Most fog light kits come with instructions for installing the lights. Unfortunately, often these instructions are confusing or make the job look complicated. Couldn’t you simply tie the fog lights into the headlights to make installation easier?
You can tie fog lights into headlights, but doing so is not recommended. You will likely blow a fuse and run the risk of fire or other damage to the electronics. Instead, the lights need to be installed with a wiring harness.
So, let’s talk about why it’s never a great idea to tie fog lights into headlights. Then, I’ll teach you how to install fog lights properly to ensure that you do not cause more harm than good.
Why Shouldn’t You Tie Fog Lights Into Headlights?
You shouldn’t tie fog lights into headlights because it could blow a fuse, introduce electrical fires, damage the harness or connectors, or create diagnostic errors in your vehicle.
So, let’s look at these electrical issues and talk about why they might happen when tying some fog lights into your headlights.
The Additional Amps Could Blow the Fuse
Tying fog lights directly into headlights will probably blow a fuse because of the additional amps of the fog lights. Unless you deal with electricity regularly, the terminology and numbers will seem meaningless. So, let’s break it down so you can understand why this amperage is critical.
As you probably know, fuses are measured in amps, while the light output is shown in watts. So, there should be a simple formula (like 12 inches equals 1 foot) that if you know the watts, you can calculate the amperage.
Unfortunately, you cannot directly convert watts to amps because watts measure energy – or the amount of work being done. Although we think of watts as a measure of electricity, watts can also measure the amount of energy used for other tasks.
For example, if you climb a ladder in 5 seconds, we can measure the watts of energy used if we know your weight and the ladder’s height.
However, amps measure the movement of the electrical charge using the unit Coulomb-per-second. So, converting watts to amps would be like calculating how many gallons are in a mile.
Luckily, if you know a third variable, the voltage, you can convert watts to amps using this formula: Watts ÷ Volts = Amps.
So if you have a 15-amp fuse and two 55-watt headlights, the formula would look like this: 110 ÷ x = 15 amps. To figure out how many volts you would need to power the lights, you’d need to find the value of x, which is 7.3. so you’d need at least 7.3 volts to power two 55-watt lights with a 15-amp fuse. For most vehicles, that’s way too much when you consider the voltage that your daytime lights require.
If you do not have enough voltage, something will go wrong, and you’ll likely blow your fuse.
You can use online calculators to determine adequate fuse sizes. The inch watts to amps conversion calendar has a simple interface, making conversion a snap.
The Additional Amps Could Lead to Electrical Fires
A fuse stops the power flow in a circuit if too much current flows through it. Although a fuse can protect a device, the fuse’s primary purpose is to safeguard the wiring. Wiring is challenging to replace, and if it is not the proper size, it can catch on fire.
Four factors affect how much current a wire can safely handle – gauge, length, composition, and bundling.
Gauge
You’ll hear people talk about gauge the most, so let’s focus on that for a minute. Gauge is the size of the wire, but what can be confusing is that as the gauge on a wire gets thicker, its gauge is lower. For example, a 10-gauge wire is wider than a 16-gauge wire.
Another quality related to gauge is resistance, which measures how easily current can flow through a circuit. As resistance increases, it becomes more difficult for the current to flow through a wire. For example, a 16-gauge wire has more resistance than a thicker 10-gauge, so more current flows through the thicker wire.
Therefore, if the headlights on your car are currently connected to the fuse with 16-gauge wires, and you want to increase the amount of power going through the wire, you will also need to change the wiring. It is easier to add wire than to replace the existing wire.
Bundling, Length, and Composition
These three factors are not as important:
- Composition refers to the material used in the wire. Car wires are typically copper, and the more delicate the copper strands are, the lower their resistance.
- Bundling might play a minor role. As more wires are bundled together, each can carry less current. Therefore, 16-gauge wires connected to headlights might not be able to hold as much energy depending on how many wires are bundled together.
- Length is also significant when thinking about resistance. Wire resistance is measured by thickness and length—a longer wire has higher resistance.
Tying fog lights to headlights is not just a matter of replacing the fuse with a larger one. Unless you replace the wires, you risk wires becoming hot, melting, and catching fire.
The Additional Amperage Might Damage the Headlight Harness or the Connectors
Although problems with wiring are not as common as blown fuses, they still occur, especially in DIY projects such as adding fog lights. The additional required power can heat the connectors or headline harness and cause them to malfunction.
Repairs to connectors and harnesses are not as simple as replacing a fuse. Replacing the connector might be a fruitless repair because it will fail again due to the increased heat.
It Could Generate Diagnostic Trouble Codes
Finally, even if you can replace the fuse with a larger one, and even if the car’s wiring gauge can handle the additional amps, you’re still not out of the woods. Depending on the vehicle’s age, fog lights tied into the headlights could trigger a diagnostic code. In an older car, a mechanic might be able to override the code.
However, the additional current could trigger the headlights to stop working in a newer model.
Install the Fog Lights With a Separate Wiring System
To install the fog lights, you will need a separate set of cables. This set is often called a harness. It will have a wire that connects to the on/off switch, a grounding wire, and a third wire that connects the relay to the battery.
Mount the Lights
Cars usually have a spot in the bumper for fog lights. You will need to detach at least part of the bumper to give you access for installation. In most cases, this is an easy task.
If your car doesn’t have one, you’ll have to create a hole for the fog lights. If the hole is slightly larger than the lights, don’t fret. Your fog lights will likely include a frame that covers any gaps.
Instead of mounting the fog lights on the bumper, you might like to place them on the front of the car. This method works best if your vehicle already has a fog light grill or rack.
The kit for the lights will include the hardware for mounting the lights to your car. In addition, the wiring will be the same regardless of how the lights will be mounted.
Install the Wiring
To begin installing the wires, you’ll need to understand where each wire goes.
A power cable will connect to the on/off light switch. You will need to run this wire into the interior. Your vehicle might have a hole you can use to run the wire, or you’ll need to drill a tiny hole if there isn’t enough space for an additional wire.
You’ll then need to find a grounding terminal for the ground wire. The terminal must be attached to an unpainted metal section under the hood or dash.
A third wire will connect the on and off switch on one end and the battery on the other.
The harness should have more detailed instructions that show exactly where you should make the connections.
Bottom Line
If you try to take a shortcut and tie your fog lights into your headlights, you will quickly learn it isn’t a shortcut. The fuses will blow constantly, and the vehicle’s electrical system could suffer additional damage. In the long run, your shortcut will mean more work than installing the fog lights with a wiring harness.