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HomeTips & GuidesHow To Wire a Fuse Box: A Guide on How To Power Accessory Devices in Your Vehicle

How To Wire a Fuse Box: A Guide on How To Power Accessory Devices in Your Vehicle

In this article, we will look at how to wire a fuse box (in case of damage), add devices (wires), and link additional fuse boxes to the battery. 

Vehicles have incredibly complex wiring systems, and all the harnesses need a safety system to protect the low-power devices from damage. This safety system is the fuse box. 

Let’s look at how to wire and connect these boxes, and we’ll begin by explaining how they work.

Table of Contents

How Fuse Boxes Work

Car fuse boxes are the electrical distribution points that link all the car’s electrical devices to the battery. 

And they connect these components via fuses plugged into the box’s sockets to protect against issues like overcurrent and shorting.

Since all electrical connections begin from this point, fuse boxes are great locations for diagnosis. 

And most, if not all, pieces in the unit have labels to indicate their types and functions.

And speaking of components, fuse boxes usually contain these three components.

A car fuse box with a cable connected for computer diagnostics

A car fuse box with a cable connected for computer diagnostics

Fuses

Fuse boxes derive their name from these pieces because they are the primary components. Fuses provide circuit protection, safeguarding electrical devices from damage. 

It is cheaper to replace fuses than components like head units and lights, so fuses take the hit to save your pocket.

And they have power ratings to indicate the device they are compatible with. So always check this variable before replacing any fuse.

Relays

Relays are electrically operated switches that indirectly activate or deactivate high electrical power circuits like headlight systems. 

They can be electromagnetic relays or use solid-state devices to do the switching.

Diodes

Diodes allow the power supply current to flow in only one direction. 

So in interconnected circuits inside the fuse box, diodes help prevent back feeding, protecting devices that can get damaged with electrical power flowing in the wrong direction.

When mounting diodes, connect them facing the right direction to allow current to flow one way. 

And don’t replace them with fuses because electric current can flow through them in both ways.

How To Wire Fuse Boxes

Identify the Location of the Fuse Box

Most vehicles feature two fuse panels, one being inside the engine bay. 

This box houses fuses for the ABS pump, radiator cooling fans, ECU, and other engine components. 

So leave this one out unless when replacing burnt fuses for these parts.

An engine bay fuse box with fuses and relays

An engine bay fuse box with fuses and relays

The second load center is usually in the dashboard, under the driver’s side knees when sitting in the car. It holds fuses for all the other car electronics, such as head units, power windows, lights, etc.

If not in that location, check your vehicle’s owner’s manual to locate it.

Fuse Box Wiring Steps

  1. Wear protective gloves when handling this device because it can have sharp edges
  2. Disconnect the fuse box from the battery’s hot wire
  3. Remove the fuse box lid by depressing the locking tab to study the wiring diagram printed inside the cover
A fuse box diagram printed on the inner side of the lid

A fuse box diagram printed on the inner side of the lid

  1. Unplug the respective cables as per the color code in the wiring chart
  2. Remove the fuse box from the vehicle, then do repairs or replacements
  3. Replace the load center’s wires if faulty (adhere to the color scheme)
  4. If replacing wires, use cables with higher amperage ratings than the components they will power to avoid overheating
  5. Insert the box or its replacement back into the vehicle
  6. Plug the fuses, fuse taps, etc., back in
  7. Install the lid above the box
  8. Connect the fuse box to the battery, then test the system

How To Add Wires/Devices To a Fuse Box

Adding new devices to your vehicle also requires wiring. And you have to access the fuse box to get power. The process involves these steps.

Locate the Fuse Box

If you are a professional electrician, finding the fuse box is easy. But if installing your stereo, reverse camera, LED lights, etc., as a DIY project, use the owner’s manual to locate this load center. Once you find it, unlock and remove the lid.

Look for Open Fuse Slots

Vehicle fuse boxes usually have fuse slots left open intentionally for system expansion. These slots come ready to use with power already connected. You can check the owner’s manual to confirm these slots.

A fuse box with open slots

A fuse box with open slots

Get a Matching Fuse Tap

We recommend using a fuse tap to access power from the empty slot to power your device.

Fuse taps provide neat connections but have thin wires that can only power individual components.

Usually, these gauge wires can only handle five amps maximum.

So avoid connecting multiple additional accessories to one fuse tap because you can burn the fuse.

And if installing a powerful amplifier for your sound system, you can bypass the fuse box and connect directly to the battery.

Such devices require thick wires due to their high electric current draw.

Examine the slot and wiring requirements, then get a matching fuse tap.

Connect The Fuse Tap Wire To Your Device’s Power Line

Use a wire stripper to strip about half an inch from the fuse tap wire, then twist it and fit the wire into a butt connector. Crimp the connector using a crimping tool to solidify the joint.

Next, install the component’s power supply wire to the other end of the butt connector. Remember to strip and twist it before crimping.

Two fuse taps with butt connectors connected

Two fuse taps with butt connectors connected

Test the connection by pulling the wires to see if it is solid. If it holds, use a heat gun to wrap the butt connector’s heat shrink tubing around the joint.

Install The Fuse Tap

Switch off the car, then insert the fuse tap into the empty slot with the metal blade side first to connect the power supply. 

Turn on the vehicle to test the installed component. If it works, insert the lid back into the box, and that’s it!

How To Connect Auxiliary Fuse Boxes

Some vehicle fuse boxes might not have open slots. It could be that you’ve used all the available ones, or the car came from the factory with no room for extra connections.

If you are in this situation, the solution is to connect an auxiliary fuse box to your vehicle.

A fuse box damaged by water

A fuse box damaged by water

Electricity and water should not mix, so find a safe place to mount the box in your car. And the load center should have a waterproof cover. Install the fuse box, then connect its power wire to the battery directly or via a fuse.

Use a crimping tool to crimp the ring connector joining the power wires from the two. Pull this joint to test it, then check the box installation for electrical faults. After that, connect fuses or fuse taps inside to power the car components.

Issues That Can Arise in the Fuse Box

Diodes rarely fail, leaving blown fuses and electrical relay failures the most typical issues.

Blown Fuse

Fuses can blow due to natural wear, overloading, poor quality amperage, or other external issues. 

And when the metal strip melts inside, repairs are impossible. The only solution is a replacement.

Replacing tiny fuses requires only your fingers or a puller tool to get them out. But bolt-in maxi fuses need screwdrivers for removal.

And when replacing the part, ensure you fit in a new fuse with the same rating as the old one.

A close-up image of fuses in a fuse box indicating their power rating

A close-up image of fuses in a fuse box indicating their power rating

Relay Failure

Relays handling high electrical current loads and multiple on/off cycles fail more often than their low amp or rarely used counterparts. 

This phenomenon is due to moving contact arcing when completing or opening the circuit.

Like fuses, always replace a relay with one featuring similar electrical ratings to prevent component damage.

Can You Replace a Fuse Box with a Breaker Box?

Fuse failure is permanent and can require multiple replacements if underlying issues exist. Although fuses are not expensive, it would be better to have reusable alternatives.

This alternative is in the form of circuit breakers, which have switches for resetting. If there is an overload or power surge, these devices trip, cutting the power supply.

But they don’t burn. After fixing the issue, you only have to switch the breaker back on.

Breaker boxes feature individual circuit breaker switches for each component, similar to the multiple fuses in a fuse box. And they provide the same level of protection.

So it is possible to replace a fuse box with a breaker box.

But circuit breakers are significantly larger than fuses and more expensive. So most car makers use fuse boxes.

Circuit breakers in a breaker box (more typical in household applications, not cars)

Circuit breakers in a breaker box (more typical in household applications, not cars)

Wrap Up

Wiring a fuse box is not challenging, even for beginners. 

But you should follow the correct procedures to avoid messing with your vehicle. The steps above should guide you through this wiring procedure. 

Contact us or comment below if you encounter any challenges in this process. We’ll be in touch to help.

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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