When selecting automotive wiring harness wire gauges, you need to understand the gauge categories to pick the best ones for the wiring system.
The naming or numbering of wire gauges is rather unconventional because the thickness increases as the number reduces.
For instance, a 1-gauge wire is thicker than a 2-gauge wire.
Here’s a guide to help you in this process. Read on to learn more.
What Is Wire Gauge Size?
A wire gauge indicates an electrical wire’s diameter and determines the electric current flow that a conductor can handle.
The lower the gauge rating, the thicker the cable, which means it is heavier and has less electrical resistance.
The typical wire gauges in automotive applications are 8, 10, 12, 14, 16, and 18. As the wire gauge increases, the wires become thinner.
So the 8-gauge cable is the thickest, while the 18-gauge wire is the narrowest.
An engine wiring harness
How To Select The Correct Wire Gauge
The factor that primarily determines the correct wire gauge is voltage drop.
Voltage drop is the potential difference lost over the cable length, and a wire’s maximum voltage drop should not exceed 2% of the value at the starting or injection point.
If more than 2%, the sensor or device receiving the power will operate less efficiently. And its lifespan will likely shorten.
Automotive Wiring Harness Wire Gauge
When considering car and household wiring cables, we usually refer to American Wire Gauge, which can go as low as 0000.
This cable is the thickest of them all and has a diameter of 0.46 inches. But it is not applicable in vehicles.
The 000-gauge wire is also not applicable for automotive wiring, so we’ll start at 00.
00-Gauge To 8-Gauge Wire
The thickest cable you’ll probably find in vehicles is the 00 gauge, but not in regular cars.
Most large trucks use 00-gauge wires to carry current from the battery pack to the starter motor.
Smaller trucks with regular diesel automotive engines or high-compression gas motors usually have 0-gauge and 1-gauge battery harness cables to power the starter.
A starter motor for a diesel truck (note the wire harness in the corrugated tube)
But low compression and displacement car engines can work with starters connected to the battery using 8-gauge wires.
Vehicles falling between this category and the small truck can have 4-gauge and 6-gauge cables.
10-Gauge To 14-Gauge Wire
Since the starter motor has the highest current draw in vehicles, all other components and electrical accessories can use thinner wire gauge sizes. And they fall in this category.
For instance, a three or one-wire alternator can supply the power it generates to charge the battery and power the car using a 10-gauge wire.
Vehicles with batteries mounted in the engine bay can also use 10-gauge wires to send power to electrical accessories mounted at the rear.
The same applies to compressors or powerful audio system amplifiers mounted in the trunk.
But you can use 8-gauge wiring for these big-draw audio systems to be on the safe side.
A car audio wiring harness
Other lower amp-draw accessories like front and rear windshield wipers, headlights, head units, fuel pumps, and primary & auxiliary fuse blocks can run on 12-gauge to 14-gauge wires.
16-Gauge To 18-Gauge Wire
Most car electrical systems get power from the battery or alternator using these cable thicknesses because they consume little power.
These include tail, interior, parking lights, and speakers.
A tail light cable assembly replacement part
But speaker wires can be thicker to reduce resistance.
The general rule of thumb is to use 16-gauge wires for a length not exceeding 15 feet, 14-gauge for up to 25 feet, and 12-gauge for up to 35 feet.
Otherwise, the sound output will not be crisp.
Here’s a table summary to guide your electrical wiring.
| Automotive Component | Automotive Wire Gauge |
| Accessory Port (Cigarette Lighter) | 14 |
| Air Conditioning System | 10 |
| Alternator | 8-10 |
| Ammeter | 10 |
| Auxiliary Driving Lights | 14 |
| CD/Radio (Head Unit) to Fuse Block | 14 |
| Clock | 18 |
| Coil Wire | 16 |
| Dome Light | 16 |
| Fuse Block-Headlight Switch | 10-12 |
| Gauges (Instrument Cluster) | 14 |
| Headlights | 12-14 |
| Heater Leads | 16 |
| Heater Switch | 10 |
| Horn | Feed-14, Ground-16 |
| Horn to Relay | 14 |
| Ignition Switch | 10-12 |
| Parking Lights | 16 |
| Starter Motor to Solenoid Relay | 12 |
| Stop and Tail Lights | 16 |
| Turn Signal Lights | 16 |
| Windshield Wipers and Washers | 14 |
Automotive Wire Harness Wiring Guide
The guide above can have inaccuracies because car components have different amp draws depending on the vehicle.
It is more accurate to wire these electrical devices using their current draw figures.
2-Gauge To 10-Gauge Wire
2-gauge wires are ideal for accessories drawing 150-200 amps at a length of 25 feet from the power source.
4-gauge wires can comfortably handle 100-amp loads for the same distance, while 8-gauge cables can transmit 40-50 amps.
But 10-gauge wires can only do 20 amps at 25 feet.
Multiple gauge wires in a vehicle
If you shorten the distance to 15-20 feet, 4-gauge wires can handle 150 to 100-amp loads, while their 6-gauge counterparts can hit 100 amps on the ammeter without overheating.
But 10-gauge wires cannot get to 100. Their rating is 30-50 amps.
8-gauge wires can conduct in the 150 to 200-amp range if only three feet long. And the same applies to 10-gauge cables.
12-Gauge To 18-Gauge Wire
At 25 feet in length, 12-gauge wiring can only handle 15-18 amps.
But this rating is significantly higher than what 14-gauge and 16-gauge wires can conduct, which are 11-12 and 8-10 amps, respectively.
An engine wiring harness
This amperage capacity increases to 20-24, 15-18, and 8-12 amps, respectively, at 15-20 feet.
Some 12-gauge wires can handle 100 amps at 3-5 feet, but their 14-gauge and 16-gauge counterparts can only transmit half that value.
For shorter distances or lower electrical current ratings (40 or less), you can use 18-gauge wires.
Factors To Consider When Selecting an Automotive Wiring Harness Wire Gauge
As you might have noticed, the primary factors to consider when selecting the correct wiring harness wire gauge for your vehicle are the electrical current requirements and wire length.
Electrical Current Requirements
The higher the electrical current draw, the thicker the cable should be, meaning a lower wire gauge.
This wider diameter will ensure the electrical line handles the electrical load with minimal resistance, which means little to no overheating.
Length
An electrical cable’s resistance increases as it gets longer.
So lengthy electrical lines, such as those running from a rear-mounted battery to a front-mounted engine block, must have a lower gauge to reduce the voltage drop associated with resistance along the line.
Solid vs. Stranded Wires in Automotive Wiring
Solid wires have a single strand as the core conductor, but stranded wires have multiple strands as the core.
The latter is better for automotive wiring because it can withstand repeated twists and turns without breaking.
Solid core wires would break easily. But they are ideal house wires because the wiring in buildings does not bend, flex, or move around.
A stranded vs. a solid core copper wire
So as you pick the correct wire gauge for your project, ensure it is a stranded wire. Generally, automotive wire harnesses have stranded cores, but it won’t hurt to confirm.
Wrap Up
Wire gauge plays a significant role in automotive wiring because it determines the current carrying capacity of the conductor.
Using thicker wires than required won’t cause harm but will be expensive.
On the other hand, using thinner wires than recommended will cause overloading that can heat and melt the cables.
So, get the sizing right, and the guide above will help you select the correct wire harness gauges. Comment below if you need further assistance.
