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Picture of the Littelfuse Minifuse

Fuse Sizes

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Here’s the answer to another popular question that we get from customers when they’re wiring their cars with our Infinitybox wiring system.

What size fuse should they use for the different outputs on the POWERCELL?

Remember why the fuse is there…  it is there to protect the wire not the end load.  There is no need to protect the headlamp with a fuse but you do need to protect the wire going to the headlamp.  A fuse is a thermally operating device that is intended to be the weakest point in an electrical system.  If the system is drawing too much current, you want the fuse to open intentionally, protecting the rest of the system.  You never want your wire to be the weakest point in your electrical system.

Fuses are designed to protect against two kinds of faults.  The most common is a short circuit.  Let’s say that the wire to your headlights gets pinched against the chassis and breaks the insulation.  You now have a very low resistance path to ground which will carry a lot of current.  You want the fuse to open before the insulation on the wire gets hot enough to cause damage.  The other scenario is a low-overload.  These are usually resistive connections to ground that will still carry enough current to damage the wire.

To properly select the size of the fuse that you need to consider two things.

First, what is the minimum gauge of wire that the fuse needs to protect?  All of the output harnesses on the Infinitybox POWERCELLs use 14-AWG wire.  But, if you spice our output wire to a wire with a smaller gauge, you need to size the fuse to protect the smaller wire.

Second, you must understand the current draw for your load connected to the output wire.  The fuse must be large enough to handle the steady-state current of the load plus any inrush that the load may have.  Anything inductive (motors, coils, solenoids, etc) have inductive inrushes that could be up to 10 times the steady-state current draw.  Incandescent bulbs also have inrushes.  Remember that the resistance of the filament in the bulb is a function of its temperature.  When the bulb is cold, the resistance is low so it can carry more current.  As the filament heats up, the resistance increases as does the current flowing through the bulb.  The inrush through a light bulb can be 5 to 10 times its steady state current.

For those who want to learn more, this link will take you to a good document from Littelfuse that talks about how to properly size a fuse for a specific application.

LED lights draw far less current than their incandescent brothers and have practically no inrush.  You can use a much smaller fuse.

Here is a simple chart to use as a reference for sizing your fuses.

Headlights- 20-amp
Starter solenoid- 20-amp
Ignition- 25-amp
Fuel pump- 25-amp
High-Beams- 20-amp
Turn-signals- 15-amp
Cooling fan- 25-amp
Power window regulator- 20-amp
Lock actuator- 15-amp
Parking lights- 15-amp
Horn- 10-amp

Use this chart as a guideline to pick your fuse sizes.  You may need to adjust depending on the gauge of wire connected to your load and the specifics of your load.  Remember, these are guidelines.  If you change the gauge of wire that is included in the kit, you have to reevaluate the size of fuse required.

Contact our technical support group at (847) 232-1991 for more details.

Picture of the Littelfuse Minifuse

Why A Fuse?

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We get this question a lot.

Why do the POWERCELL and inMOTION cells use fuses to protect the outputs?  Some say that we’re already using smart MOSFETs, why not rely on them to protect against over-current on the output wires?  Others ask, why not use circuit breakers?  There are several important and practical reasons why we rely on a traditional fuse to protect the outputs on our Infinitybox hardware.

With respect to MOSFETs, they are tried and true technology.  They are used in practically every application for switching and current control.  The MOSFETs used in the POWERCELLs are automotive-grade and designed to carry up to 270-amps.  The chances of them failing in a typical customer car are minute.  However, if a MOSFET fails, it doesn’t fail gracefully.  If a MOSFET were to fail, it fails resistive.  That means that they are going to generate a significant amount of heat, quickly.  No intelligence built into the circuitry can interrupt the current flow if the silicon die in the MOSFET package has become a resistor.  In that case, the fuse is your last line of defense to protect the system from thermal runaway.

You can use the same logic for inMOTION.  It has sensors on the board that monitor the total amount of current flowing out to the load.  inMOTION already shuts off the relay coils when it sees too much current, why not rely on that alone without the fuses?  The practical reality is that a common failure mode for relays is for the contacts to weld together, especially in inductive motor application.  If this were to happen, no intelligence on the inMOTION board can open the circuit.  Again, the fuse becomes the last line of defense.

With the above being said, you can see the need for some circuit protection component in the system if other components fail.  That leads to the debate between fuses and circuit breakers.  Circuit breakers can certainly do the job, however a fuse will out perform a breaker in all categories.

  1. Fuses are more cost effective than circuit breakers.
  2. Fuses are smaller and package better than circuit breakers.
  3. Fuses have better low-overload protection characteristics than circuit breakers.  This is very important in cases of resistive shorts like a failed MOSFET.
  4. Fuses respond quicker than circuit breakers under short-circuit conditions.

Most importantly, a fuse drives better end user behavior than a circuit breaker.  At the end of the day, there is a reason why a fuse opens or a breaker trips.  A motor has failed.  Insulation on a wire has shorted to ground.  An electronics module has failed.  In all these cases, you do not necessarily want to blindly reset a breaker and keep on driving.  The cause of the over-current condition needs to be identified and fixed before resuming normal operation of the vehicle or equipment.  Otherwise, more severe thermal events can occur in the wiring harness causing damage or injury.

See our blog for additional posts on proper fusing and other circuit protection lessons.  If you have any questions, reach out to our power distribution experts at (847) 232-1991 or email our team at sales@infinitybox.com.  You can also click this link to contact our team directly.