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Some commenters on Friday’s post about using a washer as a soldering aid noticed my sloppy splicing technique and were kind enough to educate me about the so-called “Western Union splice,” aka the “Lineman’s splice,” which is the preferred method for twisting solid-core wire leads together for inline electrical connections.

Developed during the heydey of the telegraph, the Lineman’s splice is designed for connections that will be under tension. It is commonly claimed that, properly made, a Lineman’s splice is stronger than the wires of which it is composed. In any case, it is a time-proven method, and, coolest of all, one of NASA’s Required Workmanship Standards. To wit, in a NASA-approved Lineman’s splice:

  1. The conductors shall be pre-tinned.
  2. There shall be at least 3 turns around each conductor and the wraps shall be tight with no gaps between adjacent turns.
  3. The wraps shall not overlap and the ends of the wrap shall be trimmed flush prior to soldering to prevent protruding ends.
  4. Conductors shall not overlap the insulation of the other wire.

Though the Lineman’s splice was originally used without solder, today soldering is common. And NASA insists on it:

  1. Solder shall wet all elements of the connection.
  2. The solder shall fillet between connection elements over the complete periphery
    of the connection.

This material comes from page 84 of NASA-STD 8739.4, which is a great reference if you’re interested in best practices for interconnecting cables and wires. [Thanks, Alex Barclay!]

NASA-STD 8739.4 (PDF)

Sean Michael Ragan

I am descended from 5,000 generations of tool-using primates. Also, I went to college and stuff. I write for MAKE, serve as Technical Editor for MAKE magazine, and develop original DIY content for Make: Projects.


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Comments

  1. jamesbx says:

    I read that NASA document years ago: it got me started using clear heat shrink tubing. Just have to remind myself to slide it on the wire before soldering.

    1. Travis says:

      I’ve made that mistake myself.

      I started using this method when doing auto electronics, but before I used heat shrink. It made insulating the wires with electrical tape much easier. Before, my tape would always slide off the end of the splice exposing the wires. But with this method, you can wrap each wire, and then bundle the whole group together and wrap the whole bundle neatly.

      I also found that if you are good at stripping the insulation off of the middle of a wire, this is a good method for making taps as well. Also you can splice more than 2 wires together by coiling 2 wires at the same time (I doubt that is up to NASA standards though).

  2. Avi Drissman says:

    What a badly-drawn picture. The wrappings on each side somehow reverse direction. They can’t end that way.

    1. jmin says:

      I’m glad I wasn’t the only one to notice.

      This must actually be an Escher splice.

      1. rocketguy1701 says:

        Ahh good, I’m not having a stroke after all…

      2. Shadyman says:

        [neo] Whoa. [/neo]

    2. Robotblog says:

      I spent about two minutes staring at this, wondering what the heck was going on :P

    3. firstpres says:

      Really? You get all this good information and despite the visual typo, you still understood what was going on and despite all of the authors work, you had to nit pick? Just…wow.

    4. chb says:

      is this really the point?? its a fault yes – but you know how to do it :)!

  3. Mike says:

    Very M.C. Esher-esque.

    For those of us who are visual learners, these images are confusing. It would be difficult for those wire ends to terminate the way they do in the images. It looks like they reverse direction halfway through the twists.

    1. jeonlab says:

      Right. It’s like an optical illusion. The end of the right hand side of the twisted wire goes opposite direction. Interesting discovery!

      1. jeonlab says:

        actually both sides.

    2. Actually, that drawing does show the correct wrapping direction. It is used for universal-law independent circuits like this one.

      1. darr247 says:

        Ha ha ha. Wiring up components that differ from what’s on the schematic is like ‘magic’, huh.

        1. Actually, it is exactly like magic; stage magic, to be precise. He spent a lot of time preparing props that, when connected together, created an illusion that was ‘impossible’. (FWIW, he also fully explained how he did it.)

      2. tommy says:

        agreed… one twist, 4 wraps per side, solder… on to electronics

        1. tommy says:

          arrug… forgot the shrink wrap

  4. A video of someone with skill doing this would help to understand how you do the wrappings efficiently and cleanly. How does this work with stranded wire or is that a different standard?

    1. rocketguy1701 says:

      There’s a whole category of when to use solid or stranded wire. I think you could use this splice if you had to with stranded, particularly if you make sure to twist the strands in the right direction (so they don’t loosen under tension).

      That said, I wouldn’t recommend using stranded in this application, for several reasons, but the one that leaps to the top is that a soldered connection subject to any sort of twisting or vibration will fatigue the strands at the end of the solder, leading to failure. Solid core is more resistant to that, if properly sized with reason.

      You can tin a stranded cable into a solid unit to avoid that at the splice, but you still have a solder interface farther away. So that only works if you’ve got either strain relief or no real strain in the first place (neither seems likely if you’re needing to use this splice). It’s less the tension, but the inevitable vibration that will slowly kill it.

      But then, I’m thinking in the mindset of the “it must not fail” school of engineering here. So YMMV.

    2. It may not be a standard, but what I’ve always done with stranded wire was to spread the strands on both ends a bit, slip the two ‘tufts’ together (end to end) until the ends of the strands of one are almost touching the insulation of the other (trying to make sure that all of the strands are interspersed evenly), twist the whole thing until it is solid (pre-twist the wires in the opposite direction before joining to make the soldered connection lay flat), then solder into a solid unit. I’ve found that, in a pinch, if you strip off a lot of insulation, you can often get away with not using solder. Also, if you slice along the insulation instead of cutting it off, then wrap it back around the soldered joint, it adds mechanical support.

    3. darr247 says:

      The way to do it with stranded would be to separate all the strands so you can tin them individually, then twist them back together, do the splice, and when you finish with the final solder it will be the same as a solid wire splice, with no copper exposed anywhere in the connection to oxidize.

  5. johngineer says:

    At one of my old jobs I would have to do this type of splice on occasion. Handy tip: keep a thimble in your toolbox — it helps you avoid accidentally stabbing your fingers with the end of the wire.

    1. David Dean says:

      Interesting idea. I might throw one in the toolbox for smoothing down the ends of a wrap.

    2. arigger says:

      Hmm, my fingers are too fat for most thimbles… what about a copper end-cap?

      1. johngineer says:

        In a pinch I’ve used the metal cap from a whiskey bottle too :)

  6. Paul Wilson says:

    That drawing could use some correction. The direction of the turns does not match the orientation of the ends.

  7. rocketguy1701 says:

    Nice standards document! I’m not going to apply all of it to any of my projects(overkill, even for me), but some elements of it are very informative of what to be thinking about when doing a wiring harness (in my case likely an aircraft wiring harness which is a bit more critical than my other projects).

  8. This is based on a splice invented by Western Union to splice telegraph wires. Recall that in the 1800s there was the bare minimum of tools available. The splice was designed to not only be secure electrically but also mechanically. In addition the same cathodic affects that prevent corrosion in Wire-wrap assembly also protect this splice from corrosion.

  9. eric's tinger says:

    I still hate doing it with smaller wires…anything smaller than 20awg is just too much for my toe-fingers

  10. FWIW, self-fusing silicone tape (e.g. F4 Tape, Rescue Tape, Miracle Wrap, etc.) makes high strength, well insulating, waterproof seals. To make sure that you are getting the good stuff (-65° C to 260° C, 700 PSI, 85 ppi, 400 v/mil (8,000 Volts/20mil)), check to see if it matches mil spec CID A-A-59163 and/or MIL-I-46862 Rev. C 02-98.

  11. Travis says:

    One of the first things I learned (or was at least taught) in soldering, especially in wire work. Was to always make a good solid mechanical connection first. The solder is there only for electrical connection.

    I have also found that this method puts less strain on the wire than when they are just twisted together, and is less likely to break.

    I have to admit, I still use less dependable connections when testing that are easier to disassemble.

  12. Jeff S. says:

    This is what I was taught in high school electronics, and still use to this day.

  13. Walt says:

    The example is good and it demonstrates a very strong splice. Thanks for making a note of it.

    There is a problem in your diagram at the far right end of the wire on the final twist. The direction of the wrap defies logic and physics unless you are like me, an admirer of art of M.C. Escher. Nevertheless, the concept is clear. It is fine as it is for us g-jobbers.

    Does that bug in the drawing really matter that much? Consider that a NASA or military SOP (standard operating procedure) may include or reference a picture or diagram similar to the one above for use by factory assembly staff and test/repair technicians. If the diagram above were included, that M.C. Escher twist on the final turn at the right end would become an expensive issue during an audit should verification or certification of a splice be required. QA inspectors in the DLA (Defense Logistics Agency) are required to compare an observation made during an inspection with a requirement of a standard. And as crazy as it sounds, that splice would be sited as defective and formally documented as a condition of noncompliance. Even if that inspector is intelligent (some aren’t) and realizes that the drawing has a bug, there is still a discrepancy that they will document. It is sad. I’ve been there. I’ve seen it happen. (Then there are all the stop-work forms, change authorizations, engineering analyses, change notices, re-certifications and so on once the problem is addressed . . .)

    (Man am I glad I don’t work in defense electronics anymore. :-\ )

    Anyway, your article will help others to make better repairs, even if they aren’t Escher fans. Thanks for publishing it.

  14. Is it just me, or are the ends in the 2nd picture the wrong way around?

    From the first picture and looking from center out on the 2nd picture, you see the wire coiling over the front from bottom to top. But the end is in the BACK from bottom to top.

  15. It’s easier to make this splice if you cross the wires at a 90 degree angle, with the insulated parts below and the stripped ends pointing up. Leave 2/3 of the stripped part above where it crosses. As the stripped part wraps around it gets much shorter so you need more length to wrap around the straight part. I had to do two of these today to repair a damaged power cord on a commercial wetvac. Solder and heat-shrink finished the job.

  16. aaron says:

    just looks like the farmer splice we use on pulse wire.

  17. Peter Barvoets says:

    Although not a “western union” splice, I always found the technique demonstrated here in the movie “The Train” by Burt Lancaster interesting.. http://youtu.be/Z8j9h4XYVTs

  18. 633squadron says:

    I usually splice stranded wire with Posi-Locks, or I use bullet connectors with shrink-wrap.

  19. No wonder it took NASA so long to get to the moon… Lol

  20. Brian says:

    I recall someone (a post doc I worked with?) saying that the only way to get any work done on the scientific satellite their group was making was to come in at night while the NASA auditors weren’t around.

    “Is that the correct Philips-head screwdriver for that screw?”

    Might just have been a grad student war story, though.

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  23. Superpants says:

    There is a whole document on aircraft wiring that gives loads of other useful tips;

    http://casa.gov.au/wcmswr/_assets/main/rules/1998casr/021/021c99.pdf

  24. [...] This is a simple skill, but it looks like it would be a good one to pick up. Anyone know of some video of someone doing this?http://blog.makezine.com/2012/02/28/how-to-splice-wire-to-nasa-standards/ [...]

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  26. Bryan says:

    I learned to solder from a NASA instructor but not for certification. Learning to solder wires together tool like 6 weeks. Yes, this is one of the joints we learned

    The interesting part is why. Early on NASA was very unsuccessful in launching rockets. The failures were traced to “drum roll” …bad solder joints.

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  28. Thanks for the info, interesting idea.

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