Physicists and electricians: How much heat does my extension cord generate?

[Eug]

I have a EV car charger but I must use an extension cord with it because my car charger's length is a few feet too short for convenient use.

So I bought a 10 m or 32 feet 12 AWG extension cord for it.

According to this calculator...

http://buyextensioncord.com/info_voltage_drop.shtml

...I would lose about 1.5% of the voltage over that 32 feet. Assuming the amperage is about 11.6 A, that works out to about 21 Watts over the length of the cord. Is that correct?

The reason I ask is because I only need about 20 feet, so the extra 12 -15 feet gets gathered loosely into one spot. It's not tightly coiled and its hung outside so there is good ventilation, but nonetheless I have about 12 -15 feet of cable sitting in the same spot generating about (I calculate) 10 Watts of heat. I've touched the cable after drawing that nearly 12 amps charging my car, and that cord wasn't even warm. But that was in 5C weather.

Are my calculations accurate? Also, 12 AWG stranded copper is supposed to have an impedance of about 1.65 Ohms per 1000 feet.

 

[Matthiasa]

If the assumptions you are putting are the case then the calculations are approximately correct. Most electrical parts, as well as wall voltage, have pretty large tolerances so its that plus or minus ~5-10% if not more.

It would take it running for a considerable amount of time for it to actually feel warm.

 

[Jeff7]

P = I²R
power = current * current * resistance


11.6 * 11.6 * 1.65 = 222W/1000ft
= 0.222W/ft
* 32ft
= 7.104W

 

[Matthiasa]

32ft each way... Also depends on if it is copper or aluminum, not counting that draw will fluctuate based on wall voltage.

 

[Brian Stirling]

You could make your own extension cord using 10GA wire and make it precisely the length you need. So, if you made a cord using 10GA wire at 0.0018 ohm/foot and 20 feet long the power loss would be less than 7W versus about 17W for a 10M long 12GA cord.

Of course, since you already have a cord that works the amount of time it would take to recover the cost of buying/making another cord would likely be more than 20,000 hours. So, if you charged for 5 hours per day it would take more than 4000 days or 11 years to recover the cost. Probably not worth it...


Brian

 

[DrPizza]

Since you're calculating the resistance and voltage drop for 12 gauge wire, perhaps you might also want to determine if the outlet is served by 14 gauge wire, or 12 gauge wire. If wired correctly, and it has a 15 amp breaker controlling it, then it's likely 14 gauge. If it's on a 20 amp breaker, then 12 gauge wire. There isn't magic happening inside the house - from your main panel to the outlet is likely no different than your extension cord (except it'll be made of solid wire, rather than stranded, since flexibility isn't required).

But, if your outlet is wired with 14 gauge, and there's 100 feet of wire between the main panel and the outlet, then you're looking at the lesser of two evils in considering the drop in your extension cord.

 

[edro]

What is your concern? Wasted power? Or fire from hot extension cord?
Both are ridiculous concerns, but it's good that you're thinking about them.

 

[Eug]

Ok thanks.

7 Watts x 2 = 14 Watts, so between 14 and 21 Watts depending on the calculation, and for that very lightly coiled area, that would mean about 6-10 Watts there. Why bidirectional BTW?

What is your concern? Wasted power? Or fire from hot extension cord?
Both are ridiculous concerns, but it's good that you're thinking about them.

Fire from hot extension cord. I've seen lots of pix of fires originating from coiled extension cords. However, those seem to be uber long cords (50-100 feet) that are way under-spec'd - 16 AWG - running 12-13 amp loads continuously.

DrPizza, you're right about the 14 AWG internal wiring, but the difference there is it's all relatively straight, and I think the run is probably only about 15 feet or so, since the outdoor GFCI outlet is right above the breaker box, which is in the basement below. My main concern with the extension cord was the lightly coiled spot. However, the car is charging now and I did test it again by hand along the course of the wire, including on the loosely coiled spot. All areas were dead cold, but again, it's only about 4 degrees Celsius outside.

Brian, I'm not keen on making my own cable because it's for outdoor. I can get 25 foot welder's extension cords locally though, and those are 10/3 wire, or else 8/3 on Amazon. Unfortunately, the 10/3 is CAD$100 and the 8/3 (apparently stiff as a board and massively thick) is CAD$90.

However, with the heat generation numbers provided, I think I'm good with my 12 AWG 32-foot cable. If I keep the cable loose enough and hanging on the post in the air for heat dissipation, there should be no problems at all.

 

[monkeydelmagico]

Fire from hot extension cord. I've seen lots of pix of fires originating from coiled extension cords. However, those seem to be uber long cords (50-100 feet) that are way under-spec'd - 16 AWG - running 12-13 amp loads continuously.

If 12 AWG of almost any length cannot safely handle your eco-mobile charging needs then something is terribly wrong. You been doing mods to your EV?

 

[Eug]

If 12 AWG of almost any length cannot safely handle your eco-mobile charging needs then something is terribly wrong. You been doing mods to your EV?

No mods. It's just a Level 1 12 Amp 120 V portable EV charger. I am keeping the OEM Toyota charger in my car for use at work, but bought a second one from the Nissan Leaf one off eBay just to leave at the house, so I don't have to keep putting it away in my trunk every day, since I don't have a garage at home. (I bought the Leaf one because it turns out Panasonic makes both the Nissan one and the Toyota one.)

Just being overly cautious, since the EV chargers specifically say not to use an extension cord, and everyone told me using a coiled extension cord with a continuous 12 amp current can cause overheating.

 

[monkeydelmagico]

No mods. It's just a Level 1 12 Amp 120 V portable EV charger. I am keeping the OEM Toyota charger in my car for use at work, but bought a second one from the Nissan Leaf one off eBay just to leave at the house, so I don't have to keep putting it away in my trunk every day, since I don't have a garage at home. (I bought the Leaf one because it turns out Panasonic makes both the Nissan one and the Toyota one.)

Just being overly cautious, since the EV chargers specifically say not to use an extension cord, and everyone told me using a coiled extension cord with a continuous 12 amp current can cause overheating.

Good to be cautious. Considering that charger is corded 16 awg for 25ft it's not the extension cord you need to be worried about.

 

[Eug]

Good to be cautious. Considering that charger is corded 16 awg for 25ft it's not the extension cord you need to be worried about.

Nah, not 16 AWG. Both the Nissan charger and the Toyota charger are 12 AWG. Both are 25 feet.

---

BTW, several years ago I bought from Home Depot a 100 foot 16 AWG extension cord cuz it was on sale for 10 bux. Then after never having used it, I realized it was a bad idea, and still haven't ever used it. I know enough not to run a high power device off it, but not everyone in the house would, so I'm thinking just to throw it in the garbage, esp. since nobody is going to uncoil it completely to use it.

 

[zephyrprime]

P = I²R
power = current * current * resistance


11.6 * 11.6 * 1.65 = 222W/1000ft
= 0.222W/ft
* 32ft
= 7.104W

This is correct.

"...I would lose about 1.5% of the voltage over that 32 feet. Assuming the amperage is about 11.6 A, that works out to about 21 Watts over the length of the cord. Is that correct?"

This is totally wrong. The 1.5% is the decrease in voltage not the resistance.

 

[Eug]

"...I would lose about 1.5% of the voltage over that 32 feet. Assuming the amperage is about 11.6 A, that works out to about 21 Watts over the length of the cord. Is that correct?"

This is totally wrong. The 1.5% is the decrease in voltage not the resistance.

OK thanks. Even better. But is it x2 for two directions = 14 Watts? (That would make it 6 Watts max in the lightly coiled area.)

 

[Eug]

P = I²R
power = current * current * resistance


11.6 * 11.6 * 1.65 = 222W/1000ft
= 0.222W/ft
* 32ft
= 7.104W

So for 16 AWG cable:

11.6 * 11.6 * 4.016 = 540.4 W / 1000 ft. = 0.5404 per foot.

= 17.3 Watts

Or is it x2 = 35 Watts?

 

[edro]

I vote 1x.

Matthiasa said 2x, but he also said the cord could be aluminum conductors... so his vote doesn't count.

 

[ControlD]

Pretty sure it is 1x.

I haven't done line loss calculations in about 25 years, but I don't ever remember doubling the result.

 

[Mark R]

The conductor length is 64 feet. You need to count the hot and the neutral wires together.

This gives a total resistance of 0.1 Ohms - so a voltage drop of 1.2 V. The power dissipation in the cable is given as 11.6*11.6*0.1 = 13.7 W

This is a confusing issue because some line loss tables are based on single conductors - some are based on dual conductors (hot and neutral). I have confirmed the table quoted refers to single conductors made from copper.

 

[Eug]

Ok, thanks. I'll assume it's 14 Watts.

 

[Eug]

Btw that means a 16 AWG cable is over 1 Watt per foot, or over 100 Watts for my 100 foot cable. That settles it. That 100 ft cable is going in the garbage. In an enclosed space tightly coiled up, that's almost a guaranteed fire with a large current.

 

[piasabird]

Heavy gage wire has some better properties. In the wiring in a homebrew electric automobile they use wire or cable the same size as you would for welding cables. This is because on average you might have 20 6 volt 100+ amp hr batteries. However, the wiring in your house may not be anywhere near that strong. It might be cheaper to run a heavy duty circuit from your circuit box to a location near your car in conduit to a new heavy duty outlet. A lot depends on the number of AMPS the charger uses on the input side. I imagine the charger has a transformer that increases the amps feeding into it. So the output from the charger is strongest. So ideally you want to move the charger close to the vehicle.

Why is your automobile so far away from the plug?

I am not an electrician.

 

[Matthiasa]

I vote 1x.

Matthiasa said 2x, but he also said the cord could be aluminum conductors... so his vote doesn't count.

Aluminum is pretty standard for use in wiring, it is probably copper but it doesn't have to be... For aluminum you typically go ~2 gages down so say 12 to 10 to approximate get equivalence. I have used both.

Anything of significant size over significant distance will use aluminum due to weight.

Needing to use 2x has already been explained so no need for me to repeat.

 

[Eug]

Aluminum is pretty standard for use in wiring, it is probably copper but it doesn't have to be... For aluminum you typically go ~2 gages down so say 12 to 10 to approximate get equivalence. I have used both.

Anything of significant size over significant distance will use aluminum due to weight.

Needing to use 2x has already been explained so no need for me to repeat.

The house wiring and extension cord are copper. (I can see the wires for the extension cord because it's visible through the clear plastic molded LED-lit ends.)

Heavy gage wire has some better properties. In the wiring in a homebrew electric automobile they use wire or cable the same size as you would for welding cables. This is because on average you might have 20 6 volt 100+ amp hr batteries. However, the wiring in your house may not be anywhere near that strong. It might be cheaper to run a heavy duty circuit from your circuit box to a location near your car in conduit to a new heavy duty outlet. A lot depends on the number of AMPS the charger uses on the input side. I imagine the charger has a transformer that increases the amps feeding into it. So the output from the charger is strongest. So ideally you want to move the charger close to the vehicle.

It's 120 Volts 12 Amps in, and 120 Volts 12 Amps out. The charger isn't actually a charger actually. I don't quite know the details, but apparently it just negotiates the connection, and then acts as a pass-thru.

It's common to get 240 Volt chargers, up to about 32 A (7.7 kW), or higher if you own a Tesla or something. However my car is Plug-in Prius, and apparently maxes out below 240 V 12 A (~2.8 kW). However, the battery is small and it charges in 2.5 hours anyway on 120 V 12 A (~1.4 kW), so I just stuck with 120 V. Going to 240 V would mean spending a lot more money. There is a cash rebate program here for 240 V charger installs, but I don't qualify since I bought my car used.

I believe strictly speaking I can't even install an outdoor 240 V plug and use a plug-in 240 V charger. I'd have to get a fixed install of a 240 V charger, and to put it in a convenient spot would mean running quite a bit of conduit to get there, plus I'd have to have some sort of pedestal for it. Which reminds me, the "pedestal" I used for my 120 V charger is simply a 4x4 48" fence post.

The extension cord runs from the side of the house to the post. The last 14ish feet of the 32 foot cord is loosely coiled on the back of the post. (Can't see it in this pic.) The 120 V charger is mounted on the left of the post, and then it has about 22 feet of cable until you get to the part that plugs into the car, which is that white thing with the blue cap on it.

Right now it's a mess because I'm just hanging the charger cable around the post, but this week I'm buying a hanger with a built-in EV charger holster.

It's supposed to look like this:

I'll cap off the post with a solar lamp.

Why is your automobile so far away from the plug?

Outlet just happens to be further away than ideal. I don't have a garage and the outlet is on the side of the house.

 

[Paperdoc]

The posts above have the right answers in general. I just want to raise a different point for you to monitor in future. With most heavier current draws, one of the larger sources of heat generation and energy loss is resistance at the contact points of the connectors at the ends of the cord. Over time the metal of the contacts suffers three types of degradation: (a) metal oxide buildup on the surface due to atmospheric oxidation; (b) contamination by dirt introduced because there is daily insertion of prongs; and (c) loss of contact pressure as the metal slowly fatigues and the "springiness" of the contacts is weakened. These are not normally rapid, but they are inevitable. So from time to time, examine the connectors at the cable ends. The simplest is what you've done already - feel along the cable and at the connectors after it has been in normal use for a while, looking for significant heat. If you find it, get the cable serviced, or replace it. This may not be needed for years, though.