# charging with 15 amp extension cords

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##### Well-known member
I was staying at an Airbnb with 15 amp service requiring the use of 2 extension cords plus the Nissan charger cord. What I noticed but not sure of is that if the Nissan cord is coiled in any way the junctions between the cords seem to get hotter and the charging is slower. Could coiling the cord create an electromagnetic field that takes more current?

The cords added resistance, as noted by the heat. Used heavy (10 ga) cord in the future and limit the length.
10 ga is the heaviest industrial cord you can get and should carry 30 amps for a short distance. however when the distance increases you need to increase the gauge cord to minimize the voltage drop from cord resistance, so by going with the 10 ga you are making up for the distance you need to run.
Your situation is why Nissan says don't use extension cords.

Yes, a coil is an inductor which has an impedance to AC current flow (i.e. "AC" resistance).

Plus the wires in the extension cords are probably too small for the current level. Over time the cords heat up due to the current in the wire; as the wire heats up, the resistance of the wire increases, which reduces the current and makes for slower charging. You need fatter wire cords as mentioned above.

When comparing wire gages, note that American Wire Gage is a negative logarithmic scale. So the smaller number means a thicker wire. My charger draws 12 amps when used with 120 volts. I need a 14 AWG extension cord, but 12 AWG is better. Shorter length is better.

note that American Wire Gage is a negative logarithmic scale.
So as a result there is a simple rule of thumb for doing wire resistance estimates.

You just have to remember that 20 AWG has a resistance of about 10 Ohms per 1000 ft.

Then for every 3 wire gauges down the area doubles and the resistance is reduced by half. e.g. the resistance of 1 ft of 20 AWG is 10milliOhm, and 17 AWG is 5mOhm, 14 is 2.5 mR. Going up in the other direction 23 AWG is 20 mR, and 26 AWG is 40mR per foot. Go enough for back-of-the-envelope calculatus.

I don't believe the reason a cord can get warm when coiled isn't necessarily because of any impedance but when coiled or even when cords overlap each other the heat where they touch can dramatically increase. As the heat increases the resistance increases, resulting in increased heat, resulting in more resistance, resulting in more heat, this is known as thermal runaway. Thermal runaway can result in the jacket of the cord melting and a possible fire.
A few years back my daughter plugged her 12a PHEV EVSE into an existing 100' 14g extension cord that unfortunately was coiled up on the seat of an almost new 4-wheeler inside a closed garage. About 45 minutes later someone smelled smoke in the house only to open the connecting door to a garage full of thick smoke and a small fire. They quickly emptied a small fire extinguisher on the fire and it almost went out but quickly restarted. The fire department was out quickly after that and foamed the whole area to put out the fire.
The garage was a mess, covered in a thick layer of black and the almost new 4-wheeler was totaled along with a couple nice \$1000 Honda generators. The nearby PHEV had a layer of soot on the outside and a couple pieces of the plastic trim had small melt marks but nothing that needed replacement, just a good couple of washes. The EVSE even survived with no damage.
Long story short, NEVER coil an extension cord when using high currents and always overrate your extension cords. A 12a load could have been OK with a 14g extension cord for a short period of time but not coiled and should have been 12g minimum and as others mentioned 10g would have been better and even been OK if it had been coiled a bit.

It's my believe we are going to be seeing more and more fires as more and more people get EVs and those that do will be less technical than the first gen EV buyers, resulting in things like extension cord fires. Unfortunately, others will see those fires and blame the EVs, when in reality it isn't really the EVs fault but rather failure of the people using them to read the manual or even have a little common sense on electricity.

@jjeff Wow that sucks! And this is really good advice!

I just now realize I always fully uncoil the extension cords I use, probably because I have a few and always choose one with the length (and gauge) I need. But I'll sure keep your misadventure in mind from now on!!!

I was travelling in upstate Michigan last year, and the 120V outlet at the place I was staying required that I use a 25' 15A rated extension. That got quite warm, despite being fully extended.

I installed a new 120V outlet in the garage of a Niro PHEV owner, so they could plug their EVSE directly into the new outlet. They had previously been using a 50' extension cord (15A), with most of it still coiled up on its hook. It would get quite warm.

Similarly, when the Taycan owner at my office uses the J1772 EVSE, the cable gets quite warm to the touch, even when uncoiled on the shop floor. That car pulls 9-10kW. I imagine the two Leafs that also use the same EVSE pull enough (6.6kW) to warm up the cable if it were coiled on itself in any way.

So, I concur with those that advise to not use extension cords unless absolutely necessary, use the beefiest you can find over the rated 15A, and never use them coiled.

I don't believe the reason a cord can get warm when coiled isn't necessarily because of any impedance but when coiled or even when cords overlap each other the heat where they touch can dramatically increase. As the heat increases the resistance increases, resulting in increased heat, resulting in more resistance, resulting in more heat, this is known as thermal runaway. Thermal runaway can result in the jacket of the cord melting and a possible fire.
A few years back my daughter plugged her 12a PHEV EVSE into an existing 100' 14g extension cord that unfortunately was coiled up on the seat of an almost new 4-wheeler inside a closed garage. About 45 minutes later someone smelled smoke in the house only to open the connecting door to a garage full of thick smoke and a small fire. They quickly emptied a small fire extinguisher on the fire and it almost went out but quickly restarted. The fire department was out quickly after that and foamed the whole area to put out the fire.
The garage was a mess, covered in a thick layer of black and the almost new 4-wheeler was totaled along with a couple nice \$1000 Honda generators. The nearby PHEV had a layer of soot on the outside and a couple pieces of the plastic trim had small melt marks but nothing that needed replacement, just a good couple of washes. The EVSE even survived with no damage.
Long story short, NEVER coil an extension cord when using high currents and always overrate your extension cords. A 12a load could have been OK with a 14g extension cord for a short period of time but not coiled and should have been 12g minimum and as others mentioned 10g would have been better and even been OK if it had been coiled a bit.

It's my believe we are going to be seeing more and more fires as more and more people get EVs and those that do will be less technical than the first gen EV buyers, resulting in things like extension cord fires. Unfortunately, others will see those fires and blame the EVs, when in reality it isn't really the EVs fault but rather failure of the people using them to read the manual or even have a little common sense on electricity.

I was travelling in upstate Michigan last year, and the 120V outlet at the place I was staying required that I use a 25' 15A rated extension. That got quite warm, despite being fully extended.

I installed a new 120V outlet in the garage of a Niro PHEV owner, so they could plug their EVSE directly into the new outlet. They had previously been using a 50' extension cord (15A), with most of it still coiled up on its hook. It would get quite warm.

Similarly, when the Taycan owner at my office uses the J1772 EVSE, the cable gets quite warm to the touch, even when uncoiled on the shop floor. That car pulls 9-10kW. I imagine the two Leafs that also use the same EVSE pull enough (6.6kW) to warm up the cable if it were coiled on itself in any way.

So, I concur with those that advise to not use extension cords unless absolutely necessary, use the beefiest you can find over the rated 15A, and never use them

A better choice than using the extension cord at at Airbnb is to find an Airbnb with a charger. The listing often show when they include a charger but there seems to be no filter to select for that feature. I found by using a google search "airbnb topanga canyon ev charger" you can often get those listings we EV chargers. It is also a good idea to purchase the level 2 Tesla adapter to use in case the charger is a Tesla.

As a qualified industrial electrician.

Yes, coiled cables will cause inductive resistance.
Increased resistance causes heat.
Heat causes higher resistance.
Long lengths lead to higher resistance - which leads to more heat.

If you examine any domestic coiled cable extension, it will either say 10 amp, or say it has to be fully unwound for 13 amp use.

Granny leads are not recommended for use with extension leads for exactly his reason.

If you HAVE to use an extension lead, I suggest you do what I did, all pre-made leads use 1.5mm cable, but you can buy 2.5mm 3 core flex; so I bought a 25m drum, cut it in half, and fitted a trailing lead socket to one end, and an MK (high quality) plug to the other of each giving me 2 12.5m extension leads that can easily cope with 3KW each.

It would be easy enough for those with the knowledge, skills, tools, and abilities to run a simple test to measure the voltage drop at the load of a coiled versus straight cord.
Add more coils and watch the voltage drop even more.
Wrap the coils around a steel or iron pipe and watch it drop again.
Make a video, post it on tiktok and rake in a pile of money...

It would be easy enough for those with the knowledge, skills, tools, and abilities to run a simple test to measure the voltage drop at the load of a coiled versus straight cord.
Add more coils and watch the voltage drop even more.
Wrap the coils around a steel or iron pipe and watch it drop again.
Make a video, post it on tiktok and rake in a pile of money...
It isnt quite that simple, it also varies with current and temperature - that is why all superconductors are cooled to near absolute zero.

My experience suggest this is the case and the place that seem to get the hottest was the junction between the cords. The only point, no matter what kind of cord you are dealing with, if you have too much length, do not coil the cord if you want the fastest charge?

"Yes, coiled cables will cause inductive resistance.
Increased resistance causes heat.
Heat causes higher resistance.
Long lengths lead to higher resistance - which leads to more heat."

Length doesnt cause as much heat as a joint, or rather it does, but the heat is spread out along the length, so is rarely an issue*; the greatest issue with length is the voltage drop; whereas a joint has the heat focused in a small area - which can be dangerous.

There are engineering charts that will give you the voltage drop across a range of cable cross-sections, lengths, current and voltage loads, as well as how it is used (open air, stuck to a wall, enclosed in trunking); so you can pick the best option.

I have seen 0.5m "kettle" leads catch fire, because of a long duration 3KW load and the resistance both from the terminations, but also the increased resistance of the clamp (crushing the cables also increases resistance).
Even short duration, high current loads can cause degradation and increased resistance over time.

For this reason, I never use a moulded plug in a situation where it is going to take high current loads; equipment supplied with such a lead will have the plug and "kettle" end cut off and a "proper" termination made using a high quality product - usually MK; or just thrown away if I have an already made up cable I can use.

* I have seen a 100amp rated armoured cable, overrated to nearly 300amps - and getting so hot, it was burning the paint off of the steel wall it was fixed to.

A Great Universal Stores warehouse - the entire company is populated by fuckwits at every level; from conveyor pickers to CEO; including much of their "technical staff"; I wrote up an 8 page report on serious safety regulation failures across multiple disciplines, and handed it to HSE and the Fire Brigade, who verified all of it on just one site, in one visit.

It's a bit besides the point, which is to NOT run 15A into a coiled extension cord, we'll all agree with that. I'm just wondering about the actual induction that can happen with a coiled extension cord, which has both hot and cold wires in a twisted pair. AFAIK these should cancel each other, and while I'm sure there IS an EMF, at like 15 turns for a 100 feet (2 feet diameter), at 60 Hz, there's got to be very minimal henries?

But suppose the hot and cold wires EMFs don't cancel each other...

*finds an online induction calculator* 15 turns of 14 AWG, 24 inches diameter, coiled on 6 inches = 193 µH. Although this is a VERY rough estimation.

Put that into inductive reactance 2πfL = 2π * 60Hz * 0.000193H = 0.0728Ω

So worst case scenario should be P = I²/R = 15²A / 0.0728Ω = 3090W

Wait what!?!?? 3 kW heat loss!? That's a freaking oven! Even if you half that (continuous current should actually be 12A, inductive reactance could be double) that's still a LOT! Either my calculations are wrong, or the EMFs of the hot and cold wires do cancel each other. Right?

I am unaware of any POWER cable that uses twisted pairs in a small enough cross-section for domestic use - it is used in extremely large power cabled (100mm +); however I have never done the math on how it would effect inductive resistance in a long time.

I^2 x R =~16W

I^2 x R =~16W
Phew!! I guess I thought of V²/R Thanks!

So at 16W, it's probably not noticeable across a 100 ft cord. Again, that is if the EMF of the two wires don't cancel each other. Interesting, but the fact remains: coiled extensions cords and lots of amps = risk of fire

That reminds me of that Mythbusters episode where they tried to have a christmas tree catch fire with light bulbs, and the only way they could get a spark was by overloading a coiled cheap extension cord.