Fuse rating · wire gauge · voltage drop
Car fuse and wire size calculator
How big a fuse, and how thick a wire, does a load need? Enter its power and the wire length. The calculator finds the current, the next standard fuse above it, and the thinnest wire that both carries the fuse’s current and loses no more than 3 % of the voltage.
Open the editor · ▶ Run the live demo
How the fuse and the wire are chosen
- Current = power ÷ voltage. A 55 W headlight on 12 V draws about 4.6 A.
- Fuse: the next standard blade rating above 125 % of that current, so it does not blow in normal use.
- Wire: thick enough to carry the fuse’s rating continuously — the fuse protects the wire, so it must always go first.
- Voltage drop: on a long run the wire may need to be thicker still, so the load gets at least 97 % of the voltage.
Wire ratings are conservative values for a single copper wire inside a harness (FLRY type); a wire in free air carries more. Check the ratings of the wire and the fuse you actually buy.
Typical loads on 12 V
| Load | Power | Wire length | Current | Fuse | Wire |
|---|---|---|---|---|---|
| Parking lights | 10 W | 4 m | 0.83 A | 2 A | 0.35 mm² (22 AWG) |
| Headlight bulb H7 | 55 W | 3 m | 4.58 A | 7.5 A | 0.75 mm² (18 AWG) |
| Pair of headlights | 110 W | 3 m | 9.17 A | 15 A | 1.5 mm² (16 AWG) |
| Horn | 60 W | 2 m | 5 A | 7.5 A | 0.5 mm² (20 AWG) |
| Wiper motor | 120 W | 2 m | 10 A | 15 A | 1 mm² (17 AWG) |
| Radiator fan | 250 W | 2 m | 20.83 A | 30 A | 4 mm² (12 AWG) |
| LED light bar | 300 W | 4 m | 25 A | 35 A | 6 mm² (10 AWG) |
| Audio amplifier | 1000 W | 5 m | 83.33 A | 125 A | 35 mm² (2 AWG) |
Check it in a real circuit
- Open the editor and draw battery → fuse → switch → load → ground.
- Set the fuse rating and the wire’s cross-section and length from the calculator.
- Press Simulate: click the wire to see its current and voltage drop.
- Hold a button between the wire and ground to make a short — the fuse must blow, not the wire.
Questions
What size fuse do I need?
Divide the load’s power by the voltage to get its current, add 25 % and take the next standard fuse rating. A 120 W wiper motor on 12 V draws 10 A, so it takes a 15 A fuse.
Can I just put in a bigger fuse?
No. The fuse protects the wire. A fuse bigger than the wire can carry lets the wire overheat before the fuse blows — the simulator shows the wire burning through.
Why does a longer wire need to be thicker?
Every metre of copper has resistance. On a long thin wire the load loses voltage — lamps get dim, motors slow down. A thicker wire keeps the drop small.
How do mm² and AWG compare?
Roughly: 0.5 mm² ≈ 20 AWG, 0.75 ≈ 18, 1.5 ≈ 16, 2.5 ≈ 14, 4 ≈ 12, 6 ≈ 10, 10 ≈ 8 AWG. A smaller AWG number is a thicker wire.
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