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My 18AWG stranded wire continues to come out of the terminal block it's screwed into. What are some ways that I can more reliably secure this in an industrial environment?

  • Strip extra wire and loop/fill more of the chamber before fastening down?
  • Twist strays and then tin the ends of each wire tip before fastening down? How much of a solder blob are we talking about?
  • Twist down harder? (I'm worried about cracking the solder and/or ripping off the terminal block)
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  • \$\begingroup\$ Are you tightening it to the manufacturer specified torque? Alternatively you could try a spring clamp terminal block. \$\endgroup\$ – Tyler Sep 24 '16 at 15:19
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    \$\begingroup\$ Are wire-ended ferrules an acceptable solution ? \$\endgroup\$ – K. Rmth Sep 24 '16 at 15:25
  • \$\begingroup\$ Have you seen Tinning wires that will be screwed in to a chocolate block/terminal strip? \$\endgroup\$ – Andrew Morton Sep 24 '16 at 15:25
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    \$\begingroup\$ Don't pre-tin. Solder cold flows and your connection will become loose over time. \$\endgroup\$ – D-on Sep 24 '16 at 15:26
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    \$\begingroup\$ You MUST NOT solder tin the whole portion that goes ino the connector. Tinning the VERY END is OK to stop it unravelling. | \$\endgroup\$ – Russell McMahon Sep 24 '16 at 17:43
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(This is info already mentioned in the comments on the question — but nobody wrote an actual answer, so I'm doing that. It does also agree with my small experience.)

Do not use any solder at all. Use a wire ferrule, which is a metal sleeve that slides over the (straight) strands and is crimped in place, making a solid end that can be clamped securely by a screw terminal of the type you are using.

Caveats:

  • Use the proper crimp tool, which will leave a textured surface that is readily gripped by the terminal and resists sliding out more.

  • Adding the ferrule will increase the size of the wire end, possibly too large to fit in the screw terminal. (This should not be a problem if one of the options you have considered is doubling over the wire.)

This picture shows several sizes of ferrules, two ferrules crimped on 22-gauge stranded wire, and the crimp tool I used ($21 when I bought it).

The plastic part of the ferrule guides the strands in, covers any exposed metal, provides some strain relief, and identifies the size of the ferrule. Caution: ferrule makers do not all use the same color scheme!

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  • \$\begingroup\$ Kevin gave the correct answer. When done like that the connection will not give problems. Beware however in overthightning the connection. We are discussing copper connections that starts flowing under to high pressure \$\endgroup\$ – Decapod Sep 24 '16 at 18:02
  • \$\begingroup\$ I once saw someone crimping the ferrules using cutting pliers ( the tip of the pliers exactly, to serrate the metal part of the ferrule), he never used crimping pliers. Dunno if its a method good enough for the black ferrules (small diameter wire). \$\endgroup\$ – K. Rmth Sep 24 '16 at 18:27
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Twist it together and double it back on itself. That is, strip twice the terminal depth, twist and bend over. Ensure that the wire then approximately fills the screw terminal. If it doesn't, you need a smaller terminal. There should not be any bare wire (stripped insulation) beyond the terminal itself. Good termination practise should be second nature.

Bad terminations may well initially work, but are liable to fail, heat up and perhaps cause fires. A good termination will last a lifetime.

Basic electrician termination training, this. At least it was when I did it about 30 years ago :)

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    \$\begingroup\$ This answer as well as using ferrules are both correct. I chose ferrules for this application where cost is less of an issue, and because it showed me a new part I didn't know existed. Most the time I will use this method going forward. \$\endgroup\$ – tarabyte Sep 25 '16 at 19:57
  • \$\begingroup\$ I did this because I had to get my application running quickly and Lowes didn't have ferrules. :O \$\endgroup\$ – neuronet Dec 12 '18 at 16:30
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You MUST NOT solder tin the whole portion that goes ino the connector. Tinning the VERY END is OK to stop it unravelling. See my prior answer on this here

A method of "strain relief" that works well but which is "very naughty" and which people will criticise is:

  • Strip wire several times longer than depth of terminal block hole.
  • Bend wire back over outside of insulation and wind it in a spiral so it wraps completely around the outside of the insulation several times and extends back by about the depth of the terminal block hole.
  • Insert wire into terminal block and screw down clamp or screw onto combined wire end + insulated end.

The screw or clamp both makes contact with the conductor and also clamps the insulation rather than just the conductor. I'll add a diagram if this does not get run out of town on a rail.

I was shown this method decades ago by a man who designed/built/installed taxi meters in taxi fleet cars. I tried it. It worked.

This method is contrary to a number of things you'll have been taught. It does not seem to be an utterly terrible idea in theory and proves to be a good one in practice.


A more conventional method is to either add a sleeve which can be clamped to the cable end or run the insulated part of the cable just before the outer ends under a clamp which is screwed down onto the outer.

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  • \$\begingroup\$ The problem with doubling the wire back over the insulation is that the insulation then severely limits the contact pressure that you can achieve, leading to the same problems as tinning/creepage. ... Unless you tighten down on it hard enough to achieve metal-to-metal contact anyway, in which case you negate all of the other alleged advantages of having the insulation clamped in the joint. \$\endgroup\$ – Dave Tweed Sep 24 '16 at 23:59
  • \$\begingroup\$ @DaveTweed I'm aware that the solution is nothing like what is usually recommended. If I'd heard of it being done "along the way" I'd have recommended against it. What happened was I assisted my friend for a while and was involved with taxi computer repairs. He had shown his people this method or terminating wires in std screw fastened connector blocks (no clamping bar). It seemed to work well enough that I used it and have done so since then where I had no superior / formal strain relief present. It does not seem to suffer from the 'setting' problems of soldered ends. I use a generous ... \$\endgroup\$ – Russell McMahon Sep 25 '16 at 4:54
  • \$\begingroup\$ ... lemgth of stripped wire that goes around the insulation several times so that when screwed down the screw tends to apply pressure to wire above and below the insulated part : Screw-bare wire- insulated wire - bare wire - far wall of connector. | ie much better than one wouldfeel one had any right to hope :-) \$\endgroup\$ – Russell McMahon Sep 25 '16 at 4:55
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It depends on if vibration exists and causes the wire to move how rugged the attachment must be. Normally a 5 to 10 lb. pull test is the criterion for any connection, whether it is twist wire clamped or crimp lug attached with proper two-stage crimp on insulation and conductor.

Even molded connector junctions can and often do fail due to designers under-estimating how much strain users actually apply to their DC power cords on laptops or iPads (I have had to replace mine often from grandkids and my frequent use).

If this describes your situation, then do not tin but neatly twist, shape (with tool or needle-nose or plastic barrier wall) and clamp under screw head for an air-tight pressure contact with sufficient but not excessive force to pass pull test. This will translate into a fixed screw torque level with calibrated production tools or training.Better terminal strips have barriers and crimp washer under screw head.

However in my own practice, I prefer a thin solder dip or coating so that it wicks under the insulation and a star washer. The solder wicking adds strain relief. The star washer exerts more contact pressure and the solder mitigates loose strands.

Careless loose strands can only be avoided by proxy inspection or a better design with crimped terminals and routine pull tests.

If rugged strain relief is needed to prevent frayed wire breakage, then additional cable plastic clamp strain relief is needed such that the stiffness on strain relief is about <5x stress/strain of the wire for a graduated relief. This can be a moulded jacket as well.

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  • \$\begingroup\$ This answer to another question says NOT to tin wires that are going into a screw terminal. Are you contradicting it, or is your "solder dip" different? \$\endgroup\$ – Andrew Spott Sep 24 '16 at 17:29
  • \$\begingroup\$ solder dip in a hot solder pot will fill the air gaps of twisted wires in contact with each other so that it is a very thin exterior coating,that will not compromise retention over time. The star washer also offers many redundant contacts. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Sep 24 '16 at 19:16
  • \$\begingroup\$ Ah, huh I hadn't heard of a hot solder pot before. \$\endgroup\$ – Andrew Spott Sep 24 '16 at 19:18
  • \$\begingroup\$ solder pots are used in product volume processing if you are just doing low volume, consider other solutions \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Sep 24 '16 at 19:19

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