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.

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

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.

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

First, I’m not an electrician, or anything even vaguely approaching one.

I’ve never heard that advice with extension cords, only power strips.

You could theoretically link enough extension cords together to cause problems, but it would need to be some extremely shit extension cords or a LOT of them. Resistance increases over distance, which in power cables manifests as them getting hotter. That isn’t a problem until all the sudden it is a problem.

For power strips, the main danger is that you’re potentially introducing more outlets than any of them are rated for. If you’re just using 2 3ft power bars to functionally make a 6ft cable that you’re only plugging a single thing into… you’re fine. Or at least I was, idk I did it for like a whole year. If you’re plugging 16 things into a single wall outlet via power strips you can trip the circuit breaker, or potentially much worse stuff can happen like things getting melty and starting a fire.

If you know enough of what you’re doing to math out the power draw+what your outlet/power strips are rated for you can pretty (afaik) safely daisy chain them if you wanna.

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.

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.

I’m not trying to be ignorant, I’m just curious.

I think you’re in the right community! Don’t let anyone tell you to shy away from asking curious questions. (well, unless the question is also bigoted, illegal, baiting, sealioning, or otherwise disingenuous)

I’m not an electrician in any jurisdiction, but one answer for why two 2-meter (~6 ft) extension cords in series is inadvisable compared to a single 4 meter cord is that it’s not an apples-to-apples comparison. Longer cords necessarily have to be built differently than shorter cords, not only because of electrical codes (eg the NEC in USA) or product safety specs (eg UL, CSA) but also being well-designed for their expected use. There’s also the human aspect, which all good designs must account for as well.

Here in the USA, common extension cord lengths are ~2 m (6 ft), ~7.5 m (25 ft), ~15 m (50 ft), and ~30 m (100 ft). Of those cords, the common wire gauge used might be 18 AWG (~1 mm^2), 14 AWG (~2 mm^2), 16 AWG (~1.5 mm^2), and 12 AWG (~3.5 mm^2). I’ve intentionally rounded the metric units so they’re more analogous to common wire gauges outside the USA. Finally, the insulation used can be anything from “thin, indoor only” to “heavy, abrasion and sunlight resistant”. And while the USA technically has a boat-load of AC connectors, the grand majority will use the standard 2-pin or 3-pin 120v connector, formally known as NEMA 1-15 and NEMA 5-15 respectively. What this means is that chaining extension cords is both possible and somewhat common. The problem is one of mismatched designs.

From a cursory search on the website of a major USA home improvement store, the smallest wire gauge used for a 100 ft cable is 16 AWG. The largest is 10 AWG (nb: smaller numbers mean bigger wire). That thinner cable is marketed for outdoor use. The thicker cable indicates its use “indoor/outdoor” and for heavy-duty applications. It is also branded with a major power-tool company, which would be appropriate as power tools often draw high current.

Whereas looking at 6 ft extension cords, most are 16 AWG but a few were 18 AWG (thinner than 16) or 14 AWG (thicker). But I could not find any thicker cables than that, certainly nothing that uses 10 AWG (~6 mm^2). The “heavy duty” cables of this length also used only 16 AWG wire.

Because electrical resistance is additive in series, and because Ohm’s Law governs the voltage lost at the end of a cord, the use of insufficiently large conductors can cause voltage issues for high-current appliances. Appliances for USA-spec generally require 120 Volts +/- 10%, with utilities aiming to provide 120 Volts +/- 5% from the outlets. This means a “sufficient” power cord should not have a voltage drop of more than 6 volts, give or take. Of course, a high-current appliance will also cause a larger voltage drop than a low-current device, so we only consider the former case.

For a machine that draws 12 Amps attached to a 100 ft extension cord made of 18 AWG wire, the voltage drop would be 15 volts. This is bad for the machine, which now sees a lower voltage than expected. Had the cord been made of 12 AWG wire, the drop is an acceptable 3 volts.

So if you’re operating construction tools, it would be a terrible idea to use three random 6-ft cables, and you should instead use a single 25-ft cable. Even though it’s longer than you need, the fact is that most 25 ft cables use thicker conductors, which reduces the voltage drop overall.

But there’s also that peaky human factor. Sure, there would also be more connectors which could come loose, but the really pressing issue with daisy chained cords is when people do that indoors, because they only have light-duty 6 ft cables handy. And for that Christmas tree, they need to use attach three cables together to go beneath the hallway rug.

This is essentially the worst-case scenario: using thin conductor cords, with thin insulation, underneath very flammable household surfaces, which are also trodden upon by foot traffic. Every step on that cord weakens the insulation and fatigues the conductors. Over time, the conductor becomes thinner where it’s being fatigued, and this increases the voltage drop. An unfortunate result of a voltage drop is that it generates heat. For a cable which is uniformly thin, this heat is spread over the whole length. But for localized conductor damage, the heat is pin-point… directly under a flammable rug.

In the USA, some 3300 house fires started from an extension cord. Because these cords are not within the walls, they are usually beyond the control of often-strict building/electrical codes, something that’s been critiqued by a prominent YouTuber. The US CPSC even goes so far as to create memes to promote their messaging that space heaters – a common, high-current appliance – should not be used with extension cords or strips.

Of course, from an electrical perspective, even a ten-long chain of dinky extension cords would have no problem powering just a single LED night light. But it’s reasonable to ask: 1) is this just asking to be struck down by fate, 2) are there better alternatives like thicker/longer cords, and 3) why isn’t there an outlet where you need it?

(There’s also a scenario where too long or thin of an extension cord can cause a circuit breaker to fail to trip during a short circuit, but it’s fairly esoteric and this post is quite long now)

In short, the blanket recommendation to avoid daisy-chaining cords is to avoid the nasty and sometimes fatal results when that can go wrong, even with it might not always play out that way. There’s almost always something safer than can be done than daisy chaining.

The smart answer: it can start a fire, don’t do it.

The honest answer: I’ve done it a zillion times and never started a fire. I buy heavy 12 and 10 gauge cords and only chain them to power things that don’t require a lot of amps, like LED lights.

When in doubt, a plug-in power meter is an option you can use to help monitor the situation. Plugs that draw a lot of amps can get warm, even when not using an extension cord. Also, it’s not uncommon for a device to pull a lot of amps on initial power-up, and then settle down to a lower draw.

Resistance increases over longer distance cable, and increased load from appliances. (Especially don’t with american electrics, they aren’t fused so the wire can overheat and set fire to the surroundings.)

I would also like to hear an expert opinion on this. Never really made sense to me either, but it also doesn’t sound unreasonable.

I always thought it was to stop people from plugging too many appliances into all the outlets.

I have an RV with 2x 10 gauge, 50’ extensions running power to it. I have a built-to-purpose coverat the connection point. Its fine and safe enough. Just keep slack at the connection. It cant be under pull stress.

Extension cords are rated for certain levels of resistive load based on their length. It typically has to do with the gauge of the wire used in the chord itself. Electrical resistance is typically converted into heat energy. If you daisy chain extension cords and have them plugged into appliances, then the resistive load of the chained cords can be higher than the rating of one or more of the extension cords. This can cause fire.

“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.