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When connecting a stranded wire to a solid wire in a wire nut (for example connecting a ceiling fan's internal wiring to the house wiring), I know that I should tightly twist the stranded wires and that they should lead the solid wire(s) by a little bit as I put them into the wire nut to ensure a tight connection.

I've seen many recommendations here, though, of using stranded wire as the house wiring (when pulled through conduit) with accompanying admonishments to ensuring that the stranded wires are "properly prepared" before clamping them down under a screw terminal (at an outlet, switch, or other) to ensure a safe and secure connection.

What's required to "properly prepare" these wires to make that safe connection?

I'd presume that after proper preparation, the stranded wire will look essentially like a solid wire would under the screw terminal. If that's an incorrect presumption, what should it look like?

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  • All the strands staying under the screw is the tough part. Twisting enough for good contact is just less than twisting too much and breaking some strands. Something like solder would be nice, but think that is a big no-no. One suggestion(from a question today) on using stranded wire was to wire nut a short piece of solid for under the screw.
    – crip659
    Feb 10, 2022 at 17:40
  • I saw that, @crip659, and that answer was what prompted me to ask this one. I've been wondering for a while and finally decided to pop the question.
    – FreeMan
    Feb 10, 2022 at 17:42
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    Avoid wirenut and use a wago instead
    – DDS
    Feb 11, 2022 at 11:10
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    @DDS I've just purchased my first boxes of Wago 221s and am eagerly looking forward to how much easier my next wiring project will be. However, this question is about preparing stranded wires for screw down connections. Stranded wires really aren't difficult to put into a wire nut, anyway.
    – FreeMan
    Feb 11, 2022 at 19:13
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    A word of caution: If you have devices with dual clamp-down rear terminals (not backstabs, but a screw-down plate with TWO rear entry holes per screw) ... do not mix solid and stranded under one screw. The screw will not be tight enough for the stranded wire.
    – jay613
    Oct 25, 2022 at 19:50

5 Answers 5

16

As a Brit who uses far more stranded wire than solid [I don't do first fixes, that's someone else's job], there are two ways to do it 'properly', but this can depend on how much room you have in your screw-down terminal.

  1. Proper-proper. Big terminals & junction boxes.
    Strip 15-20mm of the sheath; twist, fold in half, then insert & screw down.

  2. What actually happens. Small terminals & household plugs.
    Strip 8-10mm of sheath; twist, insert & screw down.

In each case there should be little to no 'spare' hanging out the far side of the terminal & no exposed wire on the nearside.
Stranded doesn't look like solid in a terminal after screw-down. It compresses & the exposed end flares - which is why you keep it short in small terminals. No spare, no flare.
Ideally, all your exposed portion fits exactly inside the terminal.

Pre-made cables often have copper/brass ferrules crimped onto the cable. Once you're cutting your own, you can't do this. I've seen cables soldered first - but having been in the position of re-doing a lot of these several years later, I'd say there is a fair chance the wire gets part-cut & compromised by doing this; I've nothing more than "I don't like it, it feels dodgy".
Bare wire 'just works' so long as you're not trying to share the terminal with more solid core than stranded. Stranded bunches up nicely & stays firm. Solid core can upset that if you have for instance 3 solids to 1 stranded in a big terminal; the stranded can be free to move in the gaps.
The recognised test is to see if you can move it once it's tightened down. [Empirical science FTW ;))
If not, you're good to go.

A note on twisting.
Some people like to leave the end of the sheath on, so they can quickly twist without getting spiked by spare bits of strand. I've never bothered. It takes too much time for very little difference in the end result. As I use an automatic stripper, this would just be far too much faff to bother with.
Plus, if your wire end is scrunched up enough to spike you as you twist it, you should have started with a clean end, not re-use the last one someone else made.


Notes prompted by answers elsewhere.
We don't use j-bends on terminals in the UK, ours are all straight-through. I've only ever seen them on hifi speaker terminals, where they're more faff with potential for inaccurate clamping than I'd ever like to see in a pro wiring job. There was a brief penchant for the j-type in some domestic plugs a decade ago - thankfully they seem to be gone now.
It seems that some territories in the EU mandate DIY slip-over ferrules which are then squeezed down before inserting into the terminal. I really can't comment on those, never seen one in action.


If you're going to be doing this a lot, I'd recommend a semi-automatic stripper. Set length, insert wire, squeeze. Perfect size every time, whatever the wire thickness.

Not one of this type - they need adjusting for thickness every single time. PITA.
[Had to use one of these as I didn't have my own kit with me to swap a slew of plugs at the in-laws last weekend, after they moved back to the UK from Spain… there was swearing after the 4th appliance with yet another different wire gauge;)

enter image description here

Something more like this [this isn't one I've ever used, but I can't find a pic of mine] Doesn't matter how large the wire diameter is, it 'just works'.

enter image description here

Tricks of the trade… [or old man's idle musings]
I used to some days swap 50 - 80 13A plugs a day [I worked in retail maintenance; multimedia, displays & small electricals, often converting EU imports to UK spec, on site, no workbench.]
I had a plug swap down to under two minutes ;)
Clip old plug off, strip outer, cut wires to three different lengths, strip ends & assemble. I worked "on a price" [paid by the task, not by the hour], but someone would QC us at random, so these all had to be good.

If you get a stripper, get one that will cut the whole wire first. Then your toolkit for this is a drill-driver, 'Stanley' [modelling knife, US?] & the stripper.
Check this gap, which must be big enough to cut the entire cable… cheap ones can't, good ones can.

enter image description here

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    Soldering anything that will get screwed down is a terrible idea: Solder is too soft for this and repeated heating cycles will let it gradually deform under the clamping force, compromising contact.
    – TooTea
    Feb 11, 2022 at 10:47
  • @TooTea - yup. I've seen it in factory-made cable/plugs I've had to change out. I just don't like it, it feels a bad idea even without any hard facts behind it.
    – Tetsujin
    Feb 11, 2022 at 10:53
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    @spikey_richie - LOL. I guess I've had a rich & varied career ;) From musician to sound engineer, record producer, a few years as an offset litho printer then off to IT & project management for a multinational musical instrument/electronics manufacturer. A decade as a part-time maintenance engineer. For the past decade I've worked in film/TV. Basically, for the past 20 years i've done just whatever the hell I like. There's still life in the old dog yet ;))
    – Tetsujin
    Feb 11, 2022 at 13:20
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    @spikey_richie Yeah, we'll have to start calling Tetsujin "Iron Man" ;)
    – Glen Yates
    Feb 11, 2022 at 15:34
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    @GlenYates - hehe… I see what you did there ;))
    – Tetsujin
    Feb 11, 2022 at 15:38
11

For screw terminal attachments, it's an acquired skill. You have to be self-demanding and refuse to walk away from a connection that isn't top shelf. If you continue to iterate on the problem, you will acquire the skill.

There are several ways to do it. I've seen people slide the insulation not all the way off the wire, so a short bit of insulation is at the end of the wire keeping it from fraying. Then twist good-n-plenty while shaping it into the J-hook.

I myself give myself a little extra length, and I twist hard, and then "overtwist" by grabbing the loose wire end in a tool to hold it. On my multi-tool, I stick the stranded end into the bolt shear (#6 or #8 size) and grip the tool enough to hold the wire, but not enough to shear it. (a bit of a trick, that). With the end firmly gripped, I over-twist while shaping the J-hook.

So yes, it's fidgety work.

It's not for everyone, and I wouldn't do it if I was in a hurry. It suits my style of wiring, which favors pigtailing anyway, where I can do those fidgety connections at a comfortable work bench. (and I used "domed covers" which favors that too). However, if pigtailing, you can just use solid wire for the pigtails, keeping the benefit of stranded wire everywhere else.

I have also used crimped spade or ring terminals in the rare instance I wire a ground wire. Switches and receptacles aren't made to take them, though, and you should never fully remove a screw from a switch or receptacle.

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  • For the benefit of the right-pondians… what's pigtailing?
    – Tetsujin
    Feb 11, 2022 at 11:04
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    @DanIsFiddlingByFirelight - Thanks. Not sure that's legal in the UK. It would mean you would have to try get a junction box inside your switch back-box… that's not going to fit; & using simple terminal strip in such a situation is frowned upon. [People do it, but you're not supposed to.] Traditionally, we don't use wire-nuts or spring [wago] connectors either, though that is changing.
    – Tetsujin
    Feb 11, 2022 at 13:03
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    Your answers are always thorough and well reasoned, but I have to disagree with part of your response. As I mention in my answer, wrapping the stranded wire around the screw should be reserved for "emergencies" only. The first thing any insurance adjuster will point to is unapproved work should the worst happen. Best to not go down that path unless absolutely necessary. Pigtails are fine though, presuming that the wire splice itself uses a locally approved method. Feb 11, 2022 at 17:03
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    @Tetsujin the difference is in North America we have a plethora of splice options (wire nuts, push connectors, lever-nuts, Alumiconns) and devices e.g. lamps often don't provide splice blocks, whereas in the UK your "loose connector" choices are limited but almost all devices come with highly competent lug-grade splice blocks nearly the grade of our Alumiconns (which no one uses if they can avoid it, due to cost). Feb 11, 2022 at 18:45
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    Harper - indeed - we are two countries separated by one language… & 4,372 building codes ;)
    – Tetsujin
    Feb 11, 2022 at 18:51
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I'm not really familiar with wire nuts, so this is probably not applicable to that situation. However, when it comes to ordinary screw terminals, the (continental) European answer is:

Just crimp a ferrule on.

All you need for that is a crimping tool (not too expensive and well worth buying if you play with wires now and then) and a set of ferrules for different wire diameters. (There are also double-ended ferrules for splicing wires.)

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    Small correction: To be up to code, the American users need to use an UL certified crimper with the matching ferrules from the same company. In the US the combination of crimper and ferrules got certified, not the ferrules alone.
    – Martin
    Feb 11, 2022 at 14:17
  • @Martin Does the crimper have to be regularly calibrated that it pushes hard enough as well (like torque wrenches), or is it implied by using the right type of crimper with the right type of ferrule that the connection should always be proper enough?
    – Mast
    Feb 11, 2022 at 22:38
  • @Mast not needed unless the manufacturer tells you otherwise. Most of those crimpers are quite simple, with a fixed die.
    – Martin
    Feb 12, 2022 at 17:03
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I also prefer the clamp plates, but try twisting backwards.

Most people are right-handed, and twist the strands with their right hand, away from their body (counter-clockwise). Many wires are also built with their strands lightly twisted this way, so a lot of times people just tighten up the existing twist. When wrapping around the typical screw though, it tightens by turning clockwise, so the strands get pulled in the opposite way that they are twisted, and you usually get several strands popping out and/or the end flattening. By twisting the strands "backward" (clockwise), when wrapping around the screw, the strands get pulled into the bundle and under the screw head, so you get fewer whiskers.

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There are two different "correct" answers, everything else is risky at best.

  1. If the switch/receptacle/whatever has a pressure plate under the screw such that the wire will be compressed between 2 flat surfaces: Strip just enough insulation such that the smallest amount of bare wire will be exposed on either side of the plates when the wire is pushed straight in. Certainly less than 1mm. Push the wire straight in under the plate, tighten the screw, move the wire around a bit and tighten again. Make sure the wire is compressed properly such that it will not pull out or rotate under the plates. Repeat the tighten/wiggle steps until you are confident the wire is secure. There's a bit of a touch here, make sure you don't over tighten and break the device, but you have to make sure the wire is secure.
  2. If you do not have pressure plates, then you need a ferrule as @TooTea suggests, or else another type of crimp connector.

Macgyver methods include:

  • Soldering the end so the wire effectively becomes solid. Problems with this: 1) the heat may damage a few millimeters of insulation next to the solder point, 2) the transition from "solid" to stranded will always be a strain point, and 3) this method will not pass any inspection.
  • Strip enough wire such that the bare part can wrap fully around the screw, then use the strippers to grab a little more insulation and slide it down until it almost pops off the end of the wire. This little bit of insulation will keep the stranded wire "organized" as it wraps around the screw. Problems with this: 1) the small bit of insulation doesn't always keep the wires organized, 2) it's admittedly a bit tricky to do this, you might find that you need two or three tries to get it right (and the wire gets a bit shorter at each pass), 3) this method will not pass any inspection.

I will freely admit that I've used the Macgyver method on occasion when I'm far from any supply houses and "it needs to work now". Best to not emulate this unless it's truly an emergency though.

[EDIT] Or you can pigtail the wire such that the connection is solid - as @Harper mentions.

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  • Sorry but your soldering trick is NOT a good idea. In the course of time (years) the solder will start disintegrating and cause all sorts of problems. Solder was never intended to take mechanical stress. See also @TooTea comment.
    – NL_Derek
    Feb 12, 2022 at 22:46
  • Gee, you suppose that's why I said you shouldn't do that except as an emergency? No - that can't be it... Feb 21, 2022 at 23:32

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