How to use the automotive fuse size calculator
- Choose whether you know the power in watts or the current in amps, and enter it.
- Enter the system voltage, such as 12 or 24 volts.
- Set the safety margin above the normal current. The default is an example.
- Read the normal current, the current with the margin and the next standard fuse size up.
Formula
Current = watts / volts
Target = current x (1 + margin)
Fuse = the next standard size at or above the target
The list of sizes used is 1, 2, 3, 4, 5, 7.5, 10, 15, 20, 25, 30, 35, 40, 50, 60, 70, 80 and 100 amps.
A 120 watt accessory on 12 volts
A 120 watt accessory on a 12 volt system draws 120 / 12 = 10 amps. With a 25% margin the target is 12.5 amps.
The next standard size at or above 12.5 amps is 15 amps. The size below is 10 amps, which would sit right at the normal current and might blow in use.
What a fuse is for
A fuse protects the wire, not the device. If something shorts, the current climbs far above the normal level and the wire could overheat before the device does. The fuse is chosen to open before the wire is damaged, so it must be rated no higher than the wire can carry, and high enough that the normal load does not blow it.
The margin gives room for startup surges and for a load that is slightly higher than its label. A margin of 25% is a common starting point, but motors and compressors have much larger startup currents, and the device maker's instruction may call for a particular fuse.
What this page cannot check
The wire size and the length of the run decide how much current it can carry safely and how much voltage it loses, and those are separate checks. A fuse that is correct for the device can still be too large for a thin wire. The fuse also belongs close to the power source, so that the whole length of the wire is protected.
This is informational arithmetic only. For work on a vehicle, follow the instructions of the device maker, use parts rated for automotive use and have a qualified installer check the work if you are not sure.
Assumptions and limits
- The load is steady. Surges of motors and compressors need extra care.
- The list is common blade fuse ratings. Maxi, mega and ANL fuses have other sizes.
- The wire must be rated higher than the fuse. This page does not check it.
- The figures are examples. Nothing is uploaded.
Frequently asked questions
Can I use a larger fuse than the result?
Only if the wire and the device can both safely carry it. A larger fuse protects less, so go up only on the advice of the device maker.
What margin should I use?
Use the margin the device maker recommends. If none is given, 25% is a common starting point, with a higher margin for motors.
Does it work for 24 volt systems?
Yes. Enter 24 as the voltage. The same power draws half the current at 24 volts.
Why is there no result above 100 amps?
The list stops at 100 amps. Larger circuits use other fuse types and need a professional design.