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So I have a few extension leads all are 13A rated but the cable length is too short just 1 metre. It would be cheaper for me to rewire them than buying new extension leads.

I bought a reel of the correct cable rated for 13A and have rewired one. However just have a query in regards to the wiring itself at the plug end.

enter image description here

In regards to the wires in the plug end they were previously "crimped" with just a small piece of metal (the wire wasn't left bare) if that's the right term. I didn't want to leave the wire bare either and to ensure there is a good connection, so I soldered the wire ends and then was able to nicely secure them with the screw.

However I am now wondering whether there is any safety hazard to soldering the end of the wires like that at the plug end? In terms of potential heat leading the solder to melt?

May be a silly question but it just cropped up in my mind so better to ask.


After reading and considering all replies, unfortunately I was unable to find small enough ferrules in standard stores even electrical ones, not sure whether that's mainly a US thing but it doesn't seem to be common in UK? They only had wire connector types but nothing as small to be used for the entry point holes for the wire on the plug. Sure could order online on eBay/Amazon but would be unsure of the quality.

So I left them bare rather than tinned with solder as seems like the safest alternative. Strands were secured properly. There won't be any pulling on the cable anyway as it is left stationary behind a desk.

Just to add the photo is a generic plug from Google. I done a much better job with the wire lengths (or so I hope!), as the question occurred to me after I had already soldered and been using the plug for a day or two.

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    \$\begingroup\$ The "small piece of metal on the end" is likely a ferrule. \$\endgroup\$
    – JYelton
    Jul 21, 2023 at 21:24
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    \$\begingroup\$ Almost certainly a ferrule since those terminals with the screw grinding straight into the wire are in my experience notorious for individual strands getting pushed to the side where they're not secured mechanically and for breaking strands clean off. Ferrules are the correct choice here; solder might "work" but isn't strictly recommended for those types of terminals either. \$\endgroup\$
    – vir
    Jul 21, 2023 at 21:42
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    \$\begingroup\$ Using solder is worse than clamping the ends as they are, as solder cold flows. \$\endgroup\$
    – Neil_UK
    Jul 22, 2023 at 5:12
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    \$\begingroup\$ No, it's a good question. I used to scoff at these kinds of "little details", figuring they didn't matter and weren't worth learning about - when you look at the circuit diagram they are all just lines, right? how hard can they be? - but actually, learning more and proper ways to connect things has changed my electronics life. (I don't work in electronics, just do it occasionally as a hobby) \$\endgroup\$
    – user253751
    Jul 22, 2023 at 7:09
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    \$\begingroup\$ From that photograph, your neutral (blue) wire is too long - the earth (green and yellow) wire should have most slack by far, so that if the wire is yanked out of the plug, the earth disconnects last. \$\endgroup\$ Jul 22, 2023 at 16:36

4 Answers 4

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Basically, this is in agreement with everybody else.

What you had were either uninsulated ferrules which are tubular, or a small piece of brass etc. crimped on to do the same job. These are basically the same as the aglets that you get on the end of shoelaces: they stop things fraying.

Using a ferrule is good practice whenever a screw connection is to be made, since they prevent loose strands from causing problems and- if used correctly- keep the strands parallel which is conducive to a good contact. While professionally they're applied using a crimp tool they don't have to be: a light squish with pliers to keep them in place after which, since they're very thin-walled, they will be deformed by the screws.

Ferrules, both insulated an uninsulated, are used heavily in control panel wiring and in some cases are mandated by industry standards. In the case of insulated ferrules the colour indicates the size, but there are at least two standards for this.

enter image description here

Photo from wikipedia (CC BY-SA 4.0)

Using solder to tin a wire which will then be held by a pressure connector (i.e. screw, clip etc.) is very bad news since the solder will creep with time, increasing the resistance of the joint and creating a fire risk. In addition, if the solder wicks up into the insulation it will damage it, incidentally stiffening the wire.


Considering @Ariser's comment: I wonder whether a distinction has to be made between "soldered" (as in "tin/lead alloy soaked in between the strands") and "tinned" (as in "thin layer of pure tin on individual strands")? I think the bottom line is that it's possible to do the maths (and verify by testing) for something like uncontaminated strands laid straight, but once there's an indeterminate soft filler or strands are haphazardly crossed all bets are off. Hence things like the venerable https://workmanship.nasa.gov/lib/insp/2%20books/frameset.html : NASA engineers calculated what performance was attainable, then told technicians how to work in order to best approach that performance.

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    \$\begingroup\$ Solder on stranded wires was used up to the 60s here. Then the use of solder was prohibited for this purpose. I have seen numerous badly burnt distribution boxes where the undoubted origin of the fire were tinned stranded wires. \$\endgroup\$
    – Ariser
    Jul 24, 2023 at 9:54
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    \$\begingroup\$ @Ariser Fascinating turn-around. My Dad (ex WWII RAF radar tech) taught us all to tin stranded wires before clamping. Then I gradually noticing crimp connectors with "do NOT apply solder..." instructions, which I sort of ignored at first because I knew better, until I learned about the hazards! Wow. \$\endgroup\$
    – SusanW
    Jul 24, 2023 at 12:01
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    \$\begingroup\$ @SusanW I agree. OTOH, the vast increase in the number of electrical connections and the timescale over which we have been able to observe their behaviour has increased enormously. Also the fact that we now have methodical small-signal engineering that allows us to track micro-Ohm degradation... and for that matter that we'd even consider using screw terminals for small-signal work that a few decades ago would have been soldered as routine. \$\endgroup\$ Jul 24, 2023 at 12:09
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Solder cold flows so it can work loose and cause problems with overheating, charring etc. It's worse than leaving the wires as-is.

If you don't have ferrules, leaving them as-is would be a better option. Looks like they have a floating clamp plate (no! see below) so they should hold reasonably well so long as you get the strain relief clamping right (when you wiggle the cord, the wires inside should not move at all).

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  • \$\begingroup\$ "Looks like they have a floating clamp plate" - as a user of plugs as shown in the question, I can assure you that that particular one does not. It's just a screw holding the wire in a round hole. \$\endgroup\$ Jul 24, 2023 at 19:27
  • \$\begingroup\$ @AndrewMorton Ah, I see. So the screw tip bears directly on the stranded wire. Not good, a ferrule would be considerably better. \$\endgroup\$ Jul 24, 2023 at 19:31
  • \$\begingroup\$ Exactly. At least the end of the screw is usually flat. Now that I have (a cheap) one of those square-hole crimpers and a load of ferrules, it even seems to save time to add a ferrule to a wire - it's so much easier to get it into the connector properly and you're sure there are no splayed conductors. @OP If the ferrule is too long, you can cut it shorter. \$\endgroup\$ Jul 24, 2023 at 19:41
  • \$\begingroup\$ @AndrewMorton Might have to cut off the insulator on the typical ferrules for the brown wire, or maybe just bend it over. I have a square ratchet crimping set from the usual consumer sources that was not expensive, and it came with good supply of different size ferrules of acceptable quality. \$\endgroup\$ Jul 24, 2023 at 19:48
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    \$\begingroup\$ I edited to include a link to the creep/cold flow definition; hope you don't mind. \$\endgroup\$
    – JYelton
    Aug 11, 2023 at 20:12
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Solder makes the copper strands brittle and the flux may oxidize it.

It is not recommended to solder the wire ends. It likely ends up in loose connection under the screw which is a problem. This happens due to repeated thermal expansion and cooling, as you have got materials with different thermal expansion coefficients.

It is also not recommended to leave the strands as is, as tightening them under a screw will deform the strands and may make uneven connection with hotspots.

Best option (at least after those two) is to terminate the wire with crimp ferrules like in the original mains cables.

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The original ends were likely ferrules crimped onto the ends of the wire which keep the copper from fraying and breaking.

I have connected many wires to screw terminals by "tinning" (applying solder to) the stripped ends of copper wire. Too much solder and you wind up with a rigid blob that doesn't secure well in the terminal, so one should apply just enough solder to keep the copper from fraying and separating.

If the connection generates enough heat to melt the solder, you will have bigger problems. In other words, don't worry about the use of solder versus ferrules.

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    \$\begingroup\$ But if the connection heats up enough to melt the solder on the strands, the usual cause for the heating is that the strands were soldered to begin with. It may be that excess solder is bad and just small amount is OK, but generally, to avoid problems caused by incorrect soldering, the wires should not be soldered at all. \$\endgroup\$
    – Justme
    Jul 21, 2023 at 22:11
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    \$\begingroup\$ Fair enough. I'll leave this here as a cautionary tale. Also, I have a few connections to go remove soldered ends from... :) \$\endgroup\$
    – JYelton
    Jul 21, 2023 at 23:11
  • \$\begingroup\$ Under fault conditions (short circuits downstream), the temperature of the cable can rise significantly before the fuse/breaker opens, and this is potentially even worse at terminations - solder is a much worse conductor than copper or brass, after all. Standards allow cables to reach 250C or more (ASNZS 3008 for X-90 or better insulation) under fault conditions when they would normally be only 60-90C. \$\endgroup\$ Jul 24, 2023 at 11:26
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    \$\begingroup\$ This is in addition to the threat of cold flow (the solder flows away from the joint under the pressure, leading to it coming loose over time), and fatigue at the joint, because flexing is concentrated at the point where the rigid solder meets the flexible copper, rather than being spread out. \$\endgroup\$ Jul 24, 2023 at 11:27

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