“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
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)?
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.
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.
It’s obvious you know more or less all there is to know about this topic. So much so that I suspect you have trouble explaining it to laypersons like me because it’s difficult for you to determine which parts of your knowledge are obvious common knowledge and which parts are specialist knowledge.
The super simple explanation is that the wires are too small. The water hose analogy breaks down fairly quickly, but I’ll try using it. Imagine a garden hose, with a regular nozzle on the end. But it’s not a perfect world, and our hose doesn’t transfer all the water that goes into it. Think of this as ten pinprick holes along every meter of hose. If we have ten meters of hose, that’s fine, we only need to turn on the tap a little bit to get a decent spray out of the nozzle, and a little bit will dribble out these holes. Now let’s join another hose on. We lose more water to leakage, so to get the same amount of water out of our nozzle, we have to turn on the tap more, giving it a bit more water flow. Now, our pinprick holes are not just dribbling, they’re flowing freely. Now let’s take it to the extreme- we join a thousand garden hoses together, all leaking a little bit. We have to turn the tap on A Lot More, and suddenly our pinpricks are spraying a serious amount of water everywhere. Now imagine we use a bigger hose. Let’s take it to the extreme again and say it’s a big stormwater pipe. But the key part here is that it has the same amount of holes, ten pinpricks per meter. This way, we can get heaps more water down that pipe, more than enough to give that water nozzle everything it wants. Also, because our pressure can remain low, those pinpricks are only leaking a little bit, not spraying everywhere. This is getting pretty wordy and unwieldy to type out on my phone, so I’ll try and bring it into the real world a bit more. An electrical load, like a motor (say a compressor in a fridge, a circular saw, etc) is like to our nozzle. It will pull more current (amps, or water flow) to maintain the same amount of power output (water coming out of the nozzle). As we get a longer conductor, the voltage drop (pressure reduction due to water lost to the pinpricks) gets larger, and our voltage at the end of a conductor gets lower. Power = voltage * current, so if that voltage is lower, to get the same power we need more current. More current means more heating. More heat in a small cable means melting. Physics has a way out for us, thankfully! The thicker a cable is, the less voltage drop it has, kind of like our stormwater pipe. So the voltage remains at a normal level at the motor, and consequently the motor draws a normal amount of current. This is why longer extensions are generally a lot thicker than shorter ones. If you’re interested in the math, let me know, it’s actually pretty fascinating, and ties into why long distance power lines are all super high voltage, among many other things. The basic equations are also not too hard to work with.
This is junior highschool level stuff. Not a vector or phasor in sight.
Your school taught anything at all about electricity? Mine sure didn’t.
My high school had a lot of vocational courses. I took auto shop, construction, welding, and small engine mechanics. Several of those covered electricity.
We had small level building, like a coffee table size thing but smaller.
That was kinda it.
Do you mean inductive load rather than magnetic load? Or are all inductive loads attributed to electromagnets?
Edit: also, don’t like… a lot of appliances create inductive loads?
Most inductive loads are motors. I used the term magnetic rather than inductive in the Hope of making my response less jargon filled and more intelligible. Very generally speaking inductance is the magnetic portion of the circuit or more technically it would the contribution to the circuit that causes the wave form to lag. That is specific to an AC circuit.
Nailed it in far fewer words than me.
Another bit of explanation I just thought of –
Think of an incandescent light bulb. It has a filament. You run electricity through the filament and it heats up enough to glow, producing light to see by. It does that because the thin filament has high resistance; it resists allowing current to flow through it.
Any piece of conductive material will do the same thing if you put enough current through it. Even an extension cord. It will heat up enough to glow. Being an extension cord, it will then melt the insulation and dramatically increase the likelihood of setting something on fire.
This is primarily a concern because extension cords aren’t fused, and there’s no control over how they are routed.
Most wiring in your walls come after a circuit breaker and are designed to allow for a certain amount of heating. The electrician follows a code that guarantees that the circuit breaker will trip before there’s any possibility of too much heat. This table indicates a higher ampacity rating for higher temperature ratings.
Now most extension cords are made cheaper by using lower gauge than the wiring in your walls. The general assumption is that they’re spread out, so the heat has no way to build up, and you won’t be plugging them permanently into something drawing the peak 15A allowed by the circuit breaker.
If you were to pile up a 100 foot extension cable and plug in a hairdryer, you’d probably start a fire. If it was all spread out, likely your hair dryer would just receive less than the 120V it’s expecting, and it wouldn’t get very hot.
Ironically, dinky christmas lights make very safe extension cords because they’re fused inside the plug.
And because the wire gauge is less than the wiring in the wall the breaker won’t trip before it reaches the point where it’s overloaded either.
Other way around. Low resistance - high power.
I wonder what kind of safety margin is calculated into these…
Admittedly I’ve seen some wildly different shielding or thickness in my time
For extension cords? Pretty much nothing. They can be dangerous all by themselves. You have an outlet on a 15A circuit. You can plug all sorts of things into that outlet all at the same time, especially if you’re daisy chaining cords and adapters. The sum total of current from all those things can be less than 15A, so the circuit breaker never trips, but more than what your mess of extension cords can handle. Even worse, if it’s just a little more than the extension cords can handle, you might not notice right away, and then you’ll come home later to a pile of smoldering ash.
Don’t chain extension cords.
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
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.