Of all the mind-boggling things some of us are forced to deal with when it comes to maintaining or even building classic trucks, the electrical system should rank right up there at the top-for many, it's the number one cause of headaches! And rightfully so, as even the most basic wiring isn't always that basic, especially for those who are basically clueless (excuse the pun) in the field of electronics.
As Painless' Dennis Overholser puts it in his book on the subject (Hot Rod Wiring, co-authored with Tim Remus, Wolfgang Press, 1997), "You don't need to know which way the electrons actually flow through a wire, but you do need to know how heavy the wire should be and where to connect the terminal." Ironically, though, a good number of people really don't even know that, let alone numerous other important facts about automotive wiring, otherwise there wouldn't be nearly as many vehicle fires and electrical failures. From undersized wire to insufficient connections, lack of appropriate fuses/relays to poor grounds, it doesn't take much for your truck's wiring to go awry-but with the proper know-how, you can avoid most, if not all, problems encountered ... even something as simple as a dead battery!
With the exception of a few antique OE applications that are "positive" ground, we deal mainly with 12-volt DC (direct current) systems-it may not be long, though, before we start seeing higher voltage become more popular, from elaborate audio using 16 volts to production vehicles with 24-volt systems. In the grand scheme of things, no matter what brand truck you prefer, we're all in the same boat when it comes to the electrical wiring (save for a few minor differences in charging/ignition systems); we all have to deal with those electrons, regardless of what direction they're really travelling in!
OHM'S Law Or Murphy's Law?
The manner in which our electrical systems operate is called Ohm's Law. Essentially, it's the interaction of voltage, current, and resistance. Voltage is what forces those electrons through the wires; current is the amount (amps) of those electrons being pushed by the voltage; and resistance is anything that restricts (ohms) the flow of the electrons. Sounds simple, right? If all the variables are correct, theoretically your wiring should be simple ... but even the smallest discrepancy can disrupt the flow, causing the problems we've all been forced to deal with at one time or another. Let's see if we can clear up some of the potentially hazardous situations by addressing possible problem causers.
 Electrical wire comes in all...  Electrical wire comes in all kinds of types, sizes, and colors-but for automotive applications, use insulated copper wire only. Even if you want that retro look, if you're using the old cloth-covered wiring, do yourself a favor and use the modern type that has vinyl insulation underneath. |  Above and beyond standard...  Above and beyond standard insulated wire is the TXL type. It features an additional layer of insulation (though it's actually thinner than other styles) that provides more protection in the event of a fire. After being subjected to a lighter for a minute or so, this piece still has a protective layer beneath the charred remains. |  Standard insulated wire isn't...  Standard insulated wire isn't as tolerant to direct flame-in a fraction of the time, this piece was toast, so you can imagine how quickly a fire could spread in a vehicle. |
Wire Size & Capacity
Incorrect wire size can lead to serious damage, possibly electrical fire, which we want to avoid at all costs. As you probably can imagine, the size (gauge) of wire determines how much current it can safely carry. If a wire is forced to carry too heavy a load, it will ultimately fail. Depending on what type of protection is used in-line (fuse/circuit breaker, which we'll cover shortly) of the current flow, that failure could be as minor as a blown fuse-or it could lead to your truck burning to the ground. The higher the amperage, the bigger the wire you use. Additionally, when running wires further than 4 feet, increase by one wire size-and continue to do so for every additional 3 or 4 feet beyond that. Increasing wire size also helps prevent voltage drop in these situations. For instance, copper AWG 14ga wire (small) is good for up to 20 amps; heavier 2ga can handle up to 150 amps. If you're ever uncertain about wire sizing, get yourself a reference guide either online or at an electronics supply warehouse.
Along with using the correct size wire, it's imperative you use the correct type of wire. For the best performance and protection, use insulated copper wire that's intended for automotive use only-anything else won't do. The wire in your truck should not only be able to handle the current it's carrying, it should also withstand the elements it's subjected to, mainly heat and vibration. Newer-style TXL wire is your best bet, as it features an additional layer of insulation that is much less susceptible to heat.
To minimize resistance, minimize the culprits that cause resistance-i.e., inline connectors. Use the least amount of connections between source and destination when wiring a component to ensure uninterrupted current flow.