How to Wire Off-Road Lights: Relay, Wiring Diagrams & Best Practices

A relay is an electrically operated switch that lets a low-current trigger circuit (your dashboard switch) control a high-current power circuit (your light bar). Without a relay, all of the current flowing to your lights passes through the switch and the long wire run from the dash to the lights. A 200W light bar draws about 17 amps at 12 volts. Pushing 17 amps through a thin switch wire creates significant voltage drop, heat buildup, and a genuine fire risk — especially at the switch contacts, which are not designed for sustained high-current loads.

With a relay, the switch only carries 0.2 amps to energize the relay coil. The relay then closes a set of heavy-duty contacts that complete the circuit between the battery and the lights on thick, short wires. The result is full voltage reaching the lights (brighter output), cool switch wiring, and a safe, reliable installation that will last for years.

The standard relay for off-road lights is a Bosch-style 12V 40A SPDT (Single Pole, Double Throw) relay with a 5-pin socket. Pin 30 connects to battery positive (through a fuse). Pin 87 connects to the light positive wire. Pin 85 connects to the switch positive wire. Pin 86 connects to ground. Pin 87a is the normally-closed contact and is left unused in most lighting applications.

The basic wiring path for a single light bar or pair of pod lights follows this sequence:

1. Battery positive terminal connects to an inline fuse holder (mount the fuse within 18 inches of the battery). 2. Fuse output connects to relay pin 30. 3. Relay pin 87 connects to the light bar positive wire. 4. Light bar negative wire connects to a chassis ground point (sanded bare metal, star washer, bolt tight). 5. Relay pin 86 connects to a chassis ground. 6. Relay pin 85 connects to the switch output wire. 7. Switch input wire connects to a switched 12V source (ignition-on power) or directly to battery positive through a second small fuse (3-5A) if you want the lights available with the key off.

This creates two independent circuits: the low-current switch circuit (steps 6-7) and the high-current light circuit (steps 1-4). The relay bridges them. When you flip the switch, 12V energizes the relay coil, the contacts close, and full battery voltage flows to the lights through heavy-gauge wire.

Undersized wire is the number one cause of off-road light failures and vehicle fires. Wire gauge must match the current load AND the wire run length. Longer runs need thicker wire because resistance increases with length, causing voltage drop that dims your lights and generates heat in the wire.

For the high-current side (battery to relay to lights), use the following minimums. These assume a single-run length (one way, not round trip) and are based on limiting voltage drop to under 3% at 12V.

After installing lights on dozens of Jeep builds, the same mistakes come up repeatedly. Avoiding these will save you troubleshooting time and prevent dangerous failures.

First, poor ground connections cause more intermittent light failures than any other issue. A ground wire bolted to a painted surface looks connected but has high resistance. Always sand the mounting point to bare metal, use a star washer to bite through any remaining coating, and apply a thin layer of dielectric grease over the finished connection to prevent corrosion.

Second, routing wires through the firewall without a grommet. The firewall is stamped sheet metal with sharp edges. A wire that passes through a drilled hole without a rubber grommet will eventually chafe through its insulation as the engine vibrates, creating a dead short to chassis ground that blows fuses or starts fires. Use a factory grommet pass-through whenever possible, or install a marine-grade cable gland fitting.

Third, daisy-chaining multiple lights off a single fuse and relay. Each light bar or pod pair should have its own relay and fused circuit. Sharing a relay means the combined current of all lights flows through one set of relay contacts. A 40A relay running two 200W bars (34A combined) is operating at 85% of its rated capacity with zero safety margin. Add vibration-induced contact resistance and you get a relay that welds shut or fails open unpredictably.

Fourth, using crimp connectors without proper tools. A ratcheting crimp tool with interchangeable dies produces a gas-tight, mechanically strong crimp every time. Cheap plier-style crimpers deform the connector unevenly, creating a joint that passes current initially but loosens over time as vibration works the wire free. If you cannot invest in a quality crimp tool, solder and heat-shrink your connections instead.