Auxiliary Lights Guidelines

You fitted a set of auxiliary lights to the truck, switched them on for the first night drive, and the car coming the other way flashed you before you reached the bend. The lights work, the wiring is sound, and the aim is the problem, because a driving light pointed a fraction too high blinds everyone ahead of it.

Auxiliary lights are the most misunderstood accessory on a truck. The rules that govern them are federal and state law, the aiming is a measured procedure, and the wiring runs through a relay and a fuse, not straight through the switch. Get any one of those wrong and the lights are either illegal, dangerous, or both.

This guide sets out the guidelines for auxiliary lights in full: what the types are, what the federal and state rules say, how to mount and aim them, and how to wire them safely, with the standards and the sources stated plainly.

What auxiliary lights are

Auxiliary lights are extra lamps fitted beyond the factory headlamps, and each type does a different job. The type decides the beam, the mounting height and the rules that apply, so naming the type correctly is the first step in any install.

Driving lights

Driving lights are long-range lamps that work as high-beam boosters, pushing a focused beam far down the road so the driver sees a hazard hundreds of feet before the standard high beams would reveal it. They are designed for high-speed night travel on open roads, and they must meet SAE J581, the auxiliary upper-beam lamp standard. Driving lights supplement the high beams and can never be used as a replacement for the headlamps, and they must be switched off for oncoming traffic. A driving light that is not aimed is a hazard, so the aiming step in this guide matters as much as the lamp itself, and a badly aimed driving light is worse than no driving light at all. 12V Adapters Review Amazon Source: Auxiliary driving lights, types, laws and install, LegalClarity

Fog lights

Fog lights throw a wide, flat beam with a hard upper cutoff that stays low to the ground, lighting the road surface directly ahead without bouncing off the moisture, dust or snow hanging in the air. Federal rules for commercial vehicles require fog lights to meet SAE J583, the front fog lamp standard, which governs the beam pattern and the aim to prevent glare. On passenger vehicles, fog lights mount below the headlamp centreline so the beam stays under the fog layer. Fog lights are not a substitute for the headlamps, and they must be used with the low beams, because a fog light on its own leaves the vehicle under-lit from the front and hard for other drivers to see. 12V Adapters Alternative for LED Lights Source: 49 CFR 393.24, head lamps, auxiliary driving lamps and fog lamps, eCFR

Work lights and light bars

Work lights and light bars are broad, high-output lamps made for job sites, trails and camps, not for public roads. Most are sold marked for off-road use only, which means the maker does not claim they meet any street-legal standard at all. A light bar can be a useful work light on private land, but it has no place switched on in traffic, and many states require off-road lights to be covered with opaque shields while driving on public roads. Work lights belong on a switch the driver controls, so they can be killed instantly when another vehicle appears, and they should never be wired to come on with the headlamps. 12V Adapters Alternative to USB Source: WAC 204-21-090, auxiliary driving lamps, Washington State Legislature

The rules that govern auxiliary lights

Auxiliary lights sit under two layers of law at once: the federal floor that sets the standards, and the state equipment codes that enforce them and add their own limits. Both layers matter, and the state code is the one that writes the ticket. The federal floor is the same everywhere, so the state code is where the differences live, and it is worth reading before buying rather than after fitting.

Federal standards

Federal Motor Vehicle Safety Standard 108 governs the design, performance and marking of all vehicle lighting equipment sold in the United States, and it applies to passenger cars, trucks, SUVs, motorcycles and trailers. For commercial vehicles, 49 CFR 393.24 requires auxiliary driving lamps to meet SAE J581 and front fog lamps to meet SAE J583, and it states that neither type may be used in place of the required headlamps. The lamps must be mounted so the beams are aimable and the mounting holds the aim while the vehicle runs on public roads. The federal rules set the floor for every lamp sold in the country, and a lamp that does not meet them cannot be made legal by aiming it well or wiring it neatly, so the standard comes first. Best ATV Winch Source: Wisconsin Trans 305.075, auxiliary lamps, Wisconsin Legislature

State rules

State rules add mounting heights, lamp counts and use limits on top of the federal floor, and they differ from state to state. Most states limit how many forward-facing auxiliary lamps may run at once, restrict mounting height relative to the headlamp centreline, and require off-road lights to be covered while driving on public roads. Washington requires auxiliary driving lamps to meet SAE J581 and be aimed with the hot spot at lamp-centre height, while Minnesota caps fog lamps at two, mounted between 12 and 30 inches high, and driving lamps between 16 and 42 inches. Check the equipment code for the state where the vehicle is registered and for any state it drives through, because a setup that is legal at home can still draw a ticket across the state line. Best Jump Starter Source: SAE J581, auxiliary upper-beam lamps standard, SAE International

Off-road-only markings

A lamp marked for off-road use only is not legal for street use, and that label is the first clue that the light does not meet SAE or DOT standards for public roads. SAE and DOT do not test, certify or approve individual products: manufacturers self-certify that their lights meet the published standards, so a lamp stamped SAE J581 means the maker claims it meets the fog or driving lamp standard. Check the marking on the lens before buying, and check your state equipment code before installing anything. The marking sits on the lens or the housing, so read it before the lamp is fitted, because a fitted lamp is harder to return and the marking is the only honest claim the maker makes. Best Portable Air Compressor Source: 49 CFR 393.24, auxiliary driving lamps and fog lamps, Cornell LII

Mounting and aiming auxiliary lights

Mounting and aiming decide whether auxiliary lights help the driver or blind everyone else. The aim is a measured procedure, not a guess, and it is done on level ground against a flat wall.

Mounting height and position

Mounting height follows the lamp type. Fog lights work best mounted in, on or below the bumper, roughly 10 to 24 inches above the road surface, or anywhere below the centre of the headlights, where the beam can cut just under the low-beam pattern. Driving lights typically mount at or near headlamp height, roughly 14 to 30 inches off the ground. Mount the lamps so the beams are aimable, the mounting holds the aim while driving, and the lamps sit the same distance from the vehicle centreline. Measure the height from level ground to the centre of the lamp, not from the bumper or the roof, because the standard is written in inches above the road surface. Best Engine Oil Source: Spot lamps and auxiliary driving lamps code, American Legal Publishing

Aiming driving lights

Aim driving lights by the wall method: park the vehicle on level ground 25 feet from a flat vertical wall, measure the height from the ground to the centre of each lamp, and mark that height on the wall as the level line. Turn the driving lights on and adjust the beam so the centre of the hot spot, the brightest part of the beam, sits about 1.5 inches below the level line, with both lights aimed straight ahead and equidistant from the vehicle centreline. Re-aim any time the ride height changes, after a lift kit, new tyres or a heavy load. Aim both lights to the same mark, because a pair that points in different directions leaves a dark gap in the middle of the road and a bright spot where it is not wanted. Best Automotive Multimeters vs Watts Source: Aiming fog and driving lights, Equipment World

Aiming fog lights

Aim fog lights by the same wall method, with a steeper downward angle than driving lights, because the whole point is to keep the beam under the fog layer. Park 25 feet from a flat wall on level ground, mark the lamp-centre height on the wall, and adjust the lamps so the top of the fog beam sits about 4 inches below the lamp centre at 25 feet, with the lamps aimed straight ahead. Fog lights mounted below the bumper use a smaller drop than lamps mounted above the bumper, so follow the lamp maker aiming instructions when they differ. Check the aim from the driver seat after adjusting, because a fog beam that looks right on the wall can still glare once the vehicle is loaded and the suspension settles. Best Automotive Borescope for Mechanics Source: Aiming auxiliary lights, Commercial Carrier Journal

Wiring auxiliary lights safely

Wiring decides whether auxiliary lights are safe or a fire risk. The circuit runs through a relay and a fuse, and the switch carries only a small trigger signal, never the full load.

Relay and fuse

Run battery power through a relay, not directly through the switch, because the switch cannot carry the current a set of auxiliary lights draws. The switch controls a low-current trigger signal to the relay, and the relay handles the high amperage the lamps actually draw. Fit an inline fuse close to the battery post, within 18 inches, so a short anywhere in the harness blows the fuse instead of starting a fire. Disconnect the negative battery terminal before any wiring, and work on a dead circuit every time. Size the relay and the fuse to the load, because a relay that is too small will weld its contacts shut and a fuse that is too big will not blow when the wire shorts. Best Adjustable Wrench Source: Minnesota Statutes 169.56, auxiliary lights, Office of the Revisor

Wire size and grounding

Wire size follows the load: a 14 gauge wire suits most auxiliary light installs, while a 12 gauge wire suits bigger bars and longer runs, and the ground must go to clean bare chassis metal. A poor ground causes flicker, dimming and heat, so clean the ground point down to bare metal and use a stainless bolt to hold the ring terminal. Route the power wire through a firewall grommet, never through a bare hole in the metal, and wrap the harness in split conduit where it passes near hot or moving parts. Keep the ground wire as short as the power wire, because a long ground run adds resistance and the lamp will dim even when the power feed is perfect. Best All-Season Tires Under 100 Source: How to aim headlamps and auxiliary lamps, TDIClub Forums

Switches and triggers

Switches and triggers decide when the lamps can run. Driving lights should be wired so they can only run with the high beams, because they are high-beam boosters and must go out for oncoming traffic, while fog lights run with the low beams. Use a switch the driver can reach without looking away from the road, and wire the trigger from a switched ignition source so the lamps cannot be left on with the engine off and drain the battery. Label the switch so a passenger or a new driver knows what it does, because an unmarked switch gets flipped at the wrong time and a driving light in traffic is a hazard. Best All-Season Tires Under 200 Source: How to wire motorcycle accessories safely, UMA Technology

When to use auxiliary lights

When to switch auxiliary lights on matters as much as how they are aimed. The right lamp at the wrong time is glare, and the wrong lamp at the right time is no help at all.

With high beams

Use driving lights with the high beams on open roads at night, where the extra reach reveals a hazard early enough to react. Switch them off for oncoming traffic and when following another vehicle, because a driving light is a high-beam booster and its hot spot will blind a driver ahead. Many states require auxiliary lamps projecting beyond the low beams to be extinguished within 500 feet of another vehicle, so treat the driving lights like the high beams and dip them together. Dip the driving lights with the high beams, because the two are wired together on most installs and the driver should not have to think about them separately. Best Garage Security Cameras Under 100

In fog and rain

Use fog lights in fog, heavy rain and snow, where the wide, flat beam lights the road surface without bouncing off the moisture in the air. Fog lights work with the low beams, never in place of the headlamps, and they must stay below the headlamp centreline to keep the beam under the fog layer. Switch driving lights off in fog, because a long-range beam in fog lights up the fog itself and blinds the driver. Slow down in fog as well as switching the right lights on, because no lamp can see through dense fog and the fog lights only light the road directly ahead. Best Garage Storage

Off-road and work use

Use work lights and light bars off-road, on job sites and at camp, where there is no oncoming traffic to blind and no state equipment code to break. Cover off-road lights with opaque shields before driving on public roads, because most states require it, and switch them off before rejoining the highway. A light bar left uncovered on the roof is an invitation to a ticket and a hazard to every driver coming the other way. Remove the covers only when the vehicle leaves the public road, and fit them again before it rejoins the highway, because a cover left off is the same as a light left on. Best Hard Tonneau Cover Under 500

Auxiliary light types at a glance

Installing auxiliary lights step by step

Start by checking the state equipment code and the lamp markings, because the law decides what may be fitted and where. Choose the lamp type for the job, confirm it carries the right SAE marking for street use or accept that it is off-road only, and measure the mounting height against the headlamp centreline. Mount the lamps on a rigid bracket so the aim holds while driving, and check that both lamps sit the same distance from the vehicle centreline. Buy the bracket and the hardware before the lamps, because a lamp with nowhere solid to mount is a lamp that will shake loose and lose its aim on the first rough road. Best Mud Flaps

Wire the lamps through a relay and a fuse, with the switch carrying only the trigger signal, and aim them by the 25 foot wall method before the first night drive. Park on level ground, mark the lamp-centre height on the wall, and set the driving light hot spot about 1.5 inches below the mark and the fog beam top about 4 inches below. Test the lamps on a dark road, check for glare from the driver seat of another car if you can, and re-aim after any change in ride height. Keep the aiming marks on the wall or a note of the measurements, because the next time the aim needs checking you will want the same reference instead of starting over. Best Performance Monitors Under 100

Safety essentials

Troubleshooting

Most faults with auxiliary lights trace to one of the causes below, and the fix is usually a check rather than a replacement. Best Storage Bins

Who should NOT install auxiliary lights

Do not fit off-road-only lights for street use. A lamp marked for off-road use only does not meet any street-legal standard, and switching it on in traffic is illegal in most states and dangerous for every driver ahead. Keep off-road lights covered on public roads and use them where they belong. Best Tool Storage

Do not aim auxiliary lights by eye. A light aimed a fraction too high blinds oncoming drivers, and a light aimed too low is no help at all. Use the 25 foot wall method with a measured mark, and re-aim after any change in ride height. Best Tow Ropes Under 100

Do not wire auxiliary lights without a relay and a fuse. The switch cannot carry the current, and a circuit without a fuse is a fire risk. Fit the relay, fuse close to the battery, and ground to clean bare metal. Battery Tender Junior vs NOCO Genius1

Do not run driving lights in fog. A long-range beam in fog lights up the fog itself and blinds the driver. Switch driving lights off in fog and use the fog lights with the low beams, because the wide, flat beam is the one made for the job. 12V Adapters Reviews for Cars

Final verdict

Auxiliary lights are worth fitting when the type, the rules, the aim and the wiring are all right together. Driving lights to SAE J581 supplement the high beams, fog lights to SAE J583 sit below the headlamp centreline, and both are aimed by the 25 foot wall method and wired through a relay and a fuse.

The rules are the floor, not the ceiling: check the state equipment code before installing anything, because mounting heights, lamp counts and cover requirements differ from state to state, and the state code is the one that writes the ticket.

When in doubt, aim lower, switch off sooner, and cover the off-road lights. A light that helps the driver without blinding anyone else is the whole point of the exercise.

FAQ

What is the difference between driving lights and fog lights?

Driving lights are long-range lamps that supplement the high beams and must meet SAE J581, while fog lights throw a wide, flat beam low to the ground and must meet SAE J583. Driving lights mount near headlamp height, fog lights mount below the headlamp centreline, and each is aimed differently.

Are auxiliary driving lights legal?

Yes, when they meet the right standard and follow state rules. Driving lights must meet SAE J581, mount at a legal height, and be used only with the high beams, while state codes add limits on lamp counts, mounting height and covers. Off-road-only lights are not legal for street use.

How do I aim auxiliary driving lights?

Park on level ground 25 feet from a flat wall, mark the lamp-centre height on the wall, and adjust the beam so the centre of the hot spot sits about 1.5 inches below the mark, with both lights aimed straight ahead. Re-aim after any change in ride height, tyres or load.

How do I aim fog lights?

Park 25 feet from a flat wall on level ground, mark the lamp-centre height, and adjust the lamps so the top of the fog beam sits about 4 inches below the lamp centre at 25 feet. Fog lights get a steeper downward angle than driving lights, because the beam must stay under the fog layer.

What is SAE J581 and SAE J583?

SAE J581 is the standard for auxiliary upper-beam lamps, or driving lights, and SAE J583 is the standard for front fog lamps. Federal rules for commercial vehicles require driving lamps to meet J581 and fog lamps to meet J583, and state codes reference the same standards for other vehicles.

Can I use auxiliary lights instead of headlights?

No. Auxiliary driving lamps and fog lamps may be used only in conjunction with the required headlamps, never in place of them. Federal rule 49 CFR 393.24 states this plainly, and state codes say the same. The headlamps are the required lighting equipment, and the auxiliary lamps only add to them, so a vehicle with dead headlamps is not roadworthy even with auxiliary lights fitted.

Do I need covers for off-road lights?

In most states, yes, while driving on public roads. Off-road lights must be covered with opaque shields so they cannot be switched on in traffic, and the covers come off only off-road. Check your state equipment code for the exact requirement.

How do I wire auxiliary lights?

Wire them through a relay and a fuse: the switch carries only a small trigger signal, the relay carries the full load from the battery, and the fuse sits close to the battery post. Ground to clean bare metal, route wires through a firewall grommet, and disconnect the battery before starting.

When should I switch auxiliary lights off?

Switch driving lights off for oncoming traffic and when following another vehicle, and switch them off entirely in fog. Many states require auxiliary lamps to be extinguished within 500 feet of another vehicle, so treat them like the high beams and dip them together.

Sources

• Auxiliary driving lights, types, laws and install, LegalClarity
• 49 CFR 393.24, head lamps, auxiliary driving lamps and fog lamps, eCFR
• WAC 204-21-090, auxiliary driving lamps, Washington State Legislature
• Wisconsin Trans 305.075, auxiliary lamps, Wisconsin Legislature
• SAE J581, auxiliary upper-beam lamps standard, SAE International
• 49 CFR 393.24, auxiliary driving lamps and fog lamps, Cornell LII
• Spot lamps and auxiliary driving lamps code, American Legal Publishing
• Aiming fog and driving lights, Equipment World
• Aiming auxiliary lights, Commercial Carrier Journal
• Minnesota Statutes 169.56, auxiliary lights, Office of the Revisor
• How to aim headlamps and auxiliary lamps, TDIClub Forums
• How to wire motorcycle accessories safely, UMA Technology

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