How to wire motorcycle auxiliary lights: relay, fuse and switch

Most LED lights will run off the bike's existing wiring. The question is whether they should. Here's where the line sits, and how to do it properly.

We sell motorcycle driving lights that draw anywhere from 16W to 120W, and the wiring question comes up with almost every one. The good news is that LED lights pull far less current than the halogen spots they replaced, so a lot of old advice about heavy cable and big relays is out of date. The bad news is that under-specced wiring still melts, and a light that flickers at idle gets blamed on the light when the real problem is a bad earth.

Do you need a relay?

A relay is a switch that uses a small current to control a big one. It lets the lights draw straight from the battery through short, thick cable, while the switch on your bars only handles the current needed to flip the relay, usually well under half an amp.

There's a second reason to use a relay even below those numbers. On modern bikes the tail light and licence plate circuits are monitored, and tapping them for a few amps can throw a warning light or a CAN bus error. A relay with its own battery feed avoids touching those circuits at all.

Wire and fuse sizes

Current in amps is watts divided by volts. At a running voltage of about 13.5V rather than a nominal 12V, the numbers come out slightly lower, which gives you a little margin.

Those sizes assume a run of about three metres from battery to light, which covers most bikes. Double the length and you need to go a size up, because voltage drop over a long thin cable shows up as dim lights and a hot fuse. The fuse goes as close to the battery as you can get it, within about 15 cm. Its job is to protect the cable from a short, not the light.

Wiring it, step by step

1. Disconnect the negative battery terminal. This is the step people skip, and it's the one that matters.
2. Fit the fuse holder on the positive cable, right at the battery. Don't connect it yet.
3. Run that cable to the relay's pin 30, following existing loom runs and staying clear of the exhaust, the steering head and any suspension movement.
4. Take the relay's pin 87 to the positive on each light. Join the two positives together in one splice rather than running two separate cables back to the relay.
5. Earth the lights. Use a ring terminal on a clean, bare metal point on the frame, or on the battery negative. Painted frame is not an earth.
6. Connect relay pin 86 to a switched source, so the lights can only come on with the key. This is the small-current side of the circuit.
7. Connect relay pin 85 to a good earth, or to the handlebar switch if you want the switch to control them.
8. Reconnect the battery and test before you cable-tie anything. Refit the seat and panels last.

If you're fitting the 120W combo with its wireless switch, the last two steps change. The receiver handles the switching, so no wire runs up to the bars, but the power feed still needs the fused line and the relay.

Where to take switched power

You want a circuit that is live only with the key in. The rear brake light feed and the licence plate light feed are the usual candidates, since they're easy to reach and easy to identify with a multimeter or a test light.

What you don't want is a permanent live. A light left on will flatten a bike battery overnight, and a 120W light will do it in a couple of hours. Also avoid the headlight circuit itself. On many bikes that's the heaviest load already on the loom, and adding to it means the headlight and the auxiliaries both dim together.

Mistakes we see most

• Earthing to paint. The symptom is dim lights or one light flickering. Sand the point back to bare metal, then cover it with grease or paint once the ring terminal is tight.
• Scotchlock connectors in the weather. They work for a few months, then the moisture gets in. Solder and heat shrink, or crimp and heat shrink, and use the sealed type outdoors.
• Cable run too close to the exhaust or the header. Route along the frame rails instead of across the engine.
• No slack at the steering head. Leave a loop. A cable pulled tight at full lock will break the first time you turn the bars fully.
• Fuse at the wrong end. A fuse next to the light protects nothing between the battery and that point.

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