“never plug extension cords into extension cords” is probably the most common piece of electrical related advice I’ve ever heard. But if you have, say, 2 x 2m long extension cords, and you plug one into the other, why is that considered a lot more unsafe than just using a single 4 or 5 meter cord?

Does it just boil down to that extra connection creating another opportunity for the prongs to slip out and cause a spark or short circuit? Or is there something else happening there?

For that matter - why aren’t super long extension cords (50 or more meters) considered unsafe? Does that also just come down to a matter of only having 2 connections versus 4 or more on a daisy chained cord?

Followup stupid question: is whatever causes piggybacked extension cords to be considered unsafe actually that dangerous, or is it the sort of thing that gets parroted around and misconstrued/blown out of proportion? On a scale from “smoking 20 packs of cigarettes a day” to “stubbing your toe on a really heavy piece of furniture”, how dangerous would you subjectively rate daisy chaining extension cords, assuming it was only 1 hop (2 extension cords, no more), and was kept under 5 or 10 metres?

I’m sure there’s probably somebody bashing their head against a wall at these questions, but I’m not trying to be ignorant, I’m just curious. Thank you for tolerating my stupid questions

It increases the risk of electrical overload and overheating as it adds more resistance to the circuit.

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3 points

Isn’t the added cable resistance small enough to not cause issues so soon? In case you just chain a few ( < 10 ) together.

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9 points
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It’s not just the cable resistance, but the added resistance at each connection point. Since the plugs aren’t the same piece of metal, just touching.

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18 points

Thanks for the response! Would you mind going a bit more in depth about that please? I could understand increasing the risk of overload if you were to daisychain power boards, as they add more power points to the circuit than it was designed for. But extension cords (at least in my experience) only have 2 ends - one with a single plug receptacle, and the other that plugs into a power point

Is it the actual connection between the two that adds more resistance to it? If it were the wiring, then wouldn’t that also pose a problem for longer extension cords?

In either case, what sort of resistance add are we talking about (feel free to pick random lengths of examples make it easier to explain)?

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95 points

The longer the cable, the thicker (heavier gauge) it needs to be to carry the same current without burning up. One extension cord is rated to carry the current it alone is able to carry. Put two of those in series, and both of them together are able to carry less current than either one by itself. This is how fires start.

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24 points

This is incorrect. I need to increase gauge for voltage drop. Overloading the cable via length can only happen if I have a motor or other magnetic load at the end. A motor will try to draw it’s designed wattage regardless of voltage. A wire of a given ampacity will handle that many amps regardless of the length of the conductor. The relationship is power = voltage x current and voltage = current x resistance for single phase. The fire concern on extension cords tied together indoors is you have 100% strung that shit through a doorway or window, which is a code violation. You are going to pinch it and burn your shit down. all outdoor plugs are gfci these days and on site i can have 4 or 5 extension cords tied together. i only get 109 volts at the end but a heater is a resistive load. Doesnt matter for that application.

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15 points

Nailed it in far fewer words than me.

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2 points

I wonder what kind of safety margin is calculated into these…

Admittedly I’ve seen some wildly different shielding or thickness in my time

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10 points

I don’t think what they said is actually a problem, it’s just a back-justification for the original trope. Daisy chaining them and strictly sticking to only the few appliances that would fit in one extension strip is fine. But that’s complicated to explain, it’s better to just tell people not to do it rather than expect them to understand what’s going on

A couple things that can happen…

  • plugging in too many appliances over several daisy chained power strips trips the circuit breaker because too much current is being drawn

  • if the country you live in has lax electrical safety standards then, yes, perhaps you can overload the daisy chain without tripping the main circuit which would lead to overheating

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6 points

When the breaker trips then the fundamental issue is unlikely to be present. But to be able to push enough current to cause it to break the connection needs to have a sufficiently low resistance. If that gets too high it will never break, even if you short the cables. And that will result in a fire, because the protection does not work anymore. That is the dangerous part.

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0 points

“never plug extension cords into extension cords” is probably the most common piece of electrical related advice I’ve ever heard.

Same reason power cables outside are fucking huge and the cord to your TV is tiny.

Electrical loss generates heat, so the longer the cable, the thicker it needs to be before that heat is too much.

Don’t forget a filament in an old school light bulb is just really thin wite. The thinner it is the less energy required to make it glow, which is why there’s like a 200 year old light bulb still going, it’s just a thick filament and very inefficient

So I’d never plug two of those rinky dink indoor extension cords together, it doesn’t take much length before it starts “glowing” like a light bulb filament, which happens at the plug and can burn a house down.

But…

Growing up doing rural construction with heavy gauge extension cords we never thought twice about hooking multiples up as long as it was just something quick for a few minutes at a time. Then never left it plugged into the source when not in use. You’d never do it for like a radio even because eventually it’s gonna heat up back at the aource.

Someone else already mentioned not pulling it right, we’d “doughnut” the connection so that if it did get yanked accidentally it wouldn’t unplug, but obviously it can’t be under constant stress even like that.

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1 point

So many people here talk about the thickness needed of else it heats up. That is simply wrong. The heat is the same per length and is dissipated the same way with twice the length aka the temperature is the same*. The issue is that the short circuit current could drop below the value needed to actually pop the breaker, allowing for a ton of heat to be generated where it shouldn’t be. The same way a light bulb glows bright hot but does not trip the breaker etc., now just imagine the cable to be the glowing part.

*There is another issue if you do not lay them out, that the heat has nowhere to go. Causing coiled wires to have a far lower rating compared to when they are fully extended.

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4 points

The longer the cable, the thicker it needs to be to carry the same amount of current without getting hot. This is due to resistance (Wikipedia does a good job with the details). Basically:

20’ cable be long and thick.

10’ cable be short and thin.

10’ + 10’ cables be long and thin.

Long and thin = heat then fire.

That being said, put an LED light bulb at the end, no problem. Put a gaming PC on the end, problem.

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5 points

I don’t think you’re right about heat. Two cables in series have double the resistance and therefore double the total heat generation, but that heat energy is split between the two cables so each one does not get hotter than if it was the only cable.

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0 points

Current and Voltage don’t change, but resistance does (between cable types)?

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1 point

Yes, you will have double heat output due to twice the resistance which causes twice the voltage drop and more or less the same current. But this heat output is spread across twice as much wire, so unless the extension cables are coiled together on the ground each will heat up the same amount as a single one would.

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3 points

What are you talking about? Current absolutely changes! Your own formula requires I=V/R

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37 points
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Others have pretty much mentioned it. Too thin of conductors for the total length required which can overload the cable and heat it up. If you’re just charging your phone it’s unlikely to cause a problem but the more amps you pull the riskier it gets.

Here’s a helpful chart…

Edit: Even at harbor freight (cheap hardware store) a 50ft 12 gauge extension cord is about $40 and weighs 7 pounds.

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25 points

lol “gauge”

americans will use anything except the metric system

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14 points
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16 AWG – 1.3 mm^2
14 AWG – 2 mm^2
12 AWG – 3.3 mm^2
10 AWG – 5.2 mm^2

For us from the civilised part of the world ;-)

However, as in Europe we have 230 V system, approximately half the cross section, as stated in the table above, is sufficient.

Edit: This is how the above text should be displayed:

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1 point

Both are measurements of cross-sectional AREA and are defined in terms of square millimeters (mm^2), not mm.

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12 points

I’ve always found gauge to be especially odd, because the number gets smaller as you go bigger, so at one point you can’t go any further even though you can go fatter.

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10 points

Oh, you can get bigger! Just keep adding 0s. It’s fine.

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3 points
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Removed by mod
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3 points

What is the metric unit for cables?

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3 points

for cross-sectional area? mm2.

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-11 points

It doesn’t make sense. Temperature difference does not depends on length.

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19 points

Adding length creates more resistance which creates more heat. Thinner conductors can’t handle it as well.

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-8 points
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And more surface area for heat dissipation. https://lemm.ee/comment/17115060

They cancel out.

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2 points
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Space heater is “medium duty” while a router is “heavy duty”?

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15 points

This is also known as a router

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7 points

router as in ‘tool for finer details in woodworking.’ most router motors can spin at 10000rpm at their lowest and anything with a motor, by definition, uses more power than something without a motor

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1 point
Deleted by creator
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5 points

Is it just me or is anyone else perturbed that the cable sizes in this infographic are all the same gauge?

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5 points

the cable sizes in this infographic are all the same gauge?

They’re not. They are clearly marked as different gauges, except the left most two which have different plug types… one is two prong, the other is three prong.

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1 point

Do you just mean the art showing them as the same size? Because that’s common in a lot of infovraphics to not be to scale if they are clearly labeled

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4 points

a 50ft 12 gauge extension cord is about $40

$40USD would be $58CAD.

A 50-ft 12-gauge extension cord costs $112+ CAD anywhere in Canada. A 100-ft is $200+ CAD. Like… fffffuuuuuck.

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6 points

I think it is partly a US specific problem as the quality of the extension cords really suck. Meanwhile in Eurpoe (or at least in Germany) the extension cords actually use the same wire grade as your in wall wires, so there is a basically no difference in using daisy chained extension cords versus different wall outlets (as long as the outlets are in the same curcuit)

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1 point
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Deleted by creator
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-2 points

I mean it is hard to find out if they are the exact same, since power strips often don’t specify it, but from handling both I’d say they are pretty much the same…

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3 points
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The cable specification is usually written onto or molded into the outer insulation, at least in Europe / EU.

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6 points

Not necessarily exactly the same wires, but all rated for 16A, so the circuit beaker will trip long before any wire gets a chance to heat up

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3 points

Yep, came here to write the same. We have 240V and not 120V like the US. To power the same appliance in the US you have double the Ampere and therefore higher risk of fire (correct me if I’m wrong)

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2 points

I strongly disagree… We just have higher standards regarding power wires. Since we have more voltage running through the wires we need tougher ones, but that is what regulation is for

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3 points

So the higher Ampere doesn’t require thicker cables? Genuinely asking. The higher standards and regulations are absolutely part of why you don’t hear this rule here.

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