LED Light Bar Wiring Guide: 12V and 24V Relay Diagrams & Wire Sizing
You flip the switch and your new light bar glows dimly. Or the switch is hot to the touch. Or the fuse blows instantly.
Here’s what happened: You wired it like a dome light instead of the high-current accessory it actually is. Light bars pull serious amperage—wire them wrong and you’ll melt switches, dim your lights, or start a fire.
This guide shows you exactly how to wire LED light bars safely, why most setups need relays, and how to size wire so your lights work at full brightness. By the end, you’ll know what wire gauge you need, how to wire a relay correctly, and how to avoid the 5 mistakes that cause 90% of light bar failures.
Already installed but having issues? Jump to Troubleshooting.
Understanding Light Bar Electrical Specs
Before cutting wire, you need to know what you’re working with.
Reading the Wattage Rating
Every LED light bar has a wattage rating. A typical 50" bar might be rated at 240W. Some brands advertise theoretical maximums—ignore those. Look for actual power consumption.
Here’s what matters:
Watts don’t kill switches and melt wires. Amps do.
Convert watts to amps:
Amps = Watts ÷ Volts
12V system example:
- 240W light bar ÷ 12V = 20A
24V system example:
- Same 240W light bar ÷ 24V = 10A
See the difference? A 24V system draws half the current for identical light output. That’s why RVs and marine setups use 24V—smaller wire, less voltage drop, better efficiency. Learn more about system voltage selection in our Wire Sizing Basics guide.
Actual vs Advertised Draw
Manufacturers advertise theoretical maximums. Real-world draw runs typically 10% higher.
A 240W light bar might pull 22-23A in actual use instead of the calculated 20A. Always add 10% to your calculations for safety. For that 240W bar, plan for 22A.
LED Inrush Current (The Truth)
Modern LED light bars use integrated drivers with soft-start circuits. Unlike halogen or HID lights, quality LED bars don’t have significant inrush spikes.
The 125% fuse sizing rule isn’t about inrush—it’s a heat protection standard for continuous loads from NEC and ABYC electrical codes. We’ll cover proper fuse sizing in the component selection section. See our Fuse Sizing Guide for detailed explanation.
Relay Wiring vs Direct Switch (This Matters)
Here’s where most installations go wrong.
Do I Need a Relay for a Light Bar?
If your light bar pulls more than 10A, you need a relay.
Your cab switch isn’t designed to handle sustained high current. Typical rocker switches are rated for 15-20A peak—that’s momentary capacity, not continuous duty. Run 20A through it for 10 minutes and you’ll cook the contacts.
I’ve seen switches melt into dashboards. The plastic housing literally deforms and fuses to the dash panel.
How a Relay Works
Here’s the standard automotive relay configuration:
BATTERY(+) ────[FUSE]────┬──── Relay Pin 30
│
[Relay]
│
Relay Pin 87 ────── Light Bar (+)
BATTERY(+) ────[Switch]──── Relay Pin 86
BATTERY(-) ─────────────────Relay Pin 85
Light Bar (-) ────────────── GROUND (chassis)
A relay is a remote switch for your light bar. Here’s how it works:
- The switch in your cab controls a tiny signal circuit (less than 2A)
- That signal energizes the relay coil (pins 85 and 86)
- The relay contacts close, connecting full battery power (pin 30) directly to the light bar (pin 87)
- Your cab switch only handles the baby 2A control current—it stays cool
The heavy current flows through relay contacts in the engine bay where there’s airflow and heat dissipation. Your dashboard stays safe.
Standard automotive relay pins (Bosch-style):
- Pin 30: Power IN (from battery through fuse)
- Pin 87: Power OUT (to light bar)
- Pin 86: Coil positive (from switched power)
- Pin 85: Coil negative (to ground)
Note: Pins 85 and 86 are just the relay coil terminals—polarity doesn’t matter. Most installers wire 86 to switched power and 85 to ground, but reverse works identically.
Some relays include pin 87a (normally closed contact)—ignore it for this application.
Visual Relay Wiring Diagram
The Wire Solved calculator can generate a detailed relay wiring diagram for your specific light bar installation:
- Enter your light bar’s amp draw and wire run length
- Open “Advanced Settings” at the bottom of the form
- Check “Use relay circuit (recommended for high-current devices)”
- Click “Calculate Wire Size”
- Click “View Diagram” to see a complete relay circuit diagram
The diagram shows both circuits clearly labeled:
- Control Circuit (thin wire from switch to relay coil, ~0.2A)
- Load Circuit (thick wire carrying full light bar current)
This visual reference makes it easy to see which wire goes where, with proper wire gauges, fuse placement, and relay terminal connections (30, 85, 86, 87) all labeled.
When Direct Wiring is Safe
Small light pods under 100W on a 12V system? You can skip the relay. A 40W pod pulls about 3.5A—well within switch capacity.
But relays cost $8. Using one means you never have to think about switch ratings, and you can upgrade to bigger lights later without rewiring. For $8, use one.
Step-by-Step Wiring Instructions
Let’s wire this correctly.
Planning the Route
Skip this planning step and you’ll buy the wrong wire gauge. I’ve seen $200 light bars perform like $40 bars because of undersized wire. Five minutes of measurement saves you from rewiring later.
Measure the actual path the wire will take. Don’t measure straight-line distance. Follow the route:
- Battery to firewall pass-through
- Along door jam or headliner
- Across roof or front of hood
- To light bar mounting location
Add 10% for slack and service loops. Wire that’s too tight pulls loose when doors slam or suspension articulates.
Avoid these routing paths:
- Sharp metal edges (will cut through insulation over time)
- Moving parts (steering column, suspension components, hood hinges)
- Exhaust components (heat melts insulation)
- Water collection areas (door sills, cowl drains)
Measuring Your Wire Run
CRITICAL: Wire Solved calculator wants ONE-WAY distance.
The calculator automatically doubles your input for the complete circuit (power wire + ground wire return path). If your battery is in the engine bay and the roof-mounted light bar is 18 feet away, enter 18 feet—not 36 feet.
Component Selection
Here’s what you need for a typical relay-controlled installation:
Wire: Stranded copper automotive wire with 75°C minimum insulation rating. Size it with Wire Solved based on your specific amperage and wire length. If you’re new to wire sizing, start with our Wire Sizing 101 guide to understand AWG ratings and ampacity.
Relay: 40A automotive relay rated for continuous duty (check datasheet—intermittent ratings are higher than continuous). Get a Bosch-style relay with weatherproof socket. For engine bay mounting above 70°C ambient, derate by 20-30%.
Switch: Rocker or toggle switch, 20A rated minimum. LED-illuminated switches make it easier to find at night.
Fuse: Size at 125% of continuous amp draw, rounded up to next standard fuse size. For a 20A load, that’s 25A. For intermittent loads, you can use 1:1 sizing. See our Fuse Sizing Guide for details.
Fuse Holder: Inline ATC/ATO holder for loads under 30A. ANL holder for larger applications.
Connectors: Ring terminals sized for your wire gauge. Use a ratcheting crimp tool and heat shrink every connection.
Protection: Split loom or wire wrap for any exposed sections under the hood, through the firewall, or along the roof.
Installation Sequence
Step 1: Mount the light bar
Get it bolted where you want it. Route the light bar’s pigtail wire to where you’ll mount the relay (engine bay).
Step 2: Pick a relay location
Mount the relay near the battery, not near the light bar. You want short, thick wire where all the current flows.
The relay should be:
- In the engine bay (good airflow for heat dissipation)
- Away from exhaust manifolds and turbochargers
- Protected from direct water spray
- Accessible if you need to swap it
Step 3: Run the main power wire
From battery positive terminal → fuse → relay pin 30. This is your heavy-gauge wire sized by Wire Solved.
Fuse placement is critical: Install the fuse within 18" of the positive battery terminal. If the wire shorts anywhere between the terminal and fuse, nothing protects it. That’s how vehicle fires start.
Step 4: Wire from relay to light bar
Relay pin 87 → light bar positive terminal. Use the same heavy-gauge wire as the battery-to-relay section.
Route through factory firewall grommets. Use split loom where wire is exposed under the hood. Secure every 12" with zip ties so it can’t flop around and chafe against sharp edges.
Step 5: Run the switch control wire to cab
Battery positive → switch → relay pin 86. This can be smaller wire (14-16 AWG) because it only carries the relay coil current (under 2A).
The switch wire can follow existing harnesses through the firewall. It doesn’t carry the main current load.
Step 6: Ground the relay and light bar
- Relay pin 85: Connect to chassis ground
- Light bar negative: Connect to chassis ground
Ground wire sizing matters: Use the same gauge wire for ground as you used for power. For loads over 30A, ground to engine block or chassis point near the battery to minimize return path resistance.
Ground connection quality:
- Find clean metal (no paint, rust, powder coating, or oil)
- Bolt through with star washer
- Sand paint off the mounting spot
- Verify connection won’t vibrate loose
A poor ground causes more problems than undersized power wire. Learn more about proper grounding techniques.
Step 7: Double-check everything BEFORE connecting battery
- Are fuse connections tight?
- Is the relay in the correct orientation?
- Are grounds clean and torqued?
- Is anything touching exhaust or moving parts?
- Are all crimp connections heat-shrunk?
Step 8: Connect battery and test
Connect the fuse last. Flip the switch. Lights should come on immediately. If smoke appears, disconnect battery and trace your error.
Testing Procedure
No-load test:
- Lights turn on immediately
- No flickering or dimming
- No buzzing or clicking from relay
- Switch barely gets warm
Load test (run for 10 minutes):
- Check wire near connections (should be slightly warm, not hot)
- Check switch (should be barely warm)
- Check relay (can get warm, but not burning hot)
- Measure voltage at light bar vs battery (should be within 0.6V)
If anything gets uncomfortably hot to touch, you have a problem—either undersized wire or a bad connection creating resistance.
Multiple Light Bar Setups
Multiple light bars require different wiring approaches depending on whether you want independent or ganged control.
Independent Control
Run each bar on its own relay and switch. This gives separate control:
- Main roof bar on one switch
- Bumper pods on another
- Ditch lights on a third
Each circuit gets its own wire sizing calculation. Calculate them separately in Wire Solved.
Ganged Control (One Switch, Multiple Bars)
Want one switch to control multiple bars? Gang the relays together.
Each light bar gets its own relay and main power wire, but they share the same switch signal wire. Flip the switch and all relays close simultaneously.
Why separate relays instead of one big relay?
If one light bar fails, the others keep working. If you add more lights later, you don’t rewire everything. Plus, standard 40A relays are cheaper and more available than single high-current relays rated for 80A+.
Calculating Combined Loads
Running two 20A bars from the same fuse block? Account for combined load at the fuse block.
- Bar 1: 20A
- Bar 2: 20A
- Combined: 40A total at fuse block
The wire from fuse block to battery needs to handle 40A. Use Wire Solved to size it based on that distance and current.
Routing Best Practices
Avoiding Pinch Points
Don’t run wire where it can get pinched:
- Not between body panels
- Not under battery hold-downs
- Not anywhere doors or hoods close
- Not in suspension travel paths
I once saw someone route wire over the valve cover. It melted onto the exhaust manifold within a week. Use factory harness paths—they’re proven safe.
Loom Protection
Use split loom or wire wrap for exposed sections:
- Under the hood (heat and vibration)
- Through the firewall (sharp metal edges)
- Along the roof (UV exposure)
- Anywhere it might contact moving parts
Black split loom is cheap. Use it generously.
Weatherproofing Connections
Water intrusion corrodes connections. Seal everything:
- Heat shrink over crimped terminals
- Dielectric grease inside connectors
- Weatherproof relay sockets (not open-back types)
If you live where roads get salted in winter, be extra paranoid about sealing. Salt accelerates corrosion dramatically.
Securing to Frame
Zip-tie the wire every 12-18" to prevent flopping and chafing. Don’t overtighten—you’ll crush the insulation.
Use adhesive-backed zip-tie mounts on clean surfaces. The peel-and-stick mounts work well inside door jams and along the headliner.
Service Loops
Leave slack at both ends. If you need to disconnect the light bar later for repairs or vehicle swaps, you’ll appreciate having 6" of extra wire to work with.
Common Scenarios with Wire Solved
Let’s size wire for real installations.
Scenario 1: 50" Roof-Mounted Bar (12V)
Device: Rigid Industries E-Series 50" (240W actual draw) System: 12V Actual Draw: 240W ÷ 12V = 20A (add 10% safety margin = 22A) Wire Run: 18 feet one-way (battery in engine bay to roof) Load Type: Intermittent (not running constantly)
Wire Solved Inputs:
- Amp Draw: 22A
- Wire Length: 18 feet (one-way distance)
- System Voltage: 12V
- Voltage Drop Tolerance: 3% (tighter tolerance for better performance)
- Load Type: Intermittent
Wire Solved Results:
- Wire Gauge: 10 AWG (minimum for ampacity is 12 AWG, but upgraded to meet 3% voltage drop)
- Fuse Size: 25A (22A × 1.0 for intermittent load, rounded up to standard size)
- Voltage Drop: 2.8% with 10 AWG (would be 4.4% with 12 AWG)
Why the upgrade? 12 AWG handles the current thermally (rated for 20A), but at 18 feet you’d lose 4.4% voltage. That’s 0.53V you’re throwing away. Your $200 light bar would perform like a $150 bar. The 10 AWG upgrade keeps voltage drop under 3% for full brightness.
Shopping List:
- 40 feet of 10 AWG red stranded wire (power and ground, one-way × 2 + 10% slack)
- 25A ATC fuse and inline fuse holder
- 40A automotive relay with weatherproof socket
- 20A rocker switch
- 10× ring terminals sized for 10 AWG wire
- 25 feet split loom (1/2" diameter)
- 20× zip ties
Calculate this scenario in Wire Solved and see how changing wire length affects the recommendation.
Scenario 2: Dual Pod Setup (12V)
Device: Two 50W pods (100W total) System: 12V Actual Draw: 100W ÷ 12V = 8.3A (round to 9A with safety margin) Wire Run: 12 feet one-way (bumper-mounted) Load Type: Intermittent
Wire Solved Inputs:
- Amp Draw: 9A
- Wire Length: 12 feet
- System Voltage: 12V
- Voltage Drop Tolerance: 5%
- Load Type: Intermittent
Wire Solved Results:
- Wire Gauge: 14 AWG
- Fuse Size: 10A
- Voltage Drop: 3.2%
At this current level, skipping the relay is technically safe. But for $8, use one anyway. It protects your cab switch from wear and lets you upgrade to bigger pods later without rewiring.
Scenario 3: Same Bar on 24V System (RV)
Device: Same 240W light bar System: 24V (RV or commercial truck) Actual Draw: 240W ÷ 24V = 10A Wire Run: 18 feet Load Type: Intermittent
Wire Solved Inputs:
- Amp Draw: 10A
- Wire Length: 18 feet
- System Voltage: 24V
- Voltage Drop Tolerance: 3%
- Load Type: Intermittent
Wire Solved Results:
- Wire Gauge: 14 AWG (vs 10 AWG needed for 12V—much thinner!)
- Fuse Size: 15A
- Voltage Drop: 2.3%
Let’s see why 24V is more efficient:
Here’s the voltage drop calculation for 14 AWG wire (resistance: 0.00253 Ω/ft):
Voltage drop = 10A × 0.00253 Ω/ft × 36 ft (round trip) = 0.91V
On 24V system: 0.91V ÷ 24V = 3.8%
On 12V system: 0.91V ÷ 12V = 7.6%
You’re pushing identical 240W of light, but current is half. That means thinner wire (14 AWG vs 10 AWG), smaller fuse, and less voltage loss over the same distance. This is why large RVs use 24V systems—higher efficiency for high-power accessories over long wire runs.
What’s in a Complete Light Bar Wiring Kit?
Pre-made wiring kits are convenient but often include incorrect wire gauge for your specific installation. Here’s what a complete kit should contain:
- Power wire (sized correctly for YOUR wire length and amp draw)
- Ground wire (same gauge as power wire)
- Switch control wire (14-16 AWG is fine)
- 40A relay with socket
- Inline fuse holder and correct fuse
- Switch (20A rated)
- Ring terminals
- Split loom
- Zip ties
Better approach: Use Wire Solved to calculate exactly what gauge you need, then buy components separately. You’ll get correct wire sizing for your specific installation instead of generic “one size fits most” wire that’s often undersized.
Troubleshooting
Why Are My Light Bars Dimmer Than Expected?
Symptom: Light bar turns on but doesn’t seem as bright as it should be.
Diagnosis: Voltage drop. Wire is too small for the distance.
Fix:
- Measure voltage at battery with engine running: Should be 13.8-14.4V
- Measure voltage at light bar terminals with lights running: Should be within 0.6V of battery voltage
- If it’s more than 0.6V lower, your wire is undersized or you have a bad connection
Upgrade to the next larger wire gauge. Use Wire Solved to find the correct size for your wire length.
Intermittent Operation
Symptom: Lights work sometimes but cut out when you hit bumps or make turns.
Diagnosis: Loose connection or bad ground.
Fix:
- Check all crimp connections (tug firmly—they shouldn’t pull apart)
- Check ground connection (is it tight? is the metal clean and paint-free?)
- Check fuse (is it fully seated in the holder?)
- Check relay socket (are pins making good contact?)
Most intermittent problems trace to grounds. Clean the ground point with sandpaper, use a star washer, and torque it properly.
Blown Fuses
Symptom: Fuse blows immediately when you turn lights on.
Diagnosis: Either a short circuit or fuse is undersized.
Fix:
- Disconnect light bar and try again. If fuse doesn’t blow, the light bar has an internal short
- If fuse still blows, you have a short in wiring between battery and light bar
- Inspect for pinched wires or bare copper touching metal
- Check that positive and negative wires aren’t touching anywhere
Never “solve” a blown fuse by installing a bigger fuse. That’s how vehicle fires start. Find and fix the underlying problem.
Relay Clicking But No Light
Symptom: You hear the relay click when you flip the switch, but lights don’t come on.
Diagnosis: Relay is getting the control signal but can’t pass main power.
Fix:
- Check that pin 30 has battery voltage (measure with multimeter)
- With switch on, check that pin 87 has voltage when relay is energized
- If pin 30 has power but pin 87 doesn’t when energized, the relay contacts are failed—replace relay
Relays are wear items. Keep a spare in the glove box.
Switch Getting Hot
Symptom: The rocker switch in your cab is uncomfortably hot to touch after a few minutes.
Diagnosis: You didn’t use a relay. Your switch is handling full current to the light bar.
Fix:
Add a relay immediately. Before your switch melts into the dash.
The switch should only handle the 2A relay coil current. If it’s getting hot, it’s carrying the full 20A+ load directly to your light bar. That’s a fire waiting to happen. Wire it correctly with a relay.
5 Easy Mistakes (And How to Avoid Them)
These mistakes are common—experienced installers make them too. Here’s how to avoid the most frequent problems.
Mistake #1: Running Full Power Through Cab Switch
The problem: Connecting battery → switch → light bar directly, no relay.
What happens: Switch contacts overheat and fail. Even switches “rated for 20A” are rated for momentary peaks, not continuous duty. You’ll melt it.
How to avoid it: Use a relay for any light bar pulling over 10A. The switch controls the relay coil, not the light bar directly.
Mistake #2: Forgetting to Buy Enough Wire
The problem: Measuring 18 feet to the light bar and buying 18 feet of wire.
What happens: You run out of wire because you need 18 feet for positive AND 18 feet for negative ground wire.
How to avoid it: Buy double the one-way distance plus 10% for slack. For an 18-foot run, buy 40 feet minimum.
Note: Wire Solved automatically accounts for round-trip distance in voltage drop calculations. But you still need to buy wire for both power and ground paths.
Mistake #3: Fuse at the Light Bar Instead of Battery
The problem: Running unfused wire from battery and putting the fuse near the light bar.
What happens: If the wire shorts anywhere between battery and light bar, nothing stops it from melting and starting a fire.
How to avoid it: Install fuse within 18" of the positive battery terminal. This protects the entire wire run from the power source.
Mistake #4: Using Undersized Wire Because “It’s Easier to Route”
The problem: Running 16 or 18 AWG wire from battery to light bar because thinner wire is easier to snake through tight spaces.
What happens: Voltage drop kills brightness. Wire overheats. Light bar performs poorly despite being quality equipment.
How to avoid it: Size your main power wire correctly using Wire Solved. The switch control wire can be small (14-16 AWG), but the main power run needs proper sizing. Our How to Use Wire Solved guide walks you through the calculator step-by-step.
Mistake #5: Poor Crimps
The problem: Using a cheap crimp tool or not crimping firmly enough.
What happens: High resistance at crimp point = heat = melted connector = intermittent operation.
How to avoid it:
- Use a proper ratcheting crimp tool (not pliers or wire strippers)
- Strip wire to correct length for the terminal barrel
- Crimp in the designated crimp zone
- Tug-test every connection—it shouldn’t pull apart with firm pressure
- Heat shrink over every crimp for protection and strain relief
What’s Next?
- Wire Sizing 101 - Understanding AWG, ampacity, system voltage, and the 3-5% voltage drop rule
- Voltage Drop Explained - Why wire length matters for long roof-mounted runs
- Fuse Sizing Guide - Sizing the relay circuit and main power fuse correctly
- How to Use Wire Solved - Step-by-step calculator walkthrough