How to Wire Work Lights: 12V and 24V with a Relay and Switch

To wire work lights on a 12V or 24V tractor, run a fused feed from the battery positive to pin 30 of a relay, take pin 87 out to the lamps, put a cab switch on the relay coil feed to pin 86, earth pin 85, and earth each lamp back to the battery negative or a clean chassis point. The switch carries a fraction of an amp, the relay carries the lamp current.

That is the whole circuit in one sentence. The rest of this guide covers each part properly: why the relay is there, what each relay pin does, how to size the cable for the current and the length of the run, how to pick the fuse, which switch to use, the step-by-step wiring for 2 common layouts, what changes on a 24V machine, and the faults that turn up most often after a DIY install.

What a Work Light Wiring Circuit Is Made Of

A work light wiring circuit is the path that takes current from the battery, through a fuse and a relay, to the lamp, and back to the battery negative. It has 6 parts: the battery, a fuse close to the battery, a relay, a switch in the cab, the work lamps, and the earth return.

The circuit is really 2 circuits sharing one relay:

• The load circuit carries the lamp current. It runs battery positive, fuse, relay pin 30, relay pin 87, lamp, earth. This side uses heavier cable.
• The control circuit carries the relay coil current, usually well under half an amp. It runs from a switched supply, through the cab switch, to relay pin 86, through the coil, and out of pin 85 to earth. This side can use light cable.

Keeping the 2 sides separate is the point of the design. The heavy current stays on a short, direct route outside the cab, and the only thing that goes through the bulkhead to the dashboard is a thin switch wire.

Why Work Lights Need a Relay

Work lights need a relay because the combined current of a set of lamps is often more than a dashboard switch and the cab wiring behind it are built to carry, and because a long run through a switch loses voltage the lamps need.

Current is power divided by voltage. A 40W LED work lamp draws about 3.3A at 12V. Four of them draw about 13A. A pair of 55W halogen work lamps draws about 9A. Many small rocker and toggle switches are rated somewhere around 10A to 20A at 12V, but that rating assumes a good connection and cool running. Run near the limit for hours in a warm cab and the contacts pit, heat up and eventually fail.

A relay fixes 3 things at once:

• Switch life. The switch only carries the relay coil current, typically 0.1A to 0.2A on a standard 12V automotive relay, so it runs cold and lasts.
• Voltage at the lamp. The heavy cable can run the shortest route from battery to relay to lamp instead of detouring through the dashboard, so less voltage is lost on the way.
• Cab loom protection. The factory wiring behind the dash is not asked to carry extra load, which matters on modern tractors with CAN-bus electronics behind every panel.

A single small LED work lamp drawing 2A to 3A can run through a suitably rated switch without a relay, and some compact lamps come with an integral switch. Anything more than that, or anything halogen, should go through a relay. For the wider question of how much extra load a tractor can take, see adding extra lights to your tractor without overloading the circuit.

Relay Pins Explained: 30, 85, 86, 87 and 87a

Automotive relay terminals are numbered to the German standard DIN 72552, and the numbers are moulded next to each spade on the relay base. A 4-pin relay has pins 30, 85, 86 and 87. A 5-pin changeover relay adds pin 87a.

Pin	Function	Connects to	Suggested cable colour
30	Common contact, main power in	Battery positive, through the main fuse	Red (heavy)
87	Normally open output, live when the relay is on	Work lamp positive feed	Red or brown (heavy)
86	Coil positive	Output of the cab switch	Any light-gauge colour, for example green
85	Coil negative	Chassis earth	Black (light)
87a	Normally closed output, live when the relay is off (5-pin relays only)	Leave empty for a simple work light circuit	Not used

When the switch sends power to pin 86, current flows through the coil to earth at pin 85. The coil becomes a small electromagnet and pulls the internal contact across from 87a (if fitted) to 87. Pin 30 is now connected to pin 87 and the lamps light. When the switch opens, a spring pulls the contact back and the lamps go off.

On most relays the coil has no polarity and 85 and 86 can be swapped. Some relays have a diode or a resistor across the coil to absorb the voltage spike when the coil switches off. On those, polarity matters: 86 must be positive and 85 negative, or the diode can short and blow the switch fuse. If the relay has a diode symbol printed on its case, follow the pin numbers exactly.

A 5-pin relay works in a 4-pin circuit if you leave 87a empty. The extra pin is useful when you want one set of lamps on while another is off, for example rear work lamps that switch over to a separate reversing set.

Relays are rated by contact current, commonly 20A, 30A or 40A, and by coil voltage. Size the relay comfortably above the lamp load. A 30A relay is the usual choice for 2 to 4 LED lamps. Browse 12V relays for standard and changeover types with matching bases.

Cable Size by Current and Run Length

Cable size depends on 2 things: how much current the cable carries, and how far it carries it. The current decides whether the cable will overheat. The length decides how much voltage is lost before the lamp, because every metre of copper adds resistance.

Typical Current Ratings for Automotive Cable

Thin-wall automotive cable (to ISO 6722, commonly rated to 105°C) is the modern standard for vehicle wiring. Cable suppliers quote ratings along these lines for a single cable in free air:

Cross-section	Typical strand count	Typical rating
0.5 mm²	16/0.20	About 11A
1.0 mm²	32/0.20	About 16A
2.0 mm²	28/0.30	About 25A
3.0 mm²	44/0.30	About 33A
4.0 mm²	56/0.30	About 39A
6.0 mm²	84/0.30	About 50A

Older thick-wall PVC cable carries lower ratings for the same size, and cable bundled tightly in a loom or run near the engine should be derated. Treat these figures as the ceiling, not the target. Check the rating printed by your supplier.

Cable Size to Keep Voltage Drop Under About 0.5V

For lighting, voltage drop usually decides the cable size long before heat does. Aim to lose no more than about 0.5V between the battery and the lamp on a 12V system, which is roughly 4 percent. The table below uses the total circuit length: the feed out to the lamp plus the earth cable back. If the lamp earths to the chassis close by, count only the feed length.

Lamp current (12V)	Up to 3 m total	Up to 6 m total	Up to 9 m total	Up to 12 m total
5A (1 to 2 LED lamps)	1.0 mm²	1.5 mm²	2.0 mm²	2.5 mm²
10A (3 to 4 LED lamps)	1.5 mm²	2.5 mm²	4.0 mm²	6.0 mm²
15A	2.0 mm²	4.0 mm²	6.0 mm²	Split into 2 circuits
20A	2.5 mm²	6.0 mm²	Split into 2 circuits	Split into 2 circuits
25A	4.0 mm²	6.0 mm²	Split into 2 circuits	Split into 2 circuits

The figures are calculated from the resistance of copper with an allowance for warm running. On a 24V system the same current loses the same voltage but that is only half the percentage, and a 24V lamp of the same wattage draws half the current, so 24V circuits can usually drop one or two sizes.

A practical example: 4 LED lamps of 40W each on the cab roof draw about 13A at 12V. The relay sits under the bonnet, the lamps are about 4 m away, and each lamp earths back to the battery, so the total circuit is about 8 m. Worked through, 13A over 8 m in 4.0 mm² cable loses about 0.5V, right at the limit, so 4.0 mm² is the minimum for the main feed and 6.0 mm² gives some margin. If you split the roof into front and rear pairs on 2 relays, each pair draws about 6.5A and 2.5 mm² does the job.

Cable Specification for Farm Use

Use stranded automotive cable, never solid-core household cable, which cracks under vibration. Tinned copper resists corrosion better in damp, muddy conditions. Run cable in convoluted conduit or split loom where it passes near sharp edges, moving parts or heat, and fit a grommet everywhere it passes through metal.

Fuse Sizing for Work Lights

A fuse protects the cable, not the lamp. It must blow before the cable it protects can overheat, so it is sized between the normal current of the circuit and the safe rating of the cable.

A simple rule works for most work light circuits: add up the lamp current, multiply by about 1.25, and round up to the next standard fuse size, then check the result is below the cable rating. Standard blade fuse values include 5A, 7.5A, 10A, 15A, 20A, 25A and 30A.

Lamps (40W LED each)	Current at 12V	Current at 24V	Fuse at 12V	Fuse at 24V
1	3.3A	1.7A	5A	3A or 5A
2	6.7A	3.3A	10A	5A
3	10A	5A	15A	7.5A
4	13.3A	6.7A	20A	10A
6	20A	10A	25A	15A

LED lamps draw a short inrush current when they switch on, so a fuse sized exactly to the running current can blow for no obvious reason. The 1.25 factor allows for that. Halogen lamps also draw more for a moment when the filament is cold.

Put the main fuse as close to the battery positive as you practically can, ideally within 150 mm to 300 mm. Any cable between the battery and the fuse has no protection at all, and a chafe there is the classic cause of a wiring fire. A sealed inline holder or a weatherproof blade fuse holder suits farm machines. The control circuit for the relay coil needs its own small fuse too, usually 3A to 5A, if it is not already fed from a fused circuit. See fuses and fuse holders for inline, blade and maxi holders, and our guide to fuses and circuit protection for vehicle lighting for the detail.

Switch Options for Work Lights

Because the switch only drives the relay coil, almost any switch rated for the system voltage will do. The choice comes down to fit, feel and whether you want an indicator.

Rocker Switches

A rocker switch fits a rectangular cut-out and is the most common choice for added work lights. Illuminated versions have a third terminal for an earth so the lamp in the switch lights when it is on. Many come with a symbol on the face, and the work light symbol is the usual choice.

Toggle Switches

A toggle switch mounts through a round hole, often 12 mm, and suits a small bracket or panel. Fit a rubber boot if it is exposed to weather.

Push Switches

A push switch can be latching (press on, press off) or momentary. Use a latching type for work lights. Momentary push buttons need a latching relay or module behind them.

Factory Switch Positions

Many tractors have blank switch positions or a pre-wired work light circuit with a spare relay socket. Using them gives a factory look, but check the handbook or wiring diagram first, because a factory circuit may already be loaded and may sit on a body controller that monitors current.

How the Switch Is Wired

Feed the switch from an ignition-switched fused supply so the lights cannot be left on with the key out. Switch output goes to relay pin 86. If the switch is illuminated, its earth terminal goes to a good earth behind the dash. Terminal markings vary between makers, so check the diagram on the switch or its packaging before connecting. See the range of 12V switches for rocker, toggle and illuminated types.

Layout 1: One Switch, One Relay, 2 Work Lights

This is the standard layout for a pair of lamps on the cab roof, front or rear. It suits up to 4 LED lamps on a 30A relay if the cable is sized for the total current.

1. Disconnect the battery negative. Do this before anything else and keep the lead away from the terminal.
2. Plan the routes. Decide where the relay goes (a dry spot near the battery, away from exhaust heat), where the cable will enter the cab or climb to the roof, and where the switch will sit. Measure the real cable lengths, not the straight-line distance.
3. Mount the relay. Fix the relay base to a panel with the spades pointing down so water drains off rather than into the base.
4. Run the main feed. Run heavy red cable from the battery positive to pin 30. Fit the inline fuse holder within 300 mm of the battery, but leave the fuse out for now.
5. Run the lamp feed. Run cable of the same size from pin 87 to the lamps. Split to 2 lamps with a sealed junction or a correctly crimped splice close to the lamps, not back at the relay.
6. Wire the switch. Take a fused ignition-switched supply to the switch input. Run light cable from the switch output to pin 86.
7. Earth the coil. Run light black cable from pin 85 to a clean chassis earth point.
8. Earth the lamps. Run black cable of the same size as the feed from each lamp negative to the battery negative, or to a cleaned chassis point near the lamps if the chassis path back to the battery is sound.
9. Seal the connections. Use sealed connectors or adhesive-lined heat-shrink on every joint outside the cab. Grommet every hole.
10. Test. Fit the fuse, reconnect the battery, switch on. The relay should click and the lamps light. With the engine running, measure the voltage at the lamp terminals and compare it with the battery. More than about 0.5V difference points to undersized cable or a poor joint.

Layout 2: Front and Rear Sets on 2 Switches

This layout gives separate control of front and rear lamps, which is what most operators want for cultivation, loader work and hitching. It also halves the current on each circuit, so the cable can be lighter.

1. Disconnect the battery negative.
2. Mount 2 relays side by side. Label them front and rear.
3. Feed both relays from the battery. Either run 2 separate fused feeds, or run one heavier feed to a small fused distribution block with a fuse for each relay. Each relay must have its own fuse sized to its own load.
4. Run the front lamp feed from the front relay pin 87 to the front lamps, and the rear lamp feed from the rear relay pin 87 to the rear lamps.
5. Fit 2 switches. Feed both from the same fused ignition-switched supply. Switch 1 output goes to pin 86 on the front relay, switch 2 output to pin 86 on the rear relay.
6. Earth both coils (pin 85 on each relay) and every lamp, as in Layout 1.
7. Seal, tidy and test each circuit separately, then both together with the engine running.

A variation of this layout uses a 3-position switch (off, front, front and rear) feeding the 2 relay coils. A ready-made multi-relay module does the same job in one sealed box if you prefer fewer joints.

Wiring Work Lights on a 24V System

Wiring work lights on a 24V machine uses exactly the same circuit as 12V. The relay pins, the switch position and the earth layout do not change. Four things do:

• The relay must have a 24V coil. A 12V relay on 24V will overheat its coil and fail. A 24V relay on 12V may not pull in at all. The coil voltage is printed on the relay case. Choose from the 24V relays range.
• The lamps must be rated for 24V. Most LED work lamps are multi-voltage, often 9 to 32V or 10 to 30V, and work on both. Halogen lamps need 24V bulbs; a 12V bulb on 24V blows almost instantly.
• The current halves. A 40W lamp draws about 1.7A at 24V instead of 3.3A at 12V. Fuses are sized down to match, and the cable can often be one or two sizes smaller for the same run.
• Switch illumination must suit 24V. An illuminated switch built for 12V will burn out its indicator on 24V even though the contacts would cope.

On a machine with 2 batteries in series, take the feed from the 24V positive terminal (the battery whose positive is not linked to the other battery). Never take a 12V feed from the middle of a series pair to run 12V lamps. It unbalances the batteries and shortens their life. Use a proper 24V to 12V converter instead. Our guide to 12V vs 24V work lighting systems explains how to tell which voltage your machine runs.

Earth Path: The Half of the Circuit People Skimp On

The earth carries the same current as the feed, so it needs the same cable size and the same care. A poor earth is the most common reason for dim or flickering work lights after an install.

Earthing Back to the Battery

A dedicated earth cable from each lamp to the battery negative gives the most reliable result. It costs more cable but removes any doubt about the chassis, the cab mounts or painted brackets.

Earthing to the Chassis

A chassis earth saves cable and is fine if the metal path back to the battery is sound. Scrape the paint and rust back to bright metal around the bolt hole, use a ring terminal and a serrated washer, tighten firmly, then coat the joint with grease or a protective spray to keep water out.

Remember that most tractor cabs sit on rubber isolation mounts. The cab is earthed to the chassis by a strap, and if that strap is corroded or broken, any lamp earthed to the cab roof will be unreliable. Earthing roof lamps back to the battery avoids the problem.

Connectors and Weatherproofing

Every joint outside the cab sees water, mud, pressure washing and vibration. Use sealed connectors such as Deutsch DT or Superseal at the lamps, adhesive-lined heat-shrink butt splices for cable joins, and crimped rather than twisted or taped joints. Our guide to tractor electrical connectors covers the main types and how to crimp them.

Smear a little dielectric grease on relay spades and unsealed connector pins before assembly. Cable-tie the new loom to existing harnesses at regular intervals so it cannot rub, and leave a drip loop where the cable enters the relay or a lamp so water runs off the loop rather than into the connector.

Wiring Kits

A work light wiring kit is a pre-made loom with the relay, fuse holder, switch and cable already assembled for 1, 2 or 4 lamps. Kits save time and take the guesswork out of sizing for a standard install. Check the cable length suits your machine, because a kit made for a pick-up or a quad may be too short for a tractor cab roof, and check the connectors at the lamp end match your lamps. A custom loom suits machines with lamps in several zones, unusual cable runs, or a factory circuit you want to integrate with.

Common Work Light Wiring Faults

Symptom	Likely cause	What to check
Lamps flicker or dim on rough ground	Poor earth or loose crimp	Earth points, crimps, connector pins
Lamps dim at idle, brighten with revs	Charging system or voltage drop	Battery voltage with engine running, voltage at lamp
Fuse blows at switch-on	Short to earth after the relay, or fuse too small for inrush	Chafed cable at grommets and brackets, fuse rating
Relay clicks, lamps stay off	Break in the load side	Voltage at pin 30 and pin 87, lamp earth
Relay does not click	No coil feed or no coil earth	Switch fuse, voltage at pin 86, earth at pin 85
Lamps will not switch off	Relay contacts welded, or 87 and 87a swapped	Swap the relay, check pin positions
Relay buzzes or chatters	Low coil voltage or 12V relay on 24V	Coil voltage rating, switch feed voltage

To test the load side, switch on and measure from pin 30 to earth (should read battery voltage), then from pin 87 to earth (should read the same when the relay has clicked). Battery voltage at 87 but no light means the fault is between the relay and the lamp, or in the lamp earth.

Frequently asked questions

Do I need a relay for LED work lights?

A relay is recommended for any set of work lights, and essential once the total current approaches the switch rating. A single small LED lamp drawing 2A to 3A can run through a suitably rated switch, but 2 or more lamps, or any halogen lamp, should go through a relay so the switch and cab wiring stay cool.

Which relay pin goes to the switch?

Pin 86 goes to the switch output, pin 85 goes to earth, pin 30 goes to the fused battery feed and pin 87 goes to the lamps. Pin 87a, on a 5-pin relay, is left empty in a simple work light circuit.

What size wire do I need for 12V work lights?

For 2 LED work lamps drawing about 5A to 7A over a typical tractor run of 6 m out and back, 2.5 mm² cable keeps the voltage drop under about 0.5V. Four lamps on one circuit need around 4.0 mm², or split them onto 2 relays and use lighter cable on each.

Can I use a 12V relay on a 24V tractor?

No. A 12V relay coil on 24V draws twice its rated current and will overheat and fail. Use a relay with a 24V coil. The contact rating can be the same, and the lamps will draw half the current they would at 12V.

What size fuse for 2 LED work lights?

Two 40W LED work lamps draw about 6.7A at 12V, so a 10A fuse is the usual choice. On 24V the same pair draws about 3.3A and a 5A fuse suits. Always check the fuse is below the current rating of the cable it protects.