Posted February 10, 2018 by Jefferson Bryant Know How

Automotive Wiring Guide

Automotive wiring is probably the most maligned aspect of automotive mechanics, the pitfalls of car wiring repairs are many and tracing down bad connections, broken wires, and intermittent problems is about as stressful as it gets. Once you have found the problem, you don’t want to make it worse by using the wrong wire. Not all wire is created equal, in fact, automotive wire gauges and types are quite specific.

First rule – never use solid wire for your car. Solid wire is only suitable for houses and industrial use, it should never be use in your car unless you are using to for bailing wire. Automotive wiring has to be flexible. Stranded wire is flexible, where solid core wire can bend, but not over and over. Don’t use solid core wire in any automotive application.

While this is not solid core wire, it is dang close. The top wires is a smaller gauge, but notice how the bottom wire has less than half as many strands? That is cheap wire that does not bend well and is not as good as the higher-strand wire above it.

Choosing The Right Wire Size

The main issue for wiring is the sizing. Wire is used to carry electrical current, how much current it can carry is directly related to the length and the thickness or gauge of the wire. Signal wires, like from a sensor to the computer do not need to be very big, as the current (amperage) is quite low, but major power wire applications such as alternator, electric motors (windows, locks, etc), and other high-draw items need bigger wire to support the load. As the distance from the source increases, the diameter must increase as well. There is also the issue of voltage drop. Any time you run wire, there will be a certain amount of voltage drop, it just like a water hose, the longer it is, the lower the pressure on the outlet. In electrical circuits, you can combat the voltage drop through larger gauge wires. Any chart you use needs to be noted with the voltage drop of the chart. If the drop is not noted, find another chart. For our examples here, we are using the standard 3% voltage drop, which is the maximum drop acceptable for critical components in a vehicle.

The big wires that support large amounts of current can get you into trouble if you do not have the correct size. You can always go bigger, but never go smaller.

Let’s start with the basic 12-volt circuit running your power windows. A normal fuse in your fuse panel is rated at 25 amps, which means the highest capacity for the circuit is 25 amps. This typically allows for a 10-15% variance in draw, so the window motor only draws roughly 20 amps. During another project, you pinched the wire in the door and now you have to replace it. Standard automotive primary wire is 18 gauge. This is good for signal wires, but not for hi-current applications. If you replace that section of wire with a piece of 18 gauge, it could cause a fire. Instead, you need to match the original wiring size. In most cases, this will be 12 gauge, which will service 25 amps for 10-12 feet in length, which is about the right length for a door window to the power source.

The more common smaller gauge wires are what you will likely deal with the most. 22 gauge is only good for signal, like sensors.

For the next example, you are installing an audio amplifier in your car. The fuses in the amplifier are (2) 30 amp fuses, for a total of 60 amps. To determine how big the wire needs to be, you have to figure the approximate length of the wire from the amp installation point to the battery (all amps must be wired directly to the battery). For our example, we are going with 16 feet. Over 16 feet, with 3% voltage drop, a 50-amp circuit requires 4-gauge power wire. This yields plenty of headroom for the wire to supply up to 80 amps. 90 amps over the same distance would require 2-gauge wire.

Wire Gauge Chart by Length and Amps

	5 Amps	10 Amps	15 Amps	20 Amps	25 Amps	30 Amps	40 Amps	50 Amps	60 Amps	80 Amps	100 Amps
Wiring Length
0 - 5 feet	16-gauge wire	16-gauge wire	14-gauge wire	14-gauge wire	12-gauge wire	10-gauge wire	8-gauge wire	6-gauge wire	6-gauge wire	4-gauge wire	4-gauge wire
5-7 feet	16-gauge wire	16-gauge wire	14-gauge wire	14-gauge wire	12-gauge wire	10-gauge wire	8-gauge wire	6-gauge wire	6-gauge wire	4-gauge wire	4-gauge wire
7-10 feet	16-gauge wire	14-gauge wire	12-gauge wire	12-gauge wire	10-gauge wire	10-gauge wire	8-gauge wire	6-gauge wire	6-gauge wire	4-gauge wire	4-gauge wire
10-12 feet	14-gauge wire	12-gauge wire	10-gauge wire	10-gauge wire	8-gauge wire	8-gauge wire	6-gauge wire	6-gauge wire	4-gauge wire	4-gauge wire	2-gauge wire
12-15 feet	14-gauge wire	12-gauge wire	10-gauge wire	10-gauge wire	8-gauge wire	8-gauge wire	6-gauge wire	6-gauge wire	4-gauge wire	4-gauge wire	2-gauge wire
15-20 feet	12-gauge wire	10-gauge wire	10-gauge wire	8-gauge wire	6-gauge wire	6-gauge wire	6-gauge wire	4-gauge wire	4-gauge wire	2-gauge wire	2-gauge wire
20-25 feet	12-gauge wire	10-gauge wire	8-gauge wire	6-gauge wire	6-gauge wire	6-gauge wire	4-gauge wire	4-gauge wire	2-gauge wire	2-gauge wire	1-gauge wire
25-30 feet	12-gauge wire	10-gauge wire	8-gauge wire	6-gauge wire	6-gauge wire	4-gauge wire	4-gauge wire	2-gauge wire	2-gauge wire	1-gauge wire	0-gauge wire

Wire Material

What the wire is made of is just as important as the gauge. Copper is always best for any electrical application, but copper is expensive, so there are some alternatives available, including aluminum, copper-clad aluminum. The two main types of wire are OFC (Oxygen Free Copper) and CCA (Copper Clad Aluminum). Aluminum conducts about 40 percent less electricity compared to copper. This means your wiring must be larger and will heat up more. Aluminum also has this really nasty habit of corroding. This natural corrosion is hastened when you add electricity to the mix. When combined with the moisture in the air, a process called electrolysis begins, which makes reduces the ability for the wire to conduct properly. If you have ever open up a wire and found it covered in a whitish powder, that is the byproduct of electrolysis. This can happen to copper as well, but it takes much longer and the results are green tinting of the wire.

The top wire is aluminum, the bottom is OFC copper. There is a big difference in performance.

One of the keys to wiring is making sure you have good quality wire. Both wires shown are 1-gauge, but the wire on the bottom is significantly smaller despite having a large shield. That could lead to a dangerous situation. Only buy quality wire.

The same thing goes for smaller gauge wires. These are both 16-gauge primary wires, but the bottom wire is nearly half the size. Always compare your new wire to the existing wire to make sure they are the same. The jacket or shield is NOT a good comparison, you must check the wire itself.

This is even more important when it comes to signal wires. While these wires are often 18-gauge or smaller, the quality of the wire is even more important, any additional resistance in the wire will degrade the signal and your car will suffer. There are many cheap wiring spools and kits available that use CCA instead of OFC, so it is a good idea to ask before you purchase the wire. Stick with OFC and you will be good to go.

Check out all the electrical system products available on NAPAonline or trust one of our 17,000 NAPA Auto Care locations for routine maintenance and repairs. For more information on automotive wiring, chat with a knowledgeable expert at your local NAPA Auto Parts store.

Jefferson Bryant View All

A life-long gearhead, Jefferson Bryant spends more time in the shop than anywhere else. His career began in the car audio industry as a shop manager, eventually working his way into a position at Rockford Fosgate as a product designer. In 2003, he began writing tech articles for magazines, and has been working as an automotive journalist ever since. His work has been featured in Car Craft, Hot Rod, Rod & Custom, Truckin’, Mopar Muscle, and many more. Jefferson has also written 4 books and produced countless videos. Jefferson operates Red Dirt Rodz, his personal garage studio, where all of his magazine articles and tech videos are produced.