The Components
A typical aftermarket wiring harness consists of about 47 1/2 miles of wire, give or take a dozen or two miles. Thankfully they're usually different colors and labeled to boot. (GM color-coding is most common, but other color systems-and even all-black harnesses-are available.) Also included in most kits are a fuse panel, fuses, terminals, connectors, and typically a variety of relays. The instructions will tell you where and how to hook everything up, but may not spell out what everything is and does. So let's take a look at the various components and learn their purpose.
Wires And Terminals
Wires and terminals come in a variety of sizes for a reason. Larger-diameter wires have lower gauge numbers; smaller-diameter wires carry higher numbers (in other words, a 10-gauge wire is thicker than a 12-gauge wire). Higher-amperage circuits require larger-diameter wire and terminals with higher ratings. The same is true if you are running an extra long circuit or bundling multiple circuit wires together in a loom. Just like switches, these parts need to be sized to carry the appropriate amperage and need to be able to ventilate to prevent heat buildup. Contrary to some people's beliefs, soldered connections are not a necessity; a good crimp joint is fine. In fact, if you're not careful, it's very possible to overheat a solder joint, causing a brittle connection that could break due to movement and vibration. The OEMs use crimp joints for nearly everything, including many main battery connections.
Every piece in a circuit has the total amperage running through it and needs to be sized accordingly. The things to consider when selecting a wire size and insulation are the ambient temperature, the current the wire will carry, the total length of the wire, and how much voltage loss is acceptable. (There is always some loss-the longer the wire, or the smaller the wire, the more the loss.) As a general rule, sensitive circuits and headlights can tolerate 3-percent loss, and most everything else can tolerate 10-percent loss. Headlights are sensitive because the light output varies as the square of the voltage or more. So if you want the best light output you can get, use a heavier gauge wire and use a relay. Also, EFI, ECU, and stereo systems have high peak-current requirements that demand a larger wire size.
 Some companies offer wire...  Some companies offer wire in bulk for those who want to augment their harness or build their own. Quality automotive wire differs from general-purpose hardware store wire; it's higher grade and has thinner insulation that is more heat resistant. These 25- and 50ft lengths from Painless are available in a rainbow of colors. |
 This chart gives good gauge...  This chart gives good gauge guidelines to follow when planning circuit additions or rewiring from scratch. Of course, aftermarket harnesses eliminate the guesswork by selecting the wire and setting it up for you. |
 Relays are musts in modern...  Relays are musts in modern automotive wiring. High-load circuits like headlamps and starters can and will overpower the ratings of most switches, degrading them over time. Relays basically cushion these switches from those high loads, letting them operate with much less stress and heat. |