charging with 15 amp extension cords

[nenik]

Induction plays no role whatsoever in this. First, as already pointed out by @Jerther, the hot and neutral cancel off. It doesn't matter whether twisted or not, it's still a bifilar winding with negligible inductance.
Now even if the coiled extension had inductance and thus presented reactance (not resistance) that causes voltage drop for AC current, the energy "consumed" on that perceived voltage drop is not turned into heat, but rather temporarily stored into the magnetic field and returned out of phase of the AC current.

Still, coiling is bad and leads to higher temperature (and higher losses caused by warmer copper), because it keeps the power loss concentrated in smaller space, when the heat can't radiate out as easily.

 

[Jerther]

the energy "consumed" on that perceived voltage drop is not turned into heat, but rather temporarily stored into the magnetic field and returned out of phase of the AC current.

Right! and this causes current to lag vs voltage, which changes the power factor. Also good to know that twisting the pairs makes no difference here. I understand it's mainly useful in reducing noise in data lines.

Inductors are an interesting part of electronics theory

Still, coiling is bad and leads to higher temperature (and higher losses caused by warmer copper), because it keeps the power loss concentrated in smaller space, when the heat can't radiate out as easily.

Well said. +1

 

[PaulNewall]

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?

The thing you are calling "inductive resistance" should be called reactance. It causes a voltage drop, but no heating like resistance does. A coiled extension lead will not have a reactance much greater than an uncoiled lead because the live and neutral conductors are very close together inside the cable, even if they are not twisted together.
I am quite sure that the reason that coiled cables get hot is because the heat dissipated in the cable cannot escape. A coiled cable with 15 turns is like 15 cables packed tightly together and the heat dissipated due to resistance in the turns that are near the centre of the coil has to pass through the insulation of all the turns that surround them.

 

[TronJockey]

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.

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?

As far as I'm aware the only reason they suggest not using a "coiled" extension cord is that when coiled (and used at anything near max amperage) it cannot dissipate heat as well. It's not unusual for extension cords to get quite warm but when coiled they can under certain circumstances slowly become hotter until the insulation melts. Forgive me as it's been quite a few decades since my ENGR-101 but I believe the increased heating in a coiled cord should have little or nothing to do with inductance or any increases in inductive "Reactance". Reactive components in a circuit generally do not dissipate power (heat), only the wire's resistance should dissipate heat. A coiled extension cord would be like an Air-Core inductor anyway which would be negligible especially since the power and return wires are in the same cable (usually a twisted pair).
So, in summation, as far as I'm aware, coiling a power cord is dangerous only because the heat generated has a much harder time escaping.

 

[Jerther]

Oh! we had one of these reels a long time ago! Plus it was fully enclosed, but there was a security lock that prevented the reel from turning when the little compartment that contained the end of the cord in the center was open, so we couldn't use the cord unless it was fully uncoiled. Now I see why, I find it's quite clever, and I'm sooo glad we failed to hack it...