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What aspect of using a coiled extension cord makes it more dangerous than using an unrolled power cord?

Many fire safety websites say that an extension cord should not be used while coiled.

Is this due to the cord acting as an air core inductor? I can't really see why this should cause fires. If this is the case, if every second loop is in the opposite direction is it safe?

My theory is that if the cord is getting heated from high current draw, then when coiled all of this heat is in a much more condensed location, causing a greater temperature rise than if the cable was unrolled.

Is it dangerous? If so, so what is the cause? Am I missing anything? Do parameters like loop size, loop direction etc. make any appreciable difference?

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    \$\begingroup\$ Don't know your source, but in my experience dealing with fire inspectors, they would rather you sit still in the dark, because in 0.000001% of instances, turning on a light could start a fire. Take such recommendations worth a grain of salt. \$\endgroup\$
    – Matt Young
    Commented Oct 1, 2015 at 0:08
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    \$\begingroup\$ ^ I have seen warnings telling me not to use a hair dryer on wet hair. Irony? \$\endgroup\$
    – Daniel
    Commented Oct 1, 2015 at 0:13
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    \$\begingroup\$ I am not concerned from a personal point of view, we have plenty of coiled power cords, that supply power boards in a dust workshop with no issues, it is more a theoretical interest, as to if there is any credit to these warning or if it is a myth \$\endgroup\$
    – Hugoagogo
    Commented Oct 1, 2015 at 0:14
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    \$\begingroup\$ @Daniel if there was a warning saying not to use an iron on a wet shirt, now THAT would be irony. ba dum dum dissh \$\endgroup\$
    – efox29
    Commented Oct 1, 2015 at 0:14
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    \$\begingroup\$ I recommend that this question gets locked (?) as any further answers will be because of overheating, here is a real world example and comments will be such things cannot happen in practice, it is a myth where the reverse is clearly true. \$\endgroup\$
    – KalleMP
    Commented Feb 16, 2017 at 19:48

8 Answers 8

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The normal cable ratings assume that the wire can adequately disperse heat generated in the cable due to the current flowing.

If you coil it up and use close to the maximum rating then it stands a good chance of melting the plastic insulation and then causing a short.

Overheated cable on drum

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    \$\begingroup\$ @MattYoung - Yes, and your point is? The recommendations take into account that a remarkable number of people do remarkably stupid things. Present company excepted, of course. \$\endgroup\$
    – WhatRoughBeast
    Commented Oct 1, 2015 at 1:36
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    \$\begingroup\$ To quote somebody 'million to one shots happen 9 times out of 10' ... which is true for any population over 10 million.... and to make it worse 49.99999999999% of people are more stupid than average... \$\endgroup\$
    – Spoon
    Commented Oct 1, 2015 at 7:26
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    \$\begingroup\$ Proper wire use specifications include a "bundling" spec in other words how many similar conductors are included in the wire loom. A coiled extension lead is the worst case of this. Combine that with some foreign manufactured cables not being what they are supposed to be and you have a recipe for a fire. \$\endgroup\$
    – RoyC
    Commented Feb 16, 2017 at 18:14
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    \$\begingroup\$ @MattYoung A 25m extension cord with 2.5mm$^2$ conductors has about 0.165Ω resistance per conductor, or 0.33Ω total. When carrying 16A, the extension cord will dissipate about 85 Watts. 85 Watts in a small space with some insulating plastic around it is gonna get hot. \$\endgroup\$
    – marcelm
    Commented May 4, 2017 at 23:55
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    \$\begingroup\$ @marcelm Your 2nd reference has comments that derating is required depending upon the number of conductors. If the heat cannot adequately escape from the bundle the temperature will rise; potentially to the point that the insulation will be damaged as shown in the example photographs. \$\endgroup\$
    – Kevin White
    Commented May 5, 2017 at 22:27
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Here's another illustration.

enter image description here

Used to charge an EV at 'only' 10A... MW

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    \$\begingroup\$ I reviewed this and although not really an answer, I decided to leave it alone. You can not put a picture on a comment and in this case the picture speaks a thousand words. \$\endgroup\$
    – RoyC
    Commented Feb 16, 2017 at 18:09
  • \$\begingroup\$ From looks at that photo (and all of the metal shavings that it attracted), either the coil was sitting under a metal working bench or it saw a bit more than 10A. I don't have a great intuition for the magnetic field generated by a coil, but something is suspicious there. I don't see how all of those shavings could have been attracted to the coil with just 10A. \$\endgroup\$
    – CHendrix
    Commented Feb 16, 2017 at 19:06
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    \$\begingroup\$ @CHendrix - Looks more like glass fiber to me. \$\endgroup\$
    – Dampmaskin
    Commented Feb 16, 2017 at 19:11
  • \$\begingroup\$ @Dampmaskin That would make much more sense... \$\endgroup\$
    – CHendrix
    Commented Feb 16, 2017 at 20:56
  • \$\begingroup\$ Charging an EV is, by a VERY LARGE margin, the highest draw to exist in a residential. \$\endgroup\$
    – Nelson
    Commented Feb 9, 2023 at 4:33
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schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. An inductive coil. Figure 2. Cancellation.

Unless you wired your equipment with single wires it would not be possible to create an air-cored inductor as shown in Figure 1.

Because your cables contain the feed and return current in very close proximity the inductance caused by the current to the load is exactly cancelled out by the current returning from the load.

The danger is that if they are carrying significant current (for the gauge of wire) they will get warm or hot. This may cause insulation breakdown or even fire.

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  • \$\begingroup\$ indeed the heating effect in extension cords is sometimes misunderstood and mis-attributed to the magnetic coil effect. See also "bifilar non-self-inducting coils" \$\endgroup\$
    – P2000
    Commented Feb 9, 2023 at 16:21
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Last week (1st week in Jan 2017) we almost had a fire from a coiled extension cable. It was connected to an electric urn that does draw high power, and the only reason it did NOT start a fire was the circuit breaker tripped in time. I have kept a segment of the cable in question were it melted several loops together before the wires touched inside the mess. this is a pic of the end of the loop - basically one solid lump now, except for effects of being cut thru

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Current flowing in a cable generates heat. This causes the temperature of the conductors to rise until the heat lost balances the heat generated. If the temperature gets too high the insulation on the cable softens and eventually melts.

When you pack lots of cables that are all carrying current (whether multiple seperate cables or multiple loops of the same cable) together heat dissipation suffers resulting in a higher temperature at a given current.

Reels are particulally bad because they tightly pack together a large number of passes of the cable. Excess cable in a loose jumble on the ground is far less likely to overheat than excess cable wound tightly on a reel.

You get away with it most of the time because most of the loads people plug into extension leads are small and/or intermittent. From time to time though the right combination of circumstances come together and melts one.

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It's all about the cooling

All the failures you see above are the wires overheating. You have many wires in close proximity, all getting warm. This dense "clump" of wires simply cannot dissipate the heat, and they have a "meltdown".

The National Electrical Code talks about this, in the various parts of NEC 310.15. Here is the "Cables bunched together" (coiled=raceway) derate table.

enter image description here

You see these burned up coils with 20+ loops of cable... that's 40+ conductors bundled on the reel, which calls for derating cable capacity to 35%. Now, many cables can't run at 90C, so you have to derate from the temperature they are good for. Say your 16 AWG extension cord is good for 60 degrees C, NEC 310.15(B)16 doesn't have a figure for that, but we can extrapolate and get 11A. Derate that to 35%, and we have 3.85 amps. That's all you should be putting through it when it's coiled up on the reel like that!

Of course, people are pulling 10-12 amps, that's why it burnt up.

But if you're pulling 1-2 amps through that coiled up cord, that's no problem as you can see.

Are those derates a burden for house wiring? No. Small house wiring derates off the high 90C number, but is statutorially required to use the 60C number (NEC 240.4) which is below 70% anyway, so the 70% derate doesn't really hurt. That lets you have 9 active conductors or 4 circuits in a cableway. 4 circuits because in 120/240V split-phase power, heat from 3 conductors (hot-neutral-hot) will never exceed the heat output of 2 conductors, so counts as two.

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The technical answer to why coiled electrical cords melt is based on heat transfer methods; radiation, conduction, convection. All three heat transfer methods play a role, but especially radiation. Air has less ability to convect heat away when coiled. Conduction works by direct contact, which is elevated by coils touching. Radiation especially elevates heat transfer when two closely opposing surfaces both radiate heat towards one another, creating a phenomenon called 'radiation feedback.' Radiating heat bouncing back and forth between the two closely opposing surfaces raises the temperature on a logarithmic scale rather than linear scale. By understanding electricity generates heat, dissipating the heat through proper heat transfer methods is safe, substantially increasing the heat through heat transfer methods will exceed the insulating materials capacity to absorb heat, possibly reaching fire ignition temperature, is not safe.

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  • \$\begingroup\$ No, it's just the concentration of heat in a smaller volume - reduced surface area per watt means the temperature rise is greater. There's no change to the properties of the cable. Radiation feedback refers to a change in the way earth radiates heat, because of the heat, etc. \$\endgroup\$
    – tomnexus
    Commented Feb 20, 2017 at 5:46
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    \$\begingroup\$ You argue against well establish physics. My answer discusses accumulation, you refer to generation and fail to address accumulation. After heat is generated it can accumulate or dissipate, period. Three heat transfer methods exist, they alone determine accumulation or dissipation. Let me simply explain radiation feedback so you understand. Any surface emitting radiant heat energy will heat any opposing surface, which will then emit radiant heat back, bouncing or 'feeding' the energy back and forth. If both surfaces generate radiant heat, the feedback elevates the accumulation rate. \$\endgroup\$
    – K. Taylor
    Commented Feb 24, 2017 at 20:19
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    \$\begingroup\$ The only positive feedback loop involved here stems from resistance increasing with elevated temperature. The cause of the problem is, as you rightly note, the ineffectiveness of convection when air can't reach the inner loops of the cable. Black-body radiation is so small compared to the convective and conductive heat transfers that it can be completely ignored. \$\endgroup\$
    – Ben Voigt
    Commented May 12, 2017 at 14:17
  • \$\begingroup\$ The use of the term "radiation feedback" is dangerously wrong here and the comment on exponential addition is incorrect (except just maybe in very very special cases). \$\endgroup\$
    – Russell McMahon ♦
    Commented Jan 29, 2020 at 0:14
  • \$\begingroup\$ My calcs suggest a 30' extension cord at 140 F will output ~400 Watts of radiative power, (1/3 -1/2 a toaster's output) along its length. It seems like coiled up this could cause significant heating? \$\endgroup\$
    – johnDanger
    Commented Feb 10, 2023 at 21:09
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The inductance thing doesn't seem very relevant in this case, it should add impedance not reduce it, it seems to be more of a dissipation issue, the surface area is greatly reduced and so is the dissipation capacity, on the other hand for the cord to catch fire due to overheating it has to be used pretty close or beyond it's rating, it's not like plugging a 10w light bulb on a coiled extension will cause any issue

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