Splice Cable Like a Rocket Scientist





Introduction: Splice Cable Like a Rocket Scientist

About: I've been taking things apart since I was 10. My mother wasn't impressed, even though I told her I knew how to put it back together... I've been making things since I picked up my first soldering iron (By Th...

Rocket Science without the expensive college education!

Often, in the world of electronic experimentation or repair, there's a need to splice a wire or a 2-wire cable.

How many times have you seen a wire splice where the wires are sloppily twisted together and insulated with electrical tape, or worse, yet, masking tape?? A Plague Upon such slipshod methods! Don't even go there. Such practices will only result in heartache down the road.

Do you want to make a strong, reliable, secure, neat connection in a 2-wire (or more) cable? This is not that hard. Despite the title, you don't have to be a Rocket Scientist to make a neat wire splice and do it right.

The best splices are soldered, so knowing how to solder is essential in electronics. If you need to learn the basics, I suggest my Instructable on soldering.

The kind of splice we're going to use is a modified Western Union or Lineman splice. I find the modified version easier to make. NASA's preferred method is the standard lineman splice, which, unlike ordinary twisted splices, has a very high tensile strength.

While my method would not meet NASA standards, it uses less wire and is shorter than a full lineman splice.

Step 1: Tools Needed

All you need to make a professional wire splice is:

  • Wire cutters and strippers
  • Needle-nose pliers
  • A soldering iron and solder
  • Heat-shrink tubing (Various sizes). You can get this at Radio Shack or almost any store carrying electrical parts.
  • Match, lighter, or heat gun to shrink tubing
  • A few minutes of time

If you've done any kind of electrical or electronic work, you probably have all of these already.

Step 2: Prepare the Wire Ends

The important thing to do when splicing a multi-wire cable is to stagger the splices of the individual wires. This accomplishes two things: It reduces the possibility of a short circuit if your insulation method fails, and reduces the bulk of the splice so you don't have a huge unsightly glob in the cable (also making it easier to thread the cable through holes if you need to).

Let's get started. I'm using a laptop power cord as an example. If the cables you're splicing have a jacket, gently strip about 1 1/2 inches of the jacket on each cable end, being careful not to nick the inner insulation. The wires here are color-coded, so we'll be joining white to white, and red to red. On one cable end, cut one of the inner wires about 1/2" shorter than the other. Strip 1/2" from both wires. (These dimensions will vary depending on the thickness of the wires you'll be joining. 1/2" - 5/8" works well for small gauge wires.)

Now, on the other end, cut 1/2" off of the opposite wire - In other words, if you cut 1/2" off of the red wire on the first end, cut 1/2" off the white wire on the second end. Twist all the strands together tightly. When you are done, the wires should look like the second photo.

If splicing a cable containing more than two wires, stagger all the lengths by a similar amount. Of course, this will increase the total length of the splice.

The NASA method calls for the wires to be tinned (Coated with solder) before splicing. While this will probably improve the neatness of the splice, it makes the wire much stiffer and harder to bend and twist, so I did not do this on my demonstration project.

Step 3: Prepare Shrink Tubing

Heat-shrink tubing has got to be the greatest electronic invention since the transistor: Tight seal, conforms to irregular shapes, no gooey mess as one sometimes gets from tape, and more permanent than tape. If you want to make splices like a professional, this is what you want.

We are going to use two different sizes of heat shrink tubing: First, a small size to insulate the individual splices, then a larger size to cover and protect the whole repair. Choose shrink tubing the smallest diameter that will fit over your intended area, and somewhat longer than the splice. For the final jacket, I cut the tubing at least an inch longer than the total length of the spliced area. Put the large piece over the jacket and slide it several inches away from the splice area.

Put the short pieces over the longest individual wires; the object here is to get them as far away from the soldering as possible so they don't shrink prematurely.

Step 4: Splice and Solder

Now we can splice. You did remember to slip your heat shrink tubing on already, didn't you? This is the most common mistake that happens to everyone sooner or later... You just made a beautiful splice suitable for framing, and now you discover you left off the shrink tubing! Arrgh! So double-check you've got the tubing on first.

Bend each wire into a U shape and hook the mating wires together. Then twist both wires tightly around themselves, taking care not to leave any ends sticking out that could puncture the insulation.

Now solder the splice, working quickly to keep the heat away from the shrink tubing. Start the solder joint as far away from the tubing as you can. If you run into problems with the tubing shrinking prematurely, you can re-strip the wires to a slightly longer length, use a hemostat or locking tweezers as a heat sink between the soldering and the tubing, or try a wet rag as a heat sink.

Step 5: Insulate

Did your shrink tubing accidentally shrink? Dang it! You'll have to re-do that connection. If it didn't, though, you're almost done.

Now slide the small shrink tubing over the solder joint so that it is roughly centered on the joint. Shrink it down with a match, lighter, or heat gun (Hair dryers may not get hot enough). Whatever heat source you use, keep it constantly moving - you can watch the tubing shrink and remove the heat as soon as it's shrunk down. Don't overheat the tubing - It can melt if you get carried away. Do this for all the wires you spliced.

Now slide the large piece of shrink tubing over the completed splice, overlapping the jacket ends on both sides. This will add some strength and durability to the whole assembly, so I like to make sure the shrink tubing overlaps the cable jacket by at least 1/2" on both ends. Shrink this down, and bask in the knowledge that you made a professional, rocket-worthy cable splice!

The NASA document on splicing is 140 pages long, and, unless you're building an aerospace vehicle, you don't have to be quite as fussy. My splices are strong, reliable, and neat, and no one will die if they fail.

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2 Tips

Many stores (such as Walmart, Ace, Home Depot, Lowe's, etc) sell "Liquid Electrical Tape" in a small, black & yellow plastic jar with a brush-top that's very similar in size to the cans of PVC cement. I don't generally use corner-cutting stuff like that, but I bought it once to try to coat some bare spots on some old, cloth-jacket wires that I couldn't replace, and I was AMAZED at the stuff - it's a WONDERFUL sealant AND adhesive, with a consistency similar to the Flex-Seal products...
Anyway, I like to brush on a coat of it just before sliding & shrinking the heat shrink - as it shrinks, the liquid tape oozes out each end & gives me the "warm & fuzzy" feeling that it's completely weatherproof-sealed, similar to how the much more expensive "weatherproof" crimp connectors operate...
Excellent illustration; thanks for sharing! :-)

Yes, the liquid tape is great stuff and useful in the rare cases shrink tubing won't work. I've also waterproofed connections with a coating of hot-melt glue under heat shrink; same principle applies, when glue oozes out, it's sealed.



I train and certify NASA-level solder operators. Unless you can supply current source documentation to prove otherwise, I'm sorry, but this isn't the way NASA splices wires. Twisting conductors like this (called a lineman's splice) weakens the wire, though it does offer a good mechanical joint to constrain the wires prior to solder application.

The generally accepted method of two-wire, similar gage splicing uses lap joints, where the opposing conductors are placed end to end against each other, with the insulation one to two wire diameters from the end of the opposite wire conductor.

With the proper amount of flux and solder, the (undisturbed) lay of the wire strands should be barely discernable. Add some polyolefin heat shrink, and you've got a solder joint that will survive environmental testing and extended duration spaceflight.


7 replies

To learn more about NASA requirements for inline, stranded wire splicing, Google "NASA-STD-8739.4A" and check out pages 67-69.


Thanks for the name of that standard. I learned soldering in the 70's while working for the DoD from technicians who had completed the NASA soldering training. The Lineman Splice documented in the standard (19.7) but is shown using solid (not stranded) wire. I imagine the difference is that in real-world (and spacecraft) applications, vibration is the kiss of death to solid wire.

I took the liberty of also checking page 70, where this exact splice (well, better executed; it's a bit more involved than simply twisting the wires around each other) is described. So it appears that NASA does in fact use both types of splice.

I guess the choice depends on various factors, such as the projected stresses (the lineman splice is actually stronger, not weaker), and whether the bond is intended to be permanent or temporary (undoing a lineman splice involves wire cutters).

Dead ccclarke, I've read corresponding pages of NASA-STD-8739.4A and I am confused.
Is the lap slice for fixed (motionless) wires?
Lap splice reliability critically depends on solder mechanical properties, isn't it?
At the same time wires twisted in any manner must be more strong.
I can't believe that NASA is incompetent so there is reason for using lap slice but not "usual twisted" slice, but I cant see it.

Do you have a preference of heat shrink other than polyolefin, like the 3M glue type? And who is a good supplier? A lot of the ebay stuff I run into is chinese knockoff stuff that doesnt hold up well.

Haven't used the glue type; I think it's for where the splice needs to be hermetically sealed.

If you make sure to get genuine 3M you'll get good quality. There are many large electronics suppliers such as Allied, Digi-Key, Mouser, and others that should have what you need.

My main idea here was to show people something better than "Twist the wires together and put electrical tape on them."

Although none of the splices on my rockets have ever failed.

You know what's even better (worse) than twisted wire and electrical tape? Crimp on connectors and electrical tape!

1 reply

However, crimp connectors - Properly crimped! - are recommended for some aircraft uses because solder is brittle in a high-vibration environment.

Properly crimped means a very expensive mil-spec crimping tool, not the cheapie from the hardware store.

This is not Rocket Science or NASA science. This is how Skilled Trades Electricians make splices when necessary. Where do you think NASA learned this?

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I hope NASA hires Skilled Trades Electricians to do it's wiring... Although 3 astronauts once died partly due to faulty wiring.

Partly...but mostly due to a combination of pure O2 and excessive placement of Velcro. Those fuzzy nylon pads go up very rapidly in pure O2.

A static spark could start a fire in pure O2.

That is true. It was an accident waiting to happen... Just like Challenger and Columbia. Shuttle astronaut Mike Mullane calls this "Normalization of Deviance," meaning if you get away with something a few times, you start to think of it as normal, and thus not dangerous. The Shuttle had both O-ring burn-throughs and foam shedding long before they lost the vehicles.

Mullane, on his last spaceflight, flew one of the post-Challenger flights, and when asked what his future plans were, said, "I have no plans past MECO."

Very nice tutorial on splicing.

Comments provide a great selection of other methods.

How you make your splice is highly dependent upon what you are splicing.

I've been known to insulate splices with Teflon plumbers tape, cellophane and odd bit of plastic wrapping in a pinch. Especially when you are working under performance deadlines and in the middle of no where.

Dependable? NO. Sufficient to get you through the night...usually. With sound equipment it's best, when possible to avoid splicing...tends to bite you later sometimes with very odd sound changes.

I worked backstage in the Mabel Tainter Memorial Theater, Menomonie WI growing up, the building was built when it was uncertain if electrical lighting or gas would stay around, so it had both. (Fire regulations made the gas lights illegal later...)

The original building was wired in the dual strand on ceramic posts, wires 2" apart and over an inch from their support. When a fuse would blow, it was common to short the connection across the fuse with wire..."If it gets a little hot, that's o.k."

Built as a replica of the Ford theater in Washington D.C. where Lincoln was shot, the theater underwent massive upgrading a few years ago.

It had been unused for many years and scheduled to be torn down around 1960, and my parents and a few others worked to save it.

Part of the restoration was widening the seats for modern patrons. (Yes, people were smaller in the 1890's.)


If you're near Minneapolis-St.Paul anytime, it's worth seeing.


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Glad you liked it! Neat story about the theater. My local "Makerspace" is in a 108-year-old building - A department store warehouse when downtown still had department stores - and it has some of that "ceramic post" wiring! (no longer in use... as far as I know.

You are right that sometimes one has to "McGyver" stuff just to get through the next performance, etc. My favorite term is "adapter stack."

Sometimes NASA stuff blows up! Might not be the best endorsement.

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That is because NASA flies prototypes. When it's a research project, you expect things to blow up on occasion, no matter how good your wiring is. Why do you think they installed escape towers on early spacecraft?

Both the Mercury and Gemini programs had special rocket assemblies on top of the capsules that were designed to be able to rocket the capsule away from the main rocket in case of a failure. They were never needed as all the catastrophic failures were of different origins including a fire in an enclosed capsule that was filled with pure oxygen. I don't remember if Apollo had the same rockets.

But, my comments were made in jest, as in humor.

The probability of something failing continues to escalate the more complex something becomes. It's a math thing. The probabilities of each component failing are added together. So lets say you build something and each part you use has a 1 percent chance of failure. If you have 10 parts in the build then the overall chance of failure is 10 percent. If you have 100 parts then the chance of failure is 100 percent. This is why they need to reduce the failure probability of each part by as much as possible and to make redundant systems. Because of its great complexity the Space Shuttle had a fairly high probability of failure. And that is what happened. So making every wire splice with the least probable chance of failure makes the overall reliability of a build much better. That is why it's important.

Yes the Apollo capsules had an escape rocket. In addition, there was a cable slide to evacuate before launch.