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Automotive Wire Basics
By Wayne Scraba /
Printable version
Degree of DifficultyEasy
Estimated Time240 minutes
240 minutes
What to Know to Select the Best Wire for Your Car -
Pretend you’ve just purchased a new set of driving lights for your daily driver or maybe a new winch for your 4X4. After you get the basic hardware mounted you have to run wire through a switch (or two) and into the fuse panel. No big deal. You just get some wire and quickly get the job done—but not so fast. Wire might not seem like an important or complex subject. After all, one sort of wire looks pretty much the same as the next, at least at first glance. When you examine the wiring harness on a common production line motor vehicle, one thing you’ll quickly discover is that a very large percentage of the wires in the bundle are made primarily from copper. It doesn’t matter what size, color or configuration either.
Automotive Wiring Basics
Safety Note:  Never work on anything electrical that is live or conductive without first disabling the power supply.
Avoid Aluminum
The reason copper has been used as the principle wire of choice is multi-fold: It’s not brutally expensive (it was once dirt cheap, but we digress); however, more important, it does a very good job of conducting current. Only silver does a better job of moving current, but as you’ve probably surmised, pure silver wire isn’t very common due to costs. You will find though that some high-end automotive wire is actually silver-plated copper. Some manufacturers have toyed with the use of aluminum for wiring. Even some aircraft wiring was once made from the material. It’s obviously light (important in both aircraft and racecars), but when compared to the other materials of choice (copper, silver, silver plated copper), it is simply not as good a conductor.
There’s more too: The type of aluminum suited to the manufacture of wire must be soft. Soft aluminum tends to work-harden when stressed beyond its elastic limit. Eventually, it will become brittle and crack. Here’s how it happens: If a terminal is crimped to the end of a section of aluminum wire, the procedure disturbs the aluminum core. This disruption of the core initiates the work-hardening. And if the wire isn’t supported adequately (by way of cushioned clamps and/or cable ties), it is regularly subject to vibration. The work-hardening process continues. Bottom line: The wire will eventually fail. As a result, using aluminum core wire just isn’t worth it.
Common Copper
Practicality pretty much determines that copper is the wire of choice, but be forewarned: There are plenty of different types of copper wire. A good example is household wiring. A house wired with copper (which is common) will use thick, solid wire. On the flip side, the highly flexible cord for your toaster or coffee maker has a core consisting of finely stranded wire. There are big differences though: While your toaster cord is easy to bend, the wire inside the walls of your house is difficult to bend (more on this below). Copper is a very ductile material. What this means is you can wrap a piece of solid 10-gauge wire around a very tight bend without it losing much of its structural integrity. And if you straighten the wire and then re-bend it several times, it will eventually work-harden. If constantly bent and straightened, the ductility eventually diminishes. Fissures will begin to appear on the surface of the wire. If the bending process continues, the fissures will increase in size and the wire will fail.
Multi-Strand Copper Wire
There’s a better solution: Swap the solid wire for stranded copper wire. Imagine you place 19 strands of 22-gauge wire into a bundle. The bundle of little wires has approximately the same cross sectional area as the piece of solid wire. If this piece of stranded wire is bent over a tight radius (as discussed earlier), it won't have a tendency to fracture, because the reduction in size of the individual wire within the strand produces a roughly comparative reduction in stress. This means that stranded wire for automotive use is ideal. With stranded wire, you usually have a choice between something like 7-strand or 19-strand (using a small 22-gauge wire as the example). In the case of small size wire, such as 22-gauge, a total of seven strands of 30-gauge wire are used to make up the core. If 10 strands are incorporated into the mix then the strands are 43-gauge. Is one better than the other? When it comes to quality more strands inside the wire usually equates to better.
There are some other considerations too: Copper is an "active" metal. Copper readily reacts with oxygen in the air along with moisture and of course, airborne contaminants. That’s why unprotected copper items usually turn a murky green in a month or two. When it comes to copper wire, there is no practical way to completely seal the circulation of air and moisture between the individual strands in the bundle. The reality is, the majority of plastic wire insulators are not barriers to the environment, particularly when exposed to gasoline, oil and other hydrocarbons.
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