Fog lamps · relay · interlock

Fog light wiring diagram

Two 55 W fog lamps draw about 9 A — wire them through a relay, protect them with a fuse, and in most countries make them work only with the parking lights on. Here is the circuit, and a demo of what the fuse does when the wire shorts to the body.

Open the editor · ▶ Run the live demo

The circuit

Battery + → 15 A fuse → relay 30; relay 87 → both fog lamps in parallel → ground.
Fog switch → relay 86; relay 85 → ground.
Interlock: feed the fog switch from the parking-light circuit, so fog lamps cannot be on alone.
Wire: two lamps at 9.2 A on a 3 m run need 1.5 mm² to keep the voltage drop under 3 %.

When the wire chafes

Fog lamp wires run low in the bumper and chafe through on brackets. When the copper touches the body, the current jumps to hundreds of amps; the fuse blows in milliseconds and the wire survives. With no fuse — or a much bigger one — the wire itself burns. The demo below shows exactly that.

Try it

Open the “Short circuit and a fuse” demo.
Turn on the fog lamp switch: the lamp lights.
Hold “Chafed wire” — the fuse blows; replace it and try again.
In the editor, add a second lamp and a relay, and check the currents.

Questions

What fuse for fog lights?

Two 55 W lamps draw 9.2 A on 12 V; with 25 % margin that is a 15 A fuse. Two 35 W lamps need 10 A.

Do fog lights need a relay?

Yes for halogen lamps: 9 A through the dash switch and its thin wire wears the switch and drops voltage. LED fog lamps draw little and can work without one.

Why do my fog lights only work with the headlights?

Because the switch is fed from the parking or low-beam circuit — that is the legal interlock in many countries, not a fault.

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