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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.

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  • 5
    You already have good answers, which boil down to lawyers plus lack of EE knowledge. Another related factor is that it's possible for even a correctly-rated extension lead to overheat due to the high current involved, if most of it is still coiled on the reel. You should always pull all the cable out when using an extension with a high-current appliance, even if you don't need to as regards the distance. Again, 'just don't' is a clearer message.
    – peterG
    Sep 26 at 18:30
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Well, you really nailed it. It's the fact that normal everyday garden variety extension cords are usually 16 AWG, or maybe 14 AWG if you're lucky. The reality is, an extension cord, properly sized for the load, would pose no safety issue, other then potentially being damaged from grandma's proverbial rocking chair, but then again, we have AFCI breakers to the rescue!

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    The lawyers feel safer saying "never" , rather than "use only 12 gage , short, extensions". Sep 26 at 15:48
  • 27
    @blacksmith37 exactly. To paraphrase George Carlin, "Think of how dumb the average person is. Then realize half of them are dumber than that!" It's this latter group that the lawyers are concerned with...not folks who have an electrical engineering background or other knowledge. Better to say "never do this" than start couching it with exceptions.
    – BruceWayne
    Sep 26 at 16:47
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    The 'problem' is the minimum spec cable requirement for a US extension cord. The 'fix' is to tell people not to do it, ever… as opposed, to., for instance, requiring better minimum spec.
    – Tetsujin
    Sep 26 at 19:48
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    @reirab - Yes. The UK demands all cables & sockets are fully earthed & support a 13A [approx 3kW 240v] load.
    – Tetsujin
    Sep 27 at 7:23
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    @Tetsujin, you can buy 10A extensions - here's one at B&Q. It's the fusing that makes the difference, with the nuisance of always blowing fuses avoided by labelling in amps
    – Chris H
    Sep 27 at 10:23
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In addition to the already stated "shouldn't use a flimsy little 18 gauge extension cord", which would have a definite overheating problem, two specifics come to mind:

  • Tripping

Tripping over an extension cord is a real problem. Tripping over a cord that then moves a hot appliance into a dangerous position (on clothes or curtains etc.) is far worse.

  • Covered cords

The way to avoid tripping is to cover the cord, which significantly increases the overheating problem.

All can be avoided by careful consumer usage. But putting the warnings helps protect the manufacturers from liability.

In other words, the warnings are more legal than engineering - but they are good instructions anyway.

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Lawyers, pure and simple.

When you start a fire with an electric heater plugged into an exension cord, even if the extension cord is massively over-adequate and properly protected from damage, you were "violating manufacturer's instructions" and they are off the hook, legally, even if the fire had nothing to do with the extension cord. It's "automatically your fault."

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  • Instead of using an extension cord, use something that's basically a semi-permanent installation of an outlet. That's not that difficult to do according to code.
    – Nelson
    Sep 27 at 0:17
  • @Nelson It is in sane countries like Australia where all electrical work must be performed by licensed electricians.
    – nick012000
    Sep 27 at 12:39
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Quite simply, it's the law in most states as around 40 states have adopted the international fire code.

The International Code Council (ICC) covers space heaters under the International Fire Code, Section 605.10.1-4. The code lists under what occupancies space heaters can be used, it specifies that only listed and labeled portable space heaters can be used, and it states that they should be plugged into an approved receptacle. While many organizations note to avoid using an extension cord with space heaters, the 2018 ICC Fire Code Section 605.10.3 unequivocally states:

605.10.3 Extension cords. Portable electric space heaters shall not be plugged into extension cords.

Realistically, if you are using a good extension cord and not being foolish, it's highly unlikely something bad happens.

Source: https://jsheld.com/insights/articles/common-causes-of-electric-space-heater-fires-methods-of-prevention

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  • Nice find. Only the cord and plug supplied by the manufacturer have been tested by a lab.
    – DrSparks
    Sep 27 at 2:38
  • OK, but why does the International Fire Code state forbid it?
    – nick012000
    Sep 27 at 14:13
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The common failure mode I am familiar with is plugging two (or sometimes even more) heating appliances in the extension cord sockets.

The problem is, breakers don't react quickly to mild (e.g. 2x or 4x) overloads. This is both a technology limitation of the traditional fuses and an engineered feature of the newer electronic protection devices - in order to tolerate appliances with high inrush current.

This is how one can overheat (up to and including melting or catching fire) the extension cord, the wall sockets or the wires inside the wall.

Plugging a single heating appliance into a reasonable-length extension cord that is rated for the load and not burried under carpets/furniture/pile of laundry is safe.

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  • This is a good point, but wouldn't plugging two heaters into different plugs on the same receptacle or different outlets on the same circuit share the same problem (at least with respect to the wiring inside the wall?)
    – reirab
    Sep 26 at 7:33
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    @reirab they sometimes do. But, at least this is someone else's problem, not the heater manufacturer's. And this is the least probable failure point. At least where I live, the local code mandates in-wall wires WAY thicker than the appliance's power cord for the same power rating - like, 2.5mm2 vs 1.5mm2 for 16A.
    – fraxinus
    Sep 26 at 7:50
  • They should require fuses or circuit breakers on extension cords. They do it for power strips.
    – DrSparks
    Sep 26 at 22:56
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In addition to other good answers, often there usually isn't a perfect zero-resistance connection where the plug fits and holds in the socket purely by friction. If one is using an appliance with a high current draw for a period of time, if any part of the connection gets warm/hot/overheated before the wires themselves, typically its where it plugs into the outlet, or if one is using an extension cord, at one of the places where cords are plugged together.

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One thing that is not explicitly mentioned here is something that used to happen to me when welding at a client site. - we have long extension cables (heavy duty) which easily run the welders.

if, when the length is not needed, we leave them wound-up the customer experiences their earth leakage tripping more often, and the cables get much hotter ( to the point of almost melting)

This is through the induction effect that happens when you coil wire. It is always a good principle in circuits to use the shortest span possible.

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  • Use the shortest extension cord possible, or uncoil the remaining cord so it can air cool
    – FreeMan
    Sep 27 at 13:41
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One more important reason:

Coiled cable. Because your extension cord is 20 feet and you only need 5, so why not roll the rest into neat circle? Don't. Coiled cable will heat up rapidly with no way to dissipate the energy. Good quality extension cords will have a current rating "when unrolled" and "when coiled" - the second one will be much lower. Most people will go with first value and think they're ok, but are setting themselves for a fire hazard.

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    Here in Austria cable drums usually have a built-in overheating protection. I’m not sure if it’s mandatory or just very common.
    – Michael
    Sep 27 at 9:57
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It's pretty much the same logic that always, under any circumstances, forbids you to cross a road on a red light. Obviously, if you understand how the traffic is organized and carefully look for incoming cars before crossing, nothing bad will happen. However, if you get a habit of breaking the rules, there will be a risk. The same will happen with extension cords for large loads when

  • you forget to check if the cord is good for the load
  • you miscalculate or misread the labels on the equipment or the cable
  • you forget to uncoil the cord or to put it away from the heater
  • people who are not aware of the above (e.g. your kids) see you using the cords for large loads and do likewise on their own

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