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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?

JRE
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Hugoagogo
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    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. – Matt Young Oct 01 '15 at 00:08
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    ^ I have seen warnings telling me not to use a hair dryer on wet hair. Irony? – Daniel Oct 01 '15 at 00:13
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    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 – Hugoagogo Oct 01 '15 at 00:14
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    @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* – efox29 Oct 01 '15 at 00:14
  • Take a shop drop light that recoils, level it retracted in the housing, plug in a large current draw item like a shop heat gun. See what happens. Smoke, melting plastic –  May 05 '16 at 16:08
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    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. – KalleMP Feb 16 '17 at 19:48
  • What about those retractable cord reels sold in big box stores rated 10 Amps. By definition mos of their length is retracted most of the time, I've used some of them pretty hard before. I've tripped their own breakers on multiple occasions and never had one melt. Are they just THAT overbuilt that it doesn't happen? The gague isn't any higher than I'd expect. – Billy left SE for Codidact Feb 08 '21 at 23:41

8 Answers8

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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

Kevin White
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  • Something tells me that's a cheap 14 AWG cable that was overloaded to stupid levels, with some other worst case condition. – Matt Young Oct 01 '15 at 01:23
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    @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. – WhatRoughBeast Oct 01 '15 at 01:36
  • @WhatRoughBeast Simply that these types of recommendations are usually meant to cover some borderline ridiculous corner case that has an extremely low probability of occurring in reality. – Matt Young Oct 01 '15 at 01:38
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    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... – Spoon Oct 01 '15 at 07:26
  • This was pretty much what I was thinking, so all of the stuff about the "magnetic fields stacking, and causing increased heating" is rubbish? – Hugoagogo Oct 03 '15 at 03:44
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    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. – RoyC Feb 16 '17 at 18:14
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    @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. – marcelm May 04 '17 at 23:55
  • @marcelm Ok, so your 2.5mm^2 cable is just shy of 13 AWG which is rated for 7.4A and you're putting 16A through it. Yep, I'd say that qualifies as a "Hold my beer and watch this!" moment. – Matt Young May 05 '17 at 01:35
  • @MattYoung I'm not sure where you're getting that 7.4A figure, but where I live (the Netherlands) 2.5mm\$^2\$ is definitely rated for at least 16A. I hear it's even certified for 20A in Belgium, and according to [Wikipedia](https://nl.wikipedia.org/wiki/Elektriciteitsleiding#Stroombelastbaarheid_en_stroomdichtheid) it's even okay up to 27A. I'm finding [similar ratings](https://www.bluesea.com/resources/529/Allowable_Amperage_in_Conductors_-_Wire_Sizing_Chart) for AWG#13. – marcelm May 05 '17 at 09:22
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    If you look in the fine print you will see that the higher current ratings are for conductors in free air where they get good cooling. The current limit is just related to how well the heat can dissipate - if you put multiple conductors together where they can't get rid of heat the rating is much lower. – Kevin White May 05 '17 at 15:10
  • @KevinWhite The wikipedia article I linked is explicitly talking about 3-core conductors with PVC shielding. 27A @ 2.5mm\$^2\$. And 2.5mm\$^2\$ is also what we use for house wiring, with several conductors per conduit. – marcelm May 05 '17 at 18:41
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    @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. – Kevin White May 05 '17 at 22:27
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Here's another illustration.

enter image description here

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

Nick Alexeev
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Martin Winlow
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    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. – RoyC Feb 16 '17 at 18:09
  • 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. – CHendrix Feb 16 '17 at 19:06
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    @CHendrix - Looks more like glass fiber to me. – Dampmaskin Feb 16 '17 at 19:11
  • @Dampmaskin That would make much more sense... – CHendrix Feb 16 '17 at 20:56
  • Charging an EV is, by a VERY LARGE margin, the highest draw to exist in a residential. – Nelson Feb 09 '23 at 04:33
  • @Nelson: There are a couple of other "highest draw" applications comparable to EV charging (and jointly much larger than the many common draws). For example electric resistance heat (the "emergency heat" mode on a central heat pump). – Ben Voigt Feb 09 '23 at 20:24
  • @BenVoigt I can't imagine that to be possible over the span of hours unless you like turning your house into an oven or you have a heater running out in the open. The EV is going to draw at maximum amperage for hours. – Nelson Feb 10 '23 at 01:23
  • @Nelson: Or if you have a large house in a cold climate (Thankfully I don't) – Ben Voigt Feb 10 '23 at 14:48
  • @BenVoigt Logically it can't happen, because if you have a central heat pump system, your house is properly insulated. If you end up in a situation where the emergency heat turns on for as long as an EV charging (30A gives you 25miles/hour), you're talking about many hours, and that means there's something very wrong with your insulation. You literally have to do something extraordinarily stupid like leave couple windows open and then turn on the emergency heat. – Nelson Feb 12 '23 at 02:35
  • @Nelson You made a claim which appeared to be about power, not fraction of total monthly energy usage. On an answer about overheating and fire risk, where 90 minutes continuous is just as bad as 900 minutes. (Also your references to insulation are not really useful, since you are assuming the house retains heat but the EV doesn't retain energy... recharge after a daily commute won't be charging the EV nearly as long as running electric heat after being away for a week. Initial conditions matter.) – Ben Voigt Feb 13 '23 at 01:37
  • Probably not 10A... 12A. You were using a typical "travel unit" EVSE with the standard plug dongle (NEMA 5-15). Those all run 12 amps, because that is 80% of a 15A circuit. You can get a NEMA 5-20 dongle that'll go 16 amps. – Harper - Reinstate Monica Mar 22 '23 at 22:12
  • @Ben they're talking about duty cycle. A heat pump is a big draw, yes, however it is intermittent duty cycle, cycling on/off 1-5 times per hour. The EVSE is 100% duty cycle for many hours - level 1 EV charging does 4-6 miles per hour, so an EV that drove 40 miles that day will charge for 8 hours solid at 100% duty cycle. Even at 4kW level 2, it'll be 3 hours solid. – Harper - Reinstate Monica Mar 22 '23 at 22:22
  • @Harper-ReinstateMonica: With a variable-speed compressor, or in really cold weather, or after the thermostat has been set back for a while, it's completely possible that a heat pump is not cycling, or cycling less frequently that the time-to-equilibrium of wiring temperature. Which was the whole point of my previous comment which you appear to have ignored, so I'll say again: Initial conditions matter. – Ben Voigt Mar 22 '23 at 22:27
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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.

Transistor
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  • 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" – P2000 Feb 09 '23 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

ian
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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.

Peter Green
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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.

Harper - Reinstate Monica
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0

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.

K. Taylor
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  • 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. – tomnexus Feb 20 '17 at 05:46
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    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. – K. Taylor Feb 24 '17 at 20:19
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    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. – Ben Voigt May 12 '17 at 14:17
  • 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). – Russell McMahon Jan 29 '20 at 00:14
  • 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? – johnDanger Feb 10 '23 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

diegogmx
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