Splice Cable Like a Rocket Scientist

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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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312 Discussions

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ccclarke

7 days ago

So many issues with this tutorial . . .

I'm an IPC Soldering Instructor, and have built hardware to NASA level (and higher: NAVY 2-M) standards for decades. IPC was created to standardize all electronic workmanship training and inspection standards, (NASA, DoD, consumer) to conform to a single spec with three inspection levels (four if you count the Space Addendum) for each certification. There are multiple areas IPC covers, like soldering, wire and harness fabrication, QA inspection, and PCB design and manufacturing to name a few.

1. Lineman's splices (as depicted) are not NASA level - Wires are never twisted to get a mechanical bond; they're placed side-by-side for lap joints, (much stronger without bending the copper conductors too) and joints are staggered to minimize the profile of the completed splice.

2. For spacecraft, (flight hardware) splices aren't allowed anyway. Screw it up and the harness is replaced. This is one reason flight hardware is so expensive - it's built to minimize performance issues due to workmanship to the greatest extent possible.

For IPC, there are three classes of workmanship standards: A, B, and C. The tutorial shows a Class A level job - it just has to work, (like a prototype or Third World-produced consumer electronics) but would never be used in a high-reliability, (medical, aerospace) application.

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Flying Squirrel

3 months ago

Pro Tip: Tin the wires just after stripping them, then let them cool before slipping over the heat shrink. This may seem like it takes longer, but actually makes bending the wires easier (no frayed ends) and you'll spend less time heating the solder for the final joint.

Been doing this for years, and in fact just did this yesterday.

Disclaimer: I have never worked for NASA, never trained electricians, nor claim to be an expert in anything other than commenting on posts that are at least 3 years old.
Disclaimer: This tip only applies to stranded copper wire with plastic insulation, AWG 12-28ga. It may work with other wires, but YMMV.

1 reply
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gizmologistFlying Squirrel

Reply 3 months ago

I've never worked for NASA either. Yes, tinning wires before connecting them to anything is generally a good idea.

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JimmieS7

5 months ago

well here i am always looking for better ways of doing something even at age 76 and having been a part of something automotive or racing since 1957, i love the articles like this but then i become a little embarrassed to realize i've been doing something wrong for around 60yrs, i always laced wire to connect 2 wires, then if possible solder and heat shrink, of course it was years after i started doing thing that heat shrink was easily available. i can't remember having a failure but im sure it happened after all were looking at lots of build it yourself or do without years. recently i ran across a new way to connect wires and actually sat down tonight to order a few packs, it sounds great , and better and easier than what ive been doing. of course im gonna fall back on the age thing now, i forget the name but you simply stick a wire in a hole and pull a lever down to force fit it in place, then do the same on other side. takes up less space too. but i appreciate all i see and hear about how to do anything, ive always taught my children and grand children to always be open for any advice, never think you are smarter than someone else, never ever refuse to admit you are wrong or made a mistake, and last is this, if you are talking you can not listen and learn.

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ccclarke

2 years ago

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.

CC Clarke IPC CIT

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robertn78ccclarke

Reply 7 months ago

Obviously this will work fine in most personal use soldering. Even I with my training would do this in my truck. The tools are expensive. I am mil-std 2000 certified. Which was the US standard for soldering before they adopted ISO and was used on the Shuttle. 1. You cannot ever touch wires including twisting. Human oils contaminate the surface. 2. You cannot use a cutter style insulation remover. You would remove the insulator with heat from the wire. Cutters can weaken the integrity of the wire. 3.You would tin all solder surfaces in a solder pot. 4. The iron would be of commercial grade and will recover from heat loss immediately. 5. All solder joints are shinny and there are proper fillets between the soldered surfaces. There is more depending on the type of soldering you are doing. But these are some things you would have to do in the example above.
https://electromet.com/wp-content/uploads/2015/09/Electromet-MILSTD2000a.pdf

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ccclarkeccclarke

Reply 2 years ago

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

CC Clarke IPC CIT

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CaptClaudeccclarke

Reply 1 year ago

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.

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JuanR212ccclarke

Reply 2 years ago

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).

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vea_uaccclarke

Reply 2 years ago

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.

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brainfarthccclarke

Reply 2 years ago

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.

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gizmologistbrainfarth

Reply 2 years ago

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.

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gizmologistccclarke

Reply 2 years ago

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.

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PaulA23

Tip 1 year ago

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! :-)

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gizmologistPaulA23

Reply 1 year ago

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.

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TwelveFoot

2 years ago

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

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gizmologistTwelveFoot

Reply 2 years ago

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.

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michael47362

3 years ago on Step 5

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?

2 replies
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gizmologistmichael47362

Reply 3 years ago on Step 5

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

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wizodd0gizmologist

Reply 2 years ago

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.