How to Wire Motorcycle Auxiliary Lights: Relay & Fuse Guide
A complete wiring walkthrough for adding passing lamps, fog lights or driving lights to a cruiser or touring motorcycle.
Quick answers
- Always use a relay. Auxiliary lights should draw power directly from the battery through a relay, not from the headlight or handlebar switch circuit.
- Fuse the feed at the battery. Place an inline fuse within a few inches of the battery positive terminal.
- Size the fuse to the wire, not the lamp. A pair of 18W LEDs draws about 3A at 12V; a 5A fuse on 16 AWG wire is a sound combination.
- Trigger from a switched source. Use an ignition-switched accessory wire or the high beam wire so the lights cannot be left on with the engine off.
- Leave slack at the forks. Wiring must allow full steering lock in both directions without tension.
- Disconnect the battery negative first, and reconnect it last.
Do motorcycle auxiliary lights need a relay?
Yes. Auxiliary lights should be wired through a relay so that the lamp current comes straight from the battery, while a small trigger current does the switching. Without a relay, the full load runs through your handlebar switch or the headlight circuit — components never designed to carry it.
This holds even for low-draw LEDs. The reason is not only current. On many bikes made from the mid-2010s onward, lighting circuits are monitored by the vehicle's electronics, and splicing an extra load into a monitored circuit can produce warning lights or unpredictable behaviour. A relay isolates your accessory from the bike's own wiring, and a relay coil draws only around 150 mA — small enough to trigger safely from almost any switched source.
The other benefit is diagnostic. When something stops working, a relay circuit has four clearly separable points to test. A spliced-in circuit has none.
What parts do you need?
A crimping tool that actually crimps — not pliers — makes the difference between a joint that survives and one that fails in a year. Vibration finds every bad connection eventually.
What fuse size do auxiliary lights need?
Calculate the current draw, then choose a fuse rated roughly 1.5 times that figure, and make sure the wire can carry the fuse rating. Current in amps equals total wattage divided by voltage.
A fuse protects the wire, not the lamp. Fitting a 20A fuse on 18 AWG wire means the wire becomes the fuse — and it fails by melting inside a loom rather than by blowing safely. If in doubt, use heavier wire and a fuse matched to the load.
Also check your charging system has the headroom. A pair of 18W LEDs is trivial. A pair of 55W halogens on an older bike already running heated grips is not.
Relay wiring diagram explained
A standard automotive relay has four numbered terminals. On a 5-pin relay, ignore terminal 87a — it is the normally-closed contact and is not used here.
Both lamps' negative wires go to ground. Terminals 85 and 86 can be swapped on a standard relay without a diode, but keep to the convention above so the next person to work on the bike is not guessing.
Where should the trigger wire connect?
The trigger must come from a source that is only live when the ignition is on. Otherwise the lights can be left burning with the engine off, and a flat battery is the best case. Three common choices:
- Accessory circuit — many touring bikes have a factory accessory feed under the seat or behind a side cover. Cleanest option where available.
- High beam wire — the auxiliary lamps then come on with high beam and off with low. Many jurisdictions require auxiliary driving lamps to behave this way, so check before choosing something else.
- Running light or tail light feed — live whenever the ignition is on. Combine with a handlebar switch for independent control.
Because the trigger only carries the relay coil current, tapping it does not meaningfully load the source circuit. Use a proper T-tap or, better, solder and heat-shrink. Avoid the small plastic wire-piercing connectors sold in cheap kits — they cut strands and admit water.
Step-by-step installation
- Disconnect the battery negative. Do this first, every time, before touching anything electrical.
- Mount the lamps and set rough aim. Fine aiming comes later, but get them roughly level now so you know how much wire each side needs.
- Choose a relay location. Under the seat or behind a side cover, away from exhaust heat and direct spray. Mount it with the terminals facing down so water cannot sit in them.
- Run the feed wire from the battery positive to relay terminal 30, with the inline fuse holder within a few inches of the battery. Leave the fuse out for now.
- Run the output wire from terminal 87 forward to the lamps, joining both positives. Route alongside existing looms where possible.
- Ground terminal 85 and both lamp negatives. Battery negative is the reliable choice; a frame bolt works only if you scrape to bare metal, since paint and powder coat are insulators.
- Connect the trigger to terminal 86, through your handlebar switch if fitting one.
- Check fork clearance. Turn the bars fully left, then fully right. The wiring must have slack at both extremes and must not rub, pinch, or pull.
- Insert the fuse, reconnect the battery, and test. Check the lamps switch on and off correctly, and confirm nothing else on the bike has changed behaviour.
- Tidy the loom. Split loom or spiral wrap, zip ties every few inches, dielectric grease in every connector, and a final check that nothing sits against the exhaust.
Once everything works, aim the lamps properly against a wall before your first night ride. That procedure is covered in our 4.5 inch auxiliary light fitment guide, along with beam pattern and housing selection.
Common wiring mistakes
Frequently asked questions
Can I wire auxiliary lights without a relay?
It is possible with very low-draw LEDs, but not advisable. Without a relay the full lamp current passes through a switch and wiring not rated for it, and on bikes with electronically monitored lighting circuits the extra load can produce warning lights. A relay costs a few dollars and removes both risks.
What gauge wire do motorcycle auxiliary lights need?
16 AWG stranded automotive wire handles the battery feed and lamp output for any normal auxiliary light load, and 18 AWG is sufficient for the relay trigger. Heavier wire than strictly necessary costs little and runs cooler. Never use solid-core household wire, which work-hardens and cracks under vibration.
Should auxiliary lights come on with high beam or low beam?
Many jurisdictions require auxiliary driving lamps to operate only with high beam and to switch off with low beam, while front fog lamps may be permitted with low beam. Because rules differ by country and by state, check your local vehicle lighting regulations before deciding which wire to trigger from.
Where should I mount the relay?
Mount the relay under the seat or behind a side cover, away from exhaust heat and direct road spray, with the terminals facing downward so water cannot collect in them. Secure it so it cannot vibrate loose, and keep it accessible enough to swap without removing bodywork.
Why do my auxiliary lights flicker?
Flicker is almost always a ground fault or a poor connection rather than a faulty lamp. Check the ground point first — paint or powder coat under a ring terminal is the most common cause. Next check every crimp, then confirm the lamp's driver is rated for your charging system voltage, which sits above 14V once the engine is running.
Do I need a separate switch?
Not necessarily. If you trigger the relay from the high beam wire, the lamps follow high beam automatically with no extra switch. A handlebar switch is worth adding if you want the lights available independently — for daytime visibility, for example — but it adds another connection point to keep sealed.
A note on scope. This guide covers a standard relay-based auxiliary circuit. If your motorcycle uses a CAN bus system, has an aftermarket ignition or alarm, or you are unsure about any part of the procedure, have the work done by a shop. Electrical faults on a motorcycle are not forgiving.
About this guide
Written by Handxen, a manufacturer of 4.5 inch PAR36 LED lamps in spot and flood beam configurations. Wiring practice here reflects installation feedback from riders fitting auxiliary lamps to cruisers and touring motorcycles. Where a requirement varies by model or by jurisdiction, we have said so rather than generalising.
Harley-Davidson®, Indian Motorcycle®, and all other manufacturer names are registered trademarks of their respective owners. Handxen is not affiliated with, endorsed by, or sponsored by any of these companies. Always verify vehicle lighting requirements in your jurisdiction before installation, and consult a qualified technician if you are unsure about any electrical work.