWG or larger copper wire
  • Larger wire is always OK, but normally not recommended except for long distances (well over 100')

Those wire sizes are actually relatively conservative. But they allow for the same wire sizes to be used whether:

  • Inside insulated walls
  • Inside hollow walls
  • Exposed cable
  • Inside various types of conduit

Rules for temporary wiring - e.g., extension cords - is normally specified by nationally recognized testing laboratories such as UL and ETL. The rules for wire size are, in some cases, a little more relaxed because extension cords are not supposed to be used inside walls. They are supposed to be used in the open air, which allows for heat dissipation.

The big problem though is circuit types. A standard US 15A circuit will typically have many 15A receptacles on it. A standard US 20A circuit will typically have many 15A or 20A (which can accept 15A plugs, by design) receptacles on it. Many extension cords (even super-thin ones) have 2 or more receptacles on the end, plus a multiple-receptacle strip (whether it has any overload protection or not) can be plugged into any 15A receptacle, including at the end of an extension cord.

The result is that it is trivially easy to put 15A on a "13A cord". Or even to put 20A on a "13A cord"! How? Just plug that 13A cord into a 15A receptacle on a 20A circuit. Then plug a 15A heater (extremely common) and a 5A something else (small heater, computer, whatever) into the extension cord.

Now you've got 20A on a 20A circuit. The circuit breaker might never trip, because it is perfectly happy with 20A flowing through it. Meanwhile, the extension cord is melting or burning.

In the end, it comes down mostly to customer behavior. But to play it safe:

  • Except for extremely limited circumstances, only buy extension cords rated (a) for 15A and (b) 14 AWG or larger. Note that extension cords for longer distances will often have 12 AWG wire to match a 15A rating - and that's a good thing. The key is that you never know when an extension cord "just for this little lamp" will get repurposed for anything up to 15A.
  • As others have noted, extension cords should be uncoiled when in use. This is not an obvious thing, but it is extremely important.

And my favorite picture of what can go wrong:

dangerous cord

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    For those wondering why a coiled wire melts but an uncoiled doesn't - it's simple: the wire heats up a bit from use. The higher the power draw, the more it heats up. This gets especially bad if you're close to or above maximum rated current/power. And if the wire is all bunched up, the heat has nowhere to escape, thus everything gets too hot. An uncoiled wire simply has better cooling.
    – Vilx-
    Apr 8 at 17:13
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    Yikes. It looks like a bunch of fiberglass is stuck to the power cord in your photo as well, so this cord was all coiled up and then likely thrown on top of or completely covered by fiberglass insulation. The equivalent of wrapping your power cable in a warm blanket.
    – David
    Apr 9 at 16:27
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    @David I've seen those "whiskers" on other melted plastic. I believe its a chemical in the plastic that melts out and forms crystals when the plastic overheats, but before it totally melts/burns.
    – JPhi1618
    Apr 9 at 19:28

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