Overheating · resistance · overload
Why automotive wires overheat: resistance, current and heat
A wire heats up by I² × R: current squared times resistance. Too much current through a sound wire is one fault; normal current through a bad connection is another. They look alike — melted insulation — but a fuse catches only the first.
Open the editor · ▶ Run the live demo
Two different faults
| Overload | High resistance | |
|---|---|---|
| Current | Above what the wire can carry | Normal or even lower |
| Where it gets hot | Along the whole wire | At one point: a crimp, a terminal, a fuse holder |
| Does the fuse blow? | Yes, if it is sized for the wire | No — the current is normal |
| Typical cause | Extra loads on one circuit, wrong fuse, too thin a wire | Loose or corroded terminal, pliers crimp, broken strands |
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) |
A bad crimp in numbers
A good crimp has around 0.001 Ω. With 20 A through it, it dissipates 0.4 W — nothing. A loose crimp of 0.05 Ω dissipates 20 W at the same current: as much as a soldering iron, in a plastic connector. The fuse sees an ordinary 20 A and holds. The circuit works, the connector melts.
What protects against what
- The fuse protects the wire against overload and short circuits, so it is sized for the wire, not the load.
- Nothing electrical protects against a bad connection: only good crimps, clean terminals and connector housings rated for the current.
- A connector or fuse holder that is warm to the touch under normal load is already a fault.
See both faults in WireForge
- Draw a battery, a fuse, a load and ground, and give the wire a cross-section.
- Overload: increase the load power until the wire carries more than its rating — the simulator warns and, without a proper fuse, the wire burns through.
- With the right fuse the fuse blows first, every time.
- Use the calculator below to choose the fuse and wire for your load.
Questions
Why did my connector melt but the fuse is fine?
Almost always high resistance at the terminal: a loose fit, corrosion or a bad crimp. Replace the terminals on both halves, not just the one that looks burnt.
Can a bigger fuse stop a fuse from blowing repeatedly?
It stops the fuse, not the fault — and the wire is then unprotected. Find the cause or upgrade the wire and the fuse together.
How warm is too warm for a wire?
A wire within its rating gets mildly warm in a harness. Hot to touch means it is overloaded or there is a bad connection nearby.