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I am an European, so I am not sure if that matters.

All I know is that I need a cable with the right amp or wattage throughput capability. Our cables are named as 3x1.5, 3x2.5, 3x3.5 etc, I guess that is 3 cords that are 1.5 cm thick, 2.5 cm thick, etc..

So I have heard the following and wanted to debunk it if its a myth. If let say an appliance of mine uses a 3x1.5 cable which has max 4 KW throughput, do I need a thicker extension cable than the original cable that this appliance use? Or I am good with the same one as long as the capacity of the cable is higher than the appliance itself.

It seems very pointless to me, but it makes me anxious when using random extension cords on higher wattage units like oil radiators and washing machines, as I never find them thicker then the appliance cable itself.

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    \$\begingroup\$ 3x1.5 means 3 wires, each having 1.5 mm² cross-section area ... found this chart kayjayco.com/techAmpsNSAE2Metric.htm ... thinner cable will heat up, the insulation can melt and combust. ... also the voltage drop in the cable can cause the appliance to be "power starved" \$\endgroup\$ – jsotola Dec 20 '17 at 7:29
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    \$\begingroup\$ As an aside, I'll just throw in the warning that if you use an extension lead on a reel for a high-current appliance, pull all the cable off the reel, even if you don't need the full length. In the confined space of the reel, with no convective cooling, coiled cable can overheat to the point that the insulation can melt. \$\endgroup\$ – peterG Dec 20 '17 at 12:36
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    \$\begingroup\$ I'm a European \$\endgroup\$ – Hanky Panky Dec 20 '17 at 14:32
  • \$\begingroup\$ The cable doesn't directly have a 4Kw throughput... the cable has a current rating (as in 2.5mm twin and earth is rated up to 27A... depending how it's fixed... which in European terms means a device using about 5.6Kw will be all that can safely be connected to the mains supply using it. See lightwiring.co.uk/lighting-circuit-components/light-cables/… for what different fixings / coverings will do to the maximum current capacity. \$\endgroup\$ – matt Dec 20 '17 at 15:58
  • \$\begingroup\$ @peterG Lack of cooling from constrained space isn't your only problem. Your reel will be inductive when not completely pulled which can be a problem if the current gets too high. The induction will lead to more heating. \$\endgroup\$ – Mast Dec 20 '17 at 20:51
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Basically yes — as the extending cable should be the same or higher in capacity to limit the losses which get turned into heat.

It does depend on the distance — so it may be safer, more sensible and convenient to fit a new supply point where the device is to be used.

As for the sizes, the 3 * 1.5 is three cores or wires of 1.5mm2 in cross sectional area — most in industry just tend to say 1.5 or 2.5 and don’t mention the units as everybody knows... they should, of course, mention the units but ...

As for not finding extension cables that thick - they are obviously not common and are usually made specially as necessary - I have made a few in the past.

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  • \$\begingroup\$ Clearly explained. So, for an example I want to extend a washing machine cable for a metre or two, I am not sure what thickness the original cable is, it might be 3x1.5. The machine is rated for 2.5 or 3 KW, because it has it's own heaters (European as I said). Should I just make sure to get a 3x2.5 extended cable, or in this case, as it seems to me, it is not really needed to go more than a 3x1.5 as the cable supports up to 4 KW, and its only 1-2 metre extension and even 3 KW washing machine on a two metre extended cable would suffice it? \$\endgroup\$ – A. Newb Dec 21 '17 at 9:34
  • \$\begingroup\$ Always err on the side of safety, using a cable over-rated will not be a problem ie 3*2.5 but a cable too small can be catastrophic. \$\endgroup\$ – Solar Mike Dec 21 '17 at 10:00
  • \$\begingroup\$ At least in the US and Canada, the size of the conductor is always marked on the appliance cord at a minimum frequency of once per meter. Look closely for what is known as the "jacket print legend". \$\endgroup\$ – davidmneedham Dec 21 '17 at 16:17
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There are several aspects to consider:

  • The extension cable should not overheat by the current flowing through

  • The voltage drop over the cable should be low

  • The short circuit current flowing through the cable should be so large that the circuit breaker will act fast.

A very long and thin extension cable may require an extra small circuit breaker or fuse for full protection against short circuits at the far end of the cable.

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  • \$\begingroup\$ Thanks! One more question, I have an electric oil radiator, and it's cable is fine but he male schuko connector is getting hot, should I immediately change this, why does it happen only on the schuko head? \$\endgroup\$ – A. Newb Dec 21 '17 at 8:16
  • \$\begingroup\$ @A.Newb Plugs and sockets are usually weaker points than wires. Excessive warming up means that there is bad contact, either between plug and socket, or internally - between socket or plug and it's wire. You definitely need to fix that, but keep in mind that it's possible that the head of your radiator cable is faulty itself. Try it in different sockets under observation and note when it does and when doesn't get hot. Also try a different extension. \$\endgroup\$ – Agent_L Dec 21 '17 at 13:10
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First of all, a 3 x 1.5 cable should be made up of 3 cores, each having 1.5mm\$^2\$ cross-sectional area (and not 1.5 cm as you thought).

Yes, an extension cable with equal or more thickness than that of appliance's cable should be good to use. The extension cable should have equal or more current carrying capacity with respect to the appliance's wire.

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Assuming home scenario, that is, extensions not longer than few meters

The rule is not "thicker" alone. It's "same or thicker".

So you should not use 0.5mm² Christmas lights extension for a 4kW heater that comes with 2.5mm² cord. But using exactly same cable is ok.

The cable "thickness" refers to thickness of copper wires inside. External thickness is irrelevant, it can be 0.5mm² of copper in 2cm of insulation or 2.5mm² of copper in 1cm of insulation. The thickness of insulation determines how the cable is resistant to mechanical wear (that's why garden extensions appear to be ridiculously thick), to determine the current carrying capacity you need to read the small print on the side of the cable.

Cable alone doesn't determine everything, you also need to read ratings of plug and socket too.

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    \$\begingroup\$ You are wrong, for a short extension cable, the same size may do, but for a very long extension cable, a thicker cable may be necessary to avoid too much voltage drop and to maintain short circuit protection by the circuit breaker. \$\endgroup\$ – Uwe Dec 20 '17 at 13:02
  • \$\begingroup\$ @Uwe Well, the "very long" needs it's own protection, as you've said already. I've ignored things that long, because that's not typical use scenario that OP asked for. But thank you, I will mention it. When you extend dozens of meters, merely "thicker than original" won't do either, you need to calculate the drop and short circuit current. \$\endgroup\$ – Agent_L Dec 20 '17 at 14:47
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    \$\begingroup\$ At least here in the US, where 50 foot (~15m) extension cords are common, length is indeed an issue. A 50ft 16AWG cord will induce a ~5% voltage drop when powering a 120VAC @ 15A appliance... if I'm doing my math correctly, that's slightly under 2 watts of heat per foot of cord (6V drop * 15A = 90W, divided by 50ft), which ain't great. Oh, and these 50 foot cords come with no built-in protection. \$\endgroup\$ – Doktor J Dec 20 '17 at 15:36
  • \$\begingroup\$ @Uwe After rethinking, your interpretation doesn't really hold. One can take 0.25mm² Christmas lights, extend it with 200m of 0.5mm² extension (which satisfies your interpretation). That's 17A of short-circuit current on the wires alone, which flies below typical 25A breaker. So, this rule of thumb alone is not sufficient to retain protection. For a very long cable, you need some math to find out how thick cable you need and it depends on size of the breaker more than on size of the load. IMHO that supports my interpretation that it's meant to merely prevent overloading of extension. \$\endgroup\$ – Agent_L Dec 21 '17 at 13:05

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