Light bar · relay · switch
LED light bar wiring diagram
A light bar is a big load on a long wire: from the battery to the roof or the bumper and back. Wire it through a relay with a fuse at the battery, and size the wire for the voltage drop, not only the current.
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The circuit
- Battery + → fuse next to the battery → relay 30.
- Relay 87 → light bar + ; light bar − → ground near the bar.
- Dash switch → relay 86 (from switched +, so the bar cannot stay on with the ignition off); 85 → ground.
- Optional: feed the switch from the high-beam wire so the bar only works with high beam.
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) |
Wire size: the voltage drop decides
A 120 W bar draws 10 A on 12 V and takes a 15 A fuse. For a short run 1 mm² would carry it, but over 4 m it loses too much voltage: 2.5 mm² keeps the drop under 3 %. A 300 W bar needs a 35 A fuse and 6 mm² over the same distance. The calculator below works it out for your bar.
Check your wiring in WireForge
- Draw battery → fuse → relay → light bar (a Lamp of the bar’s power) → ground, and a switch on 86.
- Give the bar wire its real length, e.g. 4 m.
- Simulate and click the wire: see the current and the voltage the bar actually gets.
- Change the cross-section until the voltage drop warning disappears.
Questions
Do I need a relay for an LED light bar?
For anything above a few amps, yes: the switch and its thin wire then carry only the coil current, and the thick wire runs the short way from the battery to the bar.
Where should the fuse go?
As close to the battery as possible, on the wire to relay terminal 30. A fuse at the bar end does not protect the long wire before it.
Can I take power from the cigarette lighter?
Not for a real light bar: the socket circuit is fused at 10–20 A and shared with other loads.
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