Fog Light Wiring Diagram: Relay, Switch, and Fuse Layout
This is a free printable fog light wiring diagram: download the diagram as SVG or open it and print to paper or PDF.
A fog light wiring diagram shows how to connect auxiliary driving or fog lights to a vehicle using a relay, inline fuse, and dashboard switch, ensuring the lights only operate when the ignition is on.
Auxiliary fog lights draw significant current — a pair of 55W H3 halogen fog lights pulls around 9A at 12V, while LED equivalents of the same output draw 3–4A. Routing this current directly through a dashboard switch would require heavy wiring to the switch, and the switch contacts would carry the full load current, causing premature wear or switch failure. The standard solution is a relay-based circuit.
The wiring circuit uses a four- or five-terminal automotive relay (typically a standard ISO mini relay). The relay coil (terminals 85 and 86) is powered by a low-current switched ignition feed — often tapped from an existing fog light switch output or an ignition-live circuit at the fuse box. When the coil energises, the relay's main contacts (terminals 30 and 87) close, connecting battery positive directly to the fog light pair through a dedicated inline fuse. Terminal 87a is the normally-closed contact (present on 5-pin relays) and is left unused in a basic fog light circuit.
Fog lights are typically wired in parallel between terminal 87 and ground, so each lamp receives full battery voltage regardless of the other's state. Each individual lamp should have its own inline fuse in the positive feed, and a common earth return to a clean chassis ground point.
Legal requirements vary by jurisdiction. In many countries (UK, EU, Australia), rear fog lights are mandatory; front fog lights are permitted but must comply with beam height and aim regulations. A fog light indicator lamp on the instrument cluster is legally required in many regions — this is simply a small LED or incandescent lamp wired in parallel with the relay coil or across the switched feed.
For vehicles with CAN-bus body control modules (BCM), adding aftermarket loads to switched feeds can cause fault codes. In these cases, a relay trigger should be sourced from a true ignition-switched auxiliary output rather than an existing lamp circuit, or a CAN-compatible relay module should be used.
Fog lights are typically wired through a dedicated relay to reduce current draw through the switch and to allow the lights to be cut by the ignition or parking-light circuit. A standard relay-controlled fog light circuit uses a 30 A relay whose coil is triggered by the fog light switch, with battery-direct power feeding through a fuse to the relay's load terminals and then to the lamps. Adding a switch leg is straightforward when you understand the relay pinout and the trigger path. Build and export your fog light diagram for free at circuitdiagrammaker.com.
How to wire fog light wiring diagram
- Plan the circuit and select wire gauges Calculate expected current from your fog lights' wattage divided by 12V. Use a minimum 16AWG (1.5mm²) wire for the relay coil circuit and 12AWG (2.5mm²) or heavier for the main power feed from battery to relay terminal 30 and from terminal 87 to the lights.
- Mount the relay close to the battery or fuse box The relay should be positioned to keep the high-current wire run (terminal 30 to battery, terminal 87 to lights) as short as possible. Mount it in a sheltered location away from heat sources using the relay's mounting tab.
- Connect the relay coil to the switched ignition feed Run a wire from an ignition-live circuit (such as an existing switched fuse in the fuse box) to relay terminal 86. Connect relay terminal 85 to a clean chassis ground. This completes the coil circuit, allowing the relay to switch on when the ignition is on.
- Wire the main power feed to relay terminal 30 Run the heavy-gauge wire from the battery positive terminal through an inline fuse (sized to 125–150% of total lamp current) to relay terminal 30. The fuse must be within 300mm (12 inches) of the battery terminal.
- Connect the fog lights to relay terminal 87 Run wires from relay terminal 87 to each fog light positive terminal, wiring the lights in parallel. Install a separate inline fuse for each lamp in its individual positive feed wire.
- Earth each fog light to a clean chassis ground point Connect the negative terminal of each fog light to bare metal on the vehicle chassis using a ring terminal bolted to a clean, paint-free surface. A poor earth return is the most common cause of dim or flickering auxiliary lights.
- Install the dashboard switch and indicator lamp Wire the dashboard switch in series between the ignition-live feed and relay terminal 86, so the switch controls whether the relay coil can energise. Wire the indicator lamp across the coil supply as described in the FAQ.
Specifications
| System voltage | 12V DC nominal (actual 13.8V–14.4V with alternator charging) |
|---|---|
| Typical halogen fog light wattage | 55W per lamp (H3, H11, or HB4/9006 bulb) |
| Current draw — 2× 55W halogen at 13.8V | ~8A |
| Recommended main fuse | 15A (halogen) / 7.5A–10A (LED) |
| Minimum main feed wire gauge | 12AWG (2.5mm²) for halogen; 18AWG (1.0mm²) acceptable for LED |
| Relay coil current (typical ISO mini relay) | 150–200mA at 12V |
| Relay contact rating | 30A maximum continuous |
Safety warnings
- Always disconnect the vehicle battery negative terminal before starting any wiring work to eliminate the risk of short-circuit sparks near the battery, which can ignite hydrogen gas vented during charging.
- The inline fuse in the main battery feed is the most critical safety component in this circuit — never omit it, and never replace it with a higher rating than specified. Without it, a wiring short could cause a vehicle fire.
- Ensure fog light beam aim complies with local road traffic regulations before driving — improperly aimed fog lights can dazzle oncoming drivers. Many jurisdictions require front fog lights to be aimed below a specific height.
- Do not tap the relay coil trigger from a lamp circuit on a CAN-bus equipped vehicle without first verifying that the BCM does not monitor that circuit for unexpected current draw, which could cause fault codes or system malfunctions.
- All wiring must be protected from chafing where it passes through bodywork or near moving parts using grommets and conduit — bare wire contact with metal edges can cause short circuits and fires.
Tools needed
- Digital multimeter (for testing switched live feed and verifying earth continuity)
- Wire strippers rated for 10–18AWG automotive wire
- Ratchet crimping tool with dies for insulated spade and ring terminals
- Heat gun for heat-shrink insulation
- Drill and appropriate drill bits for mounting holes
- Circuit test lamp or non-contact voltage tester
- Electrical tape and conduit or loom sleeve for wire protection
Common mistakes
- Sourcing the relay coil feed from a permanent live circuit instead of a switched ignition feed, causing the fog lights to remain on when the ignition is off and draining the battery.
- Routing all wiring through the dashboard switch rather than using a relay, causing excessive voltage drop, switch overheating, and premature switch failure under the full load current.
- Using too small a wire gauge for the main power feed — undersized wire has high resistance, causes voltage drop that dims the lights, and can overheat under sustained current draw.
- Earthing fog light housings to a painted surface rather than bare metal, creating a high-resistance earth path that causes dim operation and flickering.
- Omitting the inline fuse on the main feed from the battery, leaving the entire wiring run unprotected against short circuits.
Troubleshooting
- Fog lights do not illuminate when switch is turned on
- Cause: No power reaching relay coil (blown fuse, open switch, or no ignition-live source), or relay contacts not closing, or no power reaching lamps from relay terminal 87 Fix: Measure voltage at relay terminal 86 with ignition on and switch on. If no voltage, trace back through the switch and fuse. If voltage present, check relay operation by measuring terminal 87 voltage. If 87 is live but lamps are dark, check lamp earths and bulb condition.
- Fog lights flicker or are dim
- Cause: High-resistance earth connection at the lamp housing, or undersized wiring causing excessive voltage drop in the supply path Fix: Measure voltage directly at the lamp terminals (positive and negative) with lights on. If voltage is below 11.5V, trace and improve the supply or earth connection. Clean earth connection points back to bare metal.
- Relay clicks but lights do not come on
- Cause: Relay contacts have closed but no power is reaching the lamp side — check the individual lamp fuses and the wire run from relay terminal 87 to the lamp positive terminals Fix: Measure relay terminal 87 voltage with switch on. If live, trace wire from 87 to each lamp fuse, then to each lamp positive. Replace any blown fuses.
Frequently asked questions
Why must fog lights be wired through a relay?
Fog lights draw high current that would overheat a standard dashboard switch if routed directly through it. A relay keeps the high-current path (battery to lights) short and heavy-gauge, while the dashboard switch only energises the relay coil on a low-current signal wire, protecting the switch and reducing voltage drop.
What size fuse should I use for fog lights?
Size the fuse at 125–150% of the expected load current. Two 55W halogen lamps at 12V draw approximately 9.2A, so a 15A fuse is appropriate. For LED fog lights drawing 4A total, a 7.5A or 10A fuse is suitable. Always place the fuse as close to the battery positive terminal as possible.
How do I wire the fog light indicator lamp on the dashboard?
Connect a small indicator lamp (or LED with series resistor) in parallel with the relay coil between the switched positive feed and ground. When the relay coil energises and the fog lights come on, the indicator lamp receives the same signal and illuminates on the dashboard.
What relay do I use for fog lights?
A standard 12V automotive ISO mini relay rated at 30A is sufficient for most halogen or LED fog light installations. The relay has four pins: 85 (coil negative/GND), 86 (coil positive/switched trigger), 30 (common/battery positive), and 87 (normally open/output to lights). Five-pin versions add an 87a normally-closed output.
Do I need to wire fog lights to a feed that turns off with the ignition?
Yes — fog lights should only operate with the ignition on to prevent accidental battery drain and to comply with road regulations in most jurisdictions. Source the relay coil trigger (terminal 86) from an ignition-switched circuit, not a permanent live, so the lights extinguish automatically when the ignition is off.
How do you wire a fog light switch into a relay circuit?
Connect the fog light switch between a switched 12 V source (usually the parking-light positive) and relay pin 86. Relay pin 85 goes to chassis ground. Battery positive feeds through a fuse to relay pin 30, and pin 87 connects to the fog light positive wire. This way the switch only carries the small relay coil current, while the relay handles the full lamp load. The fog light ground wire returns directly to the vehicle chassis.
How do you wire fog lights with a relay?
Use a standard single-pole 30 A automotive relay. Run a fused wire from the battery (or fusebox) to pin 30, then connect pin 87 to the fog lamp positive wire. Wire pin 86 to a switched 12 V source (such as the parking lights positive), and pin 85 to ground. When the parking lights are on, the relay coil energises, closing the circuit and powering the fog lights. This protects the switch and reduces voltage drop to the lamps.
Related diagrams
- 1 way light switch wiring diagram
- 2 way light switch wiring
- 2 wire led light wiring diagram
- 2 wire light fixture wiring diagram
- 2 wire light switch
- 3 gang light switch wiring