Cutting and Splicing Cables: Properly and Easily

For convenience, many electronic systems come with plug-and-play cables and consequently, are already terminated with connectors to eliminate the need for solder. Many people find these pre-terminated cables to be very beneficial for saving time and producing a clean, professional installation. However, these same cables create a dilemma when the job requires routing through engine firewalls, car roofs, trim panels and for buildings, exterior walls.

Pre-terminated cables generally have a small diameter, but the connectors on the end can sometimes double or even quadruple the size of hole required to route cabling through a roof, firewall, etc. So installers are then faced with the age old dilemma, “Do I make an excessively large hole to compensate for the connectors?” or, “Do I cut the cable and hope that I can put it back together?”

For me, there is no dilemma at all and the choice is simple… cut and splice the cable! After all it’s easy, the appearance will be superior and in the case of exterior installations, will have a greater weatherproof integrity due to the smaller diameter hole.

It’s true that many people are adverse to cutting and splicing cables because they’re worried that it will be too difficult to match the inner wires or it will be point of failure. With a little bit of education on how to do it right and some inexpensive tools, these concerns vanish. For a successful splice, just follow the basic steps below:

Tools and supplies

If you’re MacGyver, in a pinch you can make do with a pocket knife and duct tape, but for real world and permanent installations… a few more tools are highly recommended. Having the right tools are relatively inexpensive these days and become critically important for the installation of safety devices such as LED lightbars, dash lights, sirens, etc. Recommended tools and supplies are:

– Wire strippers

– Snips or scissors (some strippers have built-in snips)

– Heat shrink tubes

– Heat gun for heat shrink (a cigarette lighter or other heat sources are also acceptable)

– Solder

– Soldering iron

– Helping Hands

– Multi-meter

– Rubber grommets (for automotive installations)

Step #1 Determine cut location and make the cut

Just before snipping the cable, give some thought as to where the most advantageous cut location will be. For instance, you may want to cut the cable in the middle to make cable routing fast or you may need to cut near the end of the cable due to additional routing points that have tight tolerances. Also, always make sure your chosen cut location gives you plenty of room to work with and to correct potential soldering mistakes. Generally, I recommend making the cut no closer than 4 inches to the end of the cable or strain relief point.

Step #2 Strip cable and wire insulation

For most cables, I recommend stripping cable insulation (main outer jacket) at least one inch to allow for easy soldering and re-insulting of the inner wires. Inner wires should be stripped just far enough to allow a good solder joint, usually one-quarter inch.

Step #3 Prep and tin wires

To prep each wire, I prefer to twist stranded wires so that they tighten and straighten. This makes the wires easier to work with and allows improved solder adhesion. Some cables such as communication or video cables may have an outer shield to minimize RF interference and perform double duty by serving as a ground. With exception to coaxial cables which should never be spliced, this shielding should be prepared as other wires with twisting to tighten up the strands.

Once the wires are prepped, put a bead of solder on your iron and run the wire through the bead. The end result should give each wire end silver in color coating.

Step #4 Apply heat shrink

Always apply all heat shrink before performing any soldering. For most cables you will need at least two sizes of heat shrink, a large piece to fit over the entire cable and smaller pieces to fit each individual wire. Heat shrink size is very forgiving but generally a proper size is heat shrink that fits over wire and cable with as little play as possible.

When applying heat shrink, make sure there is a gap between it and the future solder joint. Heat shrink that is too close to soldering heat will naturally shrink and afterwards may not allow proper placement.

Another important practice is to install heat shrink on the ground wire even if originally there was no insulation at all. Even the best of solder joints produce minor imperfections and a sleeve of heat shrink over that joint will help reduce any chance of short-circuits with other wires.

Step #5 Solder joints

If you’ve tinned your wire ends, making the solder joints will be very easy and require only a small solder bead on the soldering iron. When making the joints, be sure to make smooth passes of the wires to help minimize jagged edges from forming. If any edges do form, simply use a pair of snips or side cutters to clip them off.

Step #6 Insulate joints with heat shrink

Once your satisfied with your solder joints, slide your prepositioned heat shrink up over the joints. When applying a heat source to the heat shrink, be sure to use the smallest amount of heat required to shrink your new insulation, too much heat can loosen or undo solder joints. The problem joints become invisible once heat shrink is applied. So again, use as little heat as possible.

Step #7 Test your splice and you’re done!

Before using your spliced cable or continuing to route it, it’s always a great idea to test it. An easy way to that is with a continuity tester found on most multi-meters. When in the continuity mode, simply place the test probes on each end of the cable. If your multi-meter has an audible alert, you should get a tone when the test probes touch cable ends, which indicates proper continuity.

To fully test your installation, you should also test for shorts. With the multi-meter still in the continuity mode, choose a cable end and place the test probes on various conductors. If your splice was done correctly, you should not hear any audible alert which indicates that no short circuits were found.