this post was submitted on 26 Dec 2024
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"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

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[–] douglasg14b@lemmy.world 10 points 1 day ago

Connectors come loose, which makes them dangerous.

They are uninsulated points that allow water and material ingress, and can partially or fully pull apart, causing arching. Which can cause combustion.

This is the main reason these are dangerous, which the majority of this entire thread misses. The added length or connector resistance is somewhat negligible here unless you're daisy chaining long conductors, which often isn't the case for in-home extensions.

[–] JordanZ@lemmy.world 36 points 1 day ago* (last edited 1 day ago) (15 children)

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.

[–] rekabis@lemmy.ca 4 points 1 day ago

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.

[–] douglasg14b@lemmy.world 5 points 1 day ago (2 children)

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

[–] Mesophar@lemm.ee 1 points 6 hours ago

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

[–] rekabis@lemmy.ca 5 points 1 day ago

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.

[–] lime@feddit.nu 24 points 1 day ago (4 children)

lol "gauge"

americans will use anything except the metric system

[–] Thavron@lemmy.ca 12 points 1 day ago (1 children)

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.

[–] spizzat2@lemm.ee 10 points 1 day ago (2 children)

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

[–] Thavron@lemmy.ca 8 points 1 day ago

Oh ffs I should've known.

Yup, I work with 4/0 (0000) cable pretty regularly, for things like generators or powering large systems. We have a few trunks full of cable, and it takes a crew of 2 or 3 to actually lay it because it’s so heavy. Usually one person pushing the trunk along, one focuses on uncoiling it from the trunk, and one focuses on actually laying the cable. We use five conductors at a time (one neutral, three 120v hots leads, and a ground,) so it’s a big bundle. Each cable weighs a little over a pound per foot, and there are five bundled together. So a 150’ coil can easily weigh 750-800 pounds.

[–] Successful_Try543@feddit.org 14 points 1 day ago* (last edited 17 hours ago) (7 children)

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:
Screenshot_20241227-221529_Eternity_1

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[–] Kaput@lemmy.world 3 points 1 day ago (1 children)

What is the metric unit for cables?

[–] lime@feddit.nu 3 points 1 day ago

for cross-sectional area? mm^2^.

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[–] Kolanaki@yiffit.net 137 points 2 days ago (22 children)

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

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[–] uis@lemm.ee 11 points 1 day ago* (last edited 1 day ago) (1 children)

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

As dangerous as one extension cord of their combined length. Don't forget to verify that every cord rating is above load rating. I recommend to use at least same rating as circuit breaker or get extension cord with circuit breaker built in and never decrease rating down the line without circuit breaker before it, so even if you somehow overload it, there will be protection from it.

AND NEVER COIL OR THERMALY INSULATE! Cords rely on convection for heat dissipation, and spooling and insulating reduces it, thus increasing electrical insulator temperature until it melts and spontaneously combusts. This applies to extension cords in general.

[–] tacosplease@lemmy.world 10 points 1 day ago (1 children)

Because this is the Internet, I'll be pedantic and say the interface where each cord plugs into the next probably adds some resistance as well.

So, 50 cords 1 ft each plugged into each other would have a higher resistance than the same wire at a single 50ft length.

I doubt it really matters in the practical terms of your answer and the question being asked though.

[–] uis@lemm.ee 2 points 1 day ago

*insert technically correct*

[–] BillibusMaximus@sh.itjust.works 65 points 2 days ago (17 children)

The longer the distance, the larger the diameter of the wire you need, due to resistance/heat.

Typically, extension cords are going to be manufactured with the thinnest wire they can get away with based on the safety requirements, in order to save on materials cost.

So plugging 2 short cords together might cover the same distance as 1 longer cord, but the longer cord will use thicker wire to maintain the proper margin of safety.

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[–] Free_Opinions@feddit.uk 21 points 1 day ago* (last edited 1 day ago) (2 children)

Related thing about extension cable reels that many don't know is that even if you need like 3 meters of cable you should still unspool the entire thing as otherwise it's acting as a coil and creating ~~resistance~~ heat. Most cable reels have different ratings marked on them for when they are spooled / unspooled. This is especially true when the device you're powering takes >1000 watts

[–] modeler@lemmy.world 11 points 1 day ago (2 children)

Many years ago, my mother used the electric lawn mower without unspooling all the wire. When it finally shorted, all the plastic wire insulation was in the process of turning into a melty plastic soup. A Lesson Was Learned.

The reason isn't resistance - it's that the coiled wire makes an electromagnet that stores energy in the magnetic field. The alternating current in the mains switches 50 or 60 times a second. In each cycle the magnetic field is created, destroyed then recreated in the opposite direction, then destroyed. This dumps a lot of energy (and therefore heat) into the coil.

[–] brown567@sh.itjust.works 7 points 1 day ago

The coil wouldn't make a significant magnetic field because the cable has two wires with opposite current flow in close proximity

But when the cable is coiled, its ability to dissipate heat is less, so normal resistive heating can create higher temperatures

[–] uis@lemm.ee 5 points 1 day ago

Inductance is not the reason here

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[–] tiefling@lemmy.blahaj.zone 45 points 2 days ago (8 children)

Meanwhile landlords: "one 50 year old outlet should be enough for two bedrooms right?*

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[–] Dark_Arc@social.packetloss.gg 26 points 2 days ago (4 children)

There are two things going on here.

The first is that yes, more connections causes more opportunities for the plugs to slip. So you can get short circuits or even arcing that can start a fire.

The second is that the wire in the cord has a certain rating on it. Many of those cords do not use 12 (20 amp) or 14 (15 amp) gauge wire; so, they're not rated for the full capacity of the wire in the wall. The breakers are sized to protect the wires in the wall, they don't know anything about the things plugged into them. So what can happen is you plug too much into the extension cord (particularly if it's a power strip) and the load on the extension cord is not enough to trip the breaker (because the walls are fine) but it's enough to overload the extension cord wire. In other words, the extension cord can start getting so hot it melts and possibly arcs up as the insulation fails.

You can have a fire from overloading a single power strip in just the same way. However, the more you chain together, the more likely you are to overload the power strip.

Ideally, you just think about what you're doing... But historically the easy answer is just to tell people not to chain things.

In short it's not about the distance, it's about the insulation and quality of the wire itself along with the number of connections.

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