How to choose the right fuse rating for your vehicle wiring
Choosing the correct fuse rating for your vehicle wiring circuit is vitally important.
A fuse is an essential component for vehicle wiring and remote solar power systems. It is vital that the correct fuse rating be used in any aftermarket circuits to ensure safe operation of your equipment. A fuse protects the integrity of the circuit by preventing an overcurrent situation from occurring due to short circuit or other fault. Short circuits in automotive wiring are the most common cause of vehicle fires, often resulting in total loss.
If you are replacing a fuse from an existing circuit due to a blow, replace it with the same value and continue operation of that circuit. If the fuse blows again, then it is likely that a fault condition has occurred which will need careful investigation. Do not put a higher rating fuse into the circuit as this may result in damage to the wiring - or worse, a fire!
Fuse Ratings
Fuses are marked with the current that they will pass in a continuous situation, in the same way your cabling is rated. In most situations the fuse rating should be BELOW the rating of the cable, this way when a fault occurs the fuse will blow before damage to the cable occurs. In this way the fuse is always the weakest part of a circuit. To determine a fuse rating for your aftermarket circuit you will need a couple of pieces of information.
Firstly you will need to know the current rating of your cable. You can find a handy guide to selecting the right cable gauge for your circuit here.
Once you have that you will also require the current draw of your connected load, for instance a 130W LED spotlight or light bar would draw approximately 10 Amps at 13.2V.
The fuse rating should lie between the two values. So in the example above a 10 Amp current draw on 20 Amp maximum rated cable would typically require a 15 Amp fuse rating. HOWEVER! It is important to consider temperature and its effect on fuse rating, explained below.
Temperature is important
Most fuse ratings are provided at 25 degrees Celcius which works well in an air conditioned test facility - not so well in your car engine bay! So for situations where your fuse is located in hot environments you should multiply your fuse rating by 1.25 (for temperatures up to 50 deg. C) or 1.3 for higher temperatures. So a circuit that would normally need a 20A fuse would need a 25A fuse in an engine bay that reaches 60 deg. C.
If you're having nuisance fuse blows in warmer weather, its possible heat might be a factor (if you have checked the integrity of the rest of the circuit first of course!).
How much current will it take for a given fuse to blow?
Simply put, the current should not exceed the rating of the fuse - period. However the amount of current in excess of the fuse rating will determine the speed at which the fuse blows. So a 10A fuse which has 11A running through it may take minutes to blow, however a condition where 20+ Amps are put through the fuse will result in it blowing almost instantly.
Typical aftermarket fuse wiring configuration.
When adding circuits to power DC equipment it is important to follow a wiring configuration where each load is on its own circuit with its own dedicated fuse. This means should any piece of wiring go to "ground" and short circuit, the fuse will blow and protect your vehicle and/or equipment from damage. This is best achieved using a fuse block where you have a bank of fuses, each protecting an individual circuit connection. Much like the existing manufacturer fuse block found in your car.
It's also vitally important that the main positive feed to the fuse box is also fuse protected as shown in this diagram:
