What lighting is on a semi-trailer? Overview + functions

When building or equipping a 12V semi-trailer, one of the first questions is: which lighting exactly is needed? This depends on dimensions, semi-trailer type and applicable regulations. For semi-trailer builders, engineers and buyers, it is essential to have an immediate understanding of which lights are mandatory, which are optional and how these together shape a reliable lighting system.

In this article, you will find a comprehensive and practical overview of all the lighting that can be fitted to a 12V semi-trailer. For the exact legal requirements and exceptions, please refer to the article on semi-trailer lighting regulations.

In this article, you will read:

• How a lighting system is surface mounted on a semi-trailer
• An overview of all lighting on a semi-trailer
• the function of each lamp type
• how a complete lighting system is surface-mounted
• What you need to consider in practice

How a lighting system is surface mounted on a semi-trailer

Semi-Trailer lighting consists of one coherent system. The current flows from the vehicle via the plug connection to the semi-trailer wiring harnesses and is distributed to the various light functions. The mass (ground) plays a crucial role in this for fault-free operation.

The surface mounting is as follows: vehicle → connector → wiring harnesses → light functions → mass. Interference rarely occurs in practice due to a single light, but rather due to problems in this chain, such as poor connections or moisture.

Overview of all lighting on a semi-trailer

Light / reflector	Status	Function	Application
Rear lights	Standard	Rear visibility	All semi-trailers
Brake lights	Standard	Signal when braking	All semi-trailers
Direction indicators	Standard	Direction change	All semi-trailers
Triangular reflectors	Standard	Passive visibility	All semi-trailers
Licence plate light	Standard	Lighting license plate	All semi-trailers
Rear fog light	Standard	Visibility in poor visibility	Almost all semi-trailers
Side marker lights	Depending on length	Side visibility	> 6 metres
Reversing lights	Depending on category	View when reversing	Mainly for larger semi-trailers
Side marker lights	Depending on width	Marking vehicle width	Wide-ranging semi-trailers
Work lights	Optional	Lighting of work area around or behind the semi-trailer	Frequently used in professional applications

View all certified semi-trailer lighting in our shop

Function per lamp type

Rear lights
Ensure rear visibility and must be mounted symmetrically.

Brake lights
These indicate that the semi-trailer is braking and must operate simultaneously. Find out more about brake lights on a semi-trailer.

Direction indicators
Warn of directional changes and must be clearly visible at different angles.

Reflectors
Provide passive visibility and are mandatory on all semi-trailers.

Licence plate light
Lightening the license plate without glare.

Rear fog light
Provides additional visibility in poor visibility such as fog or heavy rain.

Side marker lights
Increase side visibility, especially for longer semi-trailers.

Reversing lights
Improves visibility when manoeuvring. Mandatory on semi-trailers from category O2.

Side marker lights
Indicate the outer width of the semi-trailer. View our range of side marker lights.

Work lights
For extra lighting whilst working, read more here about connecting work lights to a 12V semi-trailer

Difference per semi-trailer type

The lighting required varies depending on the type of semi-trailer. Smaller 12V semi-trailers are often equipped with basic lighting, while larger semi-trailers require additional features such as side marker lights and reversing lights.

For a complete overview of the legal requirements by category, please refer to the article on legislation for semi-trailer lighting.

Practical tips for the workshop

• First determine which lighting is required based on category and dimensions
• Use multifunctional LED units to reduce mounting points
• Pay attention to water resistance (at least IP67)
• ensure symmetrical mounting left and right
• Avoid interference with high-quality cabling and connectors for vehicle lighting

Frequently asked questions about semi-trailer lighting

What types of lighting are there on a semi-trailer?
A semi-trailer is equipped as standard with rear lights, brake lights, direction indicators, reflectors and licence plate lights. Depending on the type, side marker lights, rear fog light, side marker lights and work lights can also be fitted.

What is the difference between standard and supplementary lighting?
Standard lighting provides basic visibility and signalling. Supplementary lighting, such as side marker lights or work lights, is used for additional visibility or specific applications. More on this in our article: all about semi-trailer lighting

When are work lights used on a semi-trailer?
Work lights are used when loading, unloading or working around the semi-trailer. This is particularly common in professional applications such as construction, transport and agriculture.

What do side marker lights do on a semi-trailer?
Side marker lights make the side of the semi-trailer visible, especially on longer vehicles. This increases safety when driving in the dark or in poor visibility.

How does the lighting work on a semi-trailer?
The lighting is controlled via the plug connection to the vehicle and distributed via the wiring harnesses to the various light functions.