How To Identify Constant And Switched Fuses For Wiring?

Without a doubt, fuses are components designed to power electrical wiring in vehicles. They offer protection to electrical circuits against potential overcurrent and short-circuiting. However, there are various sizes of automobile fuses, and each will suit different electrical equipment and applications.

Generally, modern vehicles have two types of circuits, namely constant and switched circuits; constant circuits stay functional all the time, and switched circuits are circuits that only live when the engine is running. 

So far, each of these circuits has its fuses operating functional; a constant fuse always stays on, supplying power and protection even when the engine is off. On the other hand, a switched fuse only stays on when the engine is running. 

Notwithstanding, this article will address and tender solutions to these specific queries ranging from; what is a switched fuse, what does switched fuse box looks like, how to identify constant and switched fuses for wiring, what fuse should I use for a dash cam,

what fuses are always hot in a car, what are the two types of fuses used in automobiles, and more?

What Is A Switched Fuse?

A switched fuse is a fuse that is live when the engine runs. It is mainly used in low and medium voltages to power and protect electrical devices. In other words, it is combined with the constant fuse in an electric circuit in most modern vehicles.

What Does, Switched Fuse Box, Look Like?

A switched fuse box looks like a metal or plastic box that consist of a series of colored blade fuses of different amperage. It also consists of relays and diodes connected to vehicle components that need protection.

It is mainly two in modern cars; one near the driver’s seat and the other under the hood close to the engine. 

How To Identify Constant And Switched Fuses For Wiring

Identifying constant and switched fuses for wiring isn’t that complex. You would do your job effortlessly with a simple tool such as a circuit tester or a multimeter. 

Let’s get to it as you discover constant and switched fuses in your vehicle wiring.

  • Turn off your vehicle as you pull out the key from the ignition.
  • Locate the fuse box closest to you if your vehicle has multiple fuse boxes.
  • Connect the negative battery terminal by clipping it to a bare metal point on your vehicle. The ground point must be unpainted metal.
  • Use the tip of the circuit tester/test light to contact the exposed metal prongs on both sides of each fuse. The constant fuses would light up your circuit tester.
  • To locate the switched fuses, put the key into the ignition and turn it to the on position; if your circuit tester lights up, great! That’s a switched fuse. I recommend that you pull off the key from the ignition and repeat the procedure more than once to ensure switched fuses.

What Fuse Should I Use For Dash Cam?

As regards research and experience, I recommend you use fuses between 10A to 30A. However, your dash cam requires enough power to function correctly. It is also vital to consult your manufacturer’s guide to avoid electrical issues in your vehicle.

What Fuses Are Always Hot In A Car?

The interior/dome light fuses are always hot in a car. These fuses enable the ECU to turn on or off the lights when necessary. On the other hand, fuses getting hot indicate a faulty or wrong connection. I suggest you clean the contacts in the fuse holder and ensure enough pressure on the fuse pads in the fuse box.

What Are The Two Types Of Fuses Used In Automobiles?

The two types of fuses used in automobiles include; blade-type fuses and glass tube-type fuses. Let’s explore them below!

Blade Type Fuses

Most modern vehicles use blade-type fuses with a similar design. These fuses have plastic bodies and two prongs that would slot into the socket in the fuse box. Depending on the application, you can mount them in fuse blocks or holders.

Check out the basic types of blade-type fuses;

  • Micro 2 – They are the smallest and thin in shape.
  • Micro 3 – They have 3 terminals instead of 2.
  • Low-Profile Mini – They are also small but compact with shorter terminals.
  • Mini – They have the same body shape as the Low-Profile Mini but have more extended terminals.
  • Regular – They are the standard version and also the second largest type of blade fuses.
  • Maxi – These fuses require a high current application and are the largest.

Glass Tube Type Fuses

These are cylindrical glass fuses commonly used in American manufacturing vehicles. They are ¼ inch in diameter but differ in length. Typically, these glass fuses are available in 1A to 30A ratings. And they are designed with AG suffixes. For instance, 1AG, 3AG, and 7AG, to mention a few. However, these fuses are not standard in most modern vehicles.


In my final words, vehicle fuses come in different shapes and sizes. Each is designed to power and protect a typical set of electrical components. It is, without doubt, that modern vehicles have two types of circuits: constant and switched circuits.

Similarly, your vehicle’s fuse box consists of constant and switched fuses. The constant fuses function even when the engine is off. The switched fuses would have live when the engine is running. 

However, I have detailed the procedures to find if a fuse is constant or switched. Ensure that you clean the contact of your fuses in the fuse box to avoid getting them hot.

Do well to share this write-up with others!

Sharing is caring!

Scroll to Top