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How many circuits do I need in an automotive wire harness?

Finding an ideal auto wiring harness for your vehicle is essential for successful restoration. However, knowing and considering the number of required circuits is crucial to having a successful and seamless wiring harness. We have compiled this article to answer the question: “How many circuits do I need in an automotive wire harness?” Read on to learn more!

What are the circuits in a basic auto wiring harness?

Wiring harness circuits are nothing more than the wire designed to serve a specific function. 

For example, a wire for the starter or glove box light will likely be one circuit. On the other hand, the wire for the headlights will need two circuits.

The exact number of circuits you will require for your wiring harness depends on how many electrical accessories you have.

For example, for simple car projects, such as off-road buggy restoration, you will require only a basic auto wiring harness with 12 circuits.

However, for a classic hot auto rod wiring harness complete with multiple luxury items, you will require a new 24-circuit auto wire harness.

How many circuits do I need in my car?

As mentioned above, the number of circuits you require for your auto wiring harness depends on the electrical accessories you want in your car. 

12-circuit auto wiring harness

Vehicles have twelve (12) most basic accessories; you will need twelve (12) auto wiring harness circuits designed. Each circuit has a fuse block with a designed fuse holder.

S/NCable harness circuit
4.Stop lights
5.Turn signals
8.Dash lights
9.Heat and A/C
11.Parking light
12.Hazard flasher

18-circuit wiring harness

Most auto wiring harnesses require a minimum of 12 circuits since there are 12 extremely common and vital accessories.

However, several cars have extra electrical accessories beyond those highlighted above. For example, an 18-circuit wiring harness may have six extra circuits for the following additional power accessories:

S/NCable harness circuit
1.High-level light
2.Power locks
3.Radio B+ memory
4.Power Windows
5.Electric fun
6.Electric fuel pump

24-circuit wiring harnesses

There are also 24-circuit wiring harnesses that will require the following six circuits to support the additional electrical accessories:

S/NCable harness circuit
2.Trunk light
3.Under hood light
4.Glove box light
5.Dome light

NOTE: If you have accessories you might wish to install later, we recommend buying a new auto wiring harness with more circuit functionalities (capabilities). This allows you to expand your wiring harness without purchasing a new one.

Wiring harness diagram

Wiring harness diagram

Choosing the right auto wire harness

When choosing the right wire harness in vehicles, there is no one-fits-all solution. Different types cater to different areas and functions within the car, and the right cable harness depends on the following factors:

  • The system you want to optimize
  • The make and model of your vehicle
  • The complexity of your electrical setup
  • Your specific cable harness needs

Here are some of the common types of wire harnesses that you may consider:

Universal cable harness:

This is a super versatile wire harness, as you can use it across different types of auto applications and vehicles. It is not custom-made for a specific auto make or model. Therefore, choose a universal cable harness if you are replacing a faulty harness in an older car where a specific subsystem cable harness might be hard to find. It is also ideal when working on a custom build.

Engine cable harness:

It is a wire harness designed particularly for the vehicle’s engine compartment. It takes care of the wiring of engine components, such as spark plugs, fuel injectors, etc.

Body cable harness:

This wire harness handles all the electrical accessories within the car’s body, including the sound system, lights, and power windows. 

Chassis Wire Harness:

Used for the car’s undercarriage cabling, it connects sensors, tail lights, and other accessories underneath the car.

HVAC cable harness:

This wire harness is dedicated to Heating, Ventilation, and Air Conditioning (HVAC) systems controlling elements like AC compressors and blower motors.

A car wire harness

A car wire harness

The installation of an auto wiring harness

If you don’t have experience wiring an entire vehicle, one of the toughest steps is getting started.

Depending on the auto wiring harness type, you may need to run the wires along the chassis, under the carpet, or through the firewall.

The aim of doing this is to maintain an organized look and achieve a proper electrical connection.

Choose types of auto wiring harnesses.

When it comes to starting with wiring harness installation, there are three types of auto wiring harness you should know. 

  • Auto-cabling harnesses with wires connected to the fuse panel. These wires are usually routed to the termination point of the harness, then cut to the ideal length and connected to the load.
  • Auto cabling harness with wires running from the load. These wires are also cut to the ideal length and attached to the fuse panel.
  • Exact factory replacement basic auto cable harnesses, with termination on both ends. Usually, you will need to install additional plugs.

Generally speaking, an auto wiring harness with electrical wires attached to the vehicle’s fuse panel makes routing and installing cable harnesses more straightforward.

Auto wiring harness

Auto wiring harness

Prepare tools used for wiring.

You will need the following tools for your auto wiring harness installation:

  • Crimping tool: Getting a top-quality crimping tool is essential for a solid cable harness installation. It also makes all the difference when connecting various wires in your harness using solderless connectors.
  • Wire strippers: Self-adjusting and manual wire strippers make stripping the wire ends cleaner and quicker.
  • Connectors: Adding new connectors ensures a long-lasting, secure electrical connection.
  • Heat shrink tubing: For a professional cable harness look, you must use heat shrink tubing over electrical terminals to insulate and protect your connections.
  • Cable coverings: It is a great idea to add an extra appearance and protection to your wiring harness with cable covering. These wire coverings are available in the plastic split loom, braided cloth, braided nylon, etc.
  • OEM cable holders/insulated line clamps: You can use OEM cable holders to secure your wiring harness bundles to the frame or firewall. However, if your original retainers are missing, damaged, or broken, we recommend using insulated line clamps.

The auto wiring harness installation steps:

Before you start your wiring harness installation, we recommend disconnecting your car’s battery to prevent any accidental electrical discharge. Next, follow the below steps:

Step 1: Layout your auto cable harness on the floor.

Start by familiarizing yourself with the different circuits and wires. 

Next, bundle them into major groups of electrical wires for particular areas in the vehicle, such as tail lights, headlights, engine compartments, gauges, dash switches, etc.

We recommend cutting your factory tie wraps and reorganizing the cable harness to effectively suit your vehicle’s needs. You can use “sacrificial tie wraps” for wiring harness building.

Step 2: Choose where to install your fuse block.

A fuse block is often the beginning point for all the wires in your auto cable harness. You need to screw the box to the body or bracket on a fiberglass vehicle.

 Typically, the best location to install your fuse block is underneath the dash on your car’s firewall. You can also choose to mount it under the vehicle’s seat.

The trick is to choose an easily accessible place to make it easy to run from the back to the front of the vehicle.

We recommend holding onto existing cable harnesses to reuse various connectors or pigtails that are your vehicle-specific.

Step 3: Route your cable bundles to their corresponding termination points from the mounted fuse box.

Most auto wiring harnesses need to go through metal, such as a rear bulkhead, firewall, etc., at some point.

 Therefore, we recommend using rubber grommets in places where your cabling harness bundles pass through the mentioned metal.

Some harness kits, such as cutting templates for fuse boxes, wire bulkheads, etc., are vehicle-specific. 

Therefore, you should read the instructions carefully and fully before choosing a place to pass your auto cable harness through.


The choice between a 24, 18, and 12 auto wiring harness depends mainly on car compatibility. 

Further, it is crucial to choose a high-quality auto wiring harness. 

If you are looking for cable harnesses that include strong connectors, quality wires, durable insulation, and easy installation, Clooms Tech has got you covered. 

We have an extensive inventory where you will always find an ideal auto cable harness and wiring assembly that suits your specific needs.

I am Lillian Yang, having been a sales manager for over 10 years.

I have received many positive reviews from customers. They have praised our excellent service, on-time delivery, and high-quality cable assemblies.

For your projects, please provide cable assembly files/images/smples, etc., so that I can send you a quotation within 24 hours.

Contact me now and let’s get started on building your wire harnesses!

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