Daisy-chaining extension cables is bad - why? Featured

Ohms law

Because Americans. They don't have fused plugs, so by daisy-chaining extension leads they can easily overload the ones closer to the supply and start fires. So "over there" it's considered a serious problem.

Theoretically*, with the 13A (or less) fuse in all UK plugs, is shouldn't be possible to overload anything, and the worst that will happen is the fuse will blow (or you'll trip on the tangle of wires and kill yourself that way instead).

• I'm not convinced that poundland-specials could cope with a sustained load of whatever-it-takes to blow a 13A fuse.

So, although it's not as bad as American media would have you believe, you still need to be careful. 20 wall-warts taking less than 20W max each is fine, A kettle, 3-bar electric fire, and toaster may be pushing it, even on a single extension lead.

Indeed.

Trouble is many extension leads have 0.75mm2 or 1mm2 cable. That's fine because they calculate the voltage drop across the length of cable provided and ensure it's within limits - longer cables could be 1.5mm2 to compensate. When you chain them together you chain together the voltage drops, with the result that the voltage at the far end could be sagging under load.

That may not be a problem if the devices at the far end will adapt to the voltage, but you could see load-dependent brown-outs if you connected an incandescent lamp or say a motor whose speed or torque depended on the mains voltage.

(also, you don't want to exceed the thermal limits of the cables. Having a daisy chain won't change that, unless you put them all together so they combine their heat)

Theo

Can you be a bit more explicit?

Sure, V = IR.

So the longer the cable, the more resistance in the wires and therefore the more voltage dropped across it.

Fair enough. But it is any worse to daisy-chain two 20-metre extension cables than to use a 40-metre extension cable?

I've committed the cardinal sin of daisy-chaining three extension reels

- 40 and two 20s - to run our garden shredder at the bottom of the garden close to where I'm stripping branches off fallen or pruned trees, so I don't have to lug all the branches up to nearer the house and shred them on the gravel drive.

The 40m extension is more likely to come as 2.5mm^2 cable and the 20m extensions more likely as 1.5mm^2 ... unless from Amazon ...

The outside diameters of all three cables that I have are very similar, suggesting that they are all 1.5 mm^2 or all 2.5 mm^2.

PVC filling is cheaper than copper... but the mm2 should be written on the outside.

Theo

It will just be lower power, thats all

Yes, because you have an additional plug, socket and fuse in the path, all of which might have poor or loose connections, and fuses have resistance, as they must in order to function. All additional points of failure and potential overheating.

OK for temporary use, as long as the cables are fully unreeled. A shredder is a fairly powerful beast. The problems really arise in amateur workshops, offices and such, where several extensions get chained, and often sit for years on the floor behind benches or desks, where they will not be inspected regularly. In a business, they will be inspected and PAT tested every year or two.

The main concern is that inclusion of extra plug/socket connections combined with the extra cable resistance could result in raising the earth loop impedance to a point were you could no longer rely on it being low enough to clear a fault quickly[1].

There is a secondary concern that many extension cable makers will spec the thinnest flex that they can get away with taking into account the length. Using several may result in excessive voltage drop at the point of use - especially for higher power loads. For mainly resistive loads this may just result in lower performance / less power at the appliance. For something like a mower with an induction motor, it could result in motor damage since it can draw more current that it was designed to, and overheat.

Manufacturers of things like fan heaters tend to issue dire warnings about using extensions on their appliances owing to the heat effect in cables. Yet they often provide such short leads that it is difficult to plug in a heater and position it to get maximum effect. The answer is to provide larger cables but I suppose when you are working to a price that becomes a problem yet for end users adding an extension as inappropriate as it is often the only solution.

I had one of those Stanley workshop fan heaters and there was always detectable warmth in the cable and moulded plug. When it eventually failed the moulded plug almost melted and I suspected there was a dodgy connection within but it is one of those things that’s impossible to check.

A comprehensible explanation - thanks John.

I spent some time playing with extension cables when I had the EV before the proper charger. The variability between different 13A plugs and sockets was quite marked, some staying quite cool and others getting worryingly warm for something you need to leave running unattended for hours. I was testing using an old 3kW fan heater. I was surprised also at how much warmer a cable would get just coiled on the floor, not even wound on a reel.

I ended up making my own lead with chunky 2.5 mm^2 cable, which is a bit of a squeeze in some plugs, an old MK plug of the type where you wind the conductor around a post and then clamp down with a nut, and a single plastic trailing 13A socket where I was lucky first time. Plugged into a new RCD-protected wall socket that all ran cool.

The 2.5 cable also stayed reasonably cool when lightly coiled on the floor rather than stretched out.

The only non-ideal element was the 13A plug moulded on to the charger which would always get warm, especially around the L pin, though pin, fuse and fuse holder looked clean.

nib

Isn't a warm plug - especially a moulded plug - equally likely to be a poor/loose connection at the socket?

Certainly can be...

General tip, squeeze the fuseholder contacts together a bit whenever you open up a plug, whatever type of contact it is, it will lose tension over time.

Also do as you have done, check the state of fuse and contacts, I've often seen partly oxidised fuse end caps. Generally when a plug gets warm, it's something fuse-related.

Note that if the cable grip is over-tightened, the pressure can break wire strands inside over time, possibly leading to local heating. I've seen at least three 'non-working' plugs where all the strands could be pulled out of one of the wires, the break being under the cable grip.

Can be, but I wouldn't have said 'equally'. That entry hole on the socket will usually look slightly (or sometimes very) discoloured compared to the others,