Trailer wiring is not something that should intimidate the handyman. These 12v or 24v systems are not difficult, and once equipped with some basic knowledge, you will have your trailer wired in no time. If you reach a point however where you decide the electrical side of the build is not for you, you can contact a local automotive electrician who will be able to sort the wiring for you.
Right is a trailer wiring diagram for 5/7 or 12 pin connection options. There is a trailer wiring diagram included with all our trailer plans.
Wiring Notes
Electrical wiring must:
- Be supported at intervals no more than 600mm apart.
- Be insulated at joints.
- Not chafe or contact moving parts.
- Have rubber grommets when passing through metal.
- Be earthed (ground wire) between the trailer and tow vehicle. Your trailer can not be earthed through the coupler.
Trailer Lights & Reflectors
Most countries require specific trailer lighting arrangements and positioning to meet regulations that allow your trailer to be registered for road use. Lighting requirements can often cause confusion due varying arrangements. If you have difficulties, your best point of contact is your local trailer registration office.
Typically, trailers require:
- Tail lights
- Brake lights
- Indicators
- Reflectors (front, side and rear)
- Width and heigh indicators also known as marker/clearance lights (depends on trailer size and local regulations).
How to Route Electrical Wires
There are several methods for routing electrical wires. In all cases, any ‘loose wiring’ where you need to connect to the outside of the chassis member, should be secured to the chassis using a method such as automotive P-clips.
Routing cable through chassis
This method best protects the wiring loom from being damaged but means that you need to cut holes in the chassis for in/outlets. This is not a problem if you intend on galvanising the chassis (drainage holes are required for galvanising).
Routing cable through 'tube' section
This method is best used on trailers made from enclosed section material (RHS) where you want to avoid moisture from entering the chassis though wiring cutouts. The steel tube is welded to the chassis where the wiring loom will be routed.
Tie back points
Electric brake routing
Weld thin wall tube to the back side of the axle. Route the brake wires through the tube and where the wiring exits the pipe and connects to the magnet, protect with flexible conduit as these wires need to move with the suspension.
Electrical Joints
Electrical joints are a common point of failure, so it’s important to ensure they are done right.
Solder joints provide the best connection.
- To prevent a cold solder joint, use new wire (corrosion can cause a bad joint) and apply heat to the bottom of the joint before feeding solder on to the top of the joint. See this video by Crutchfield for correct method.
- Alternatively, solder splice heat shrink connectors can be used.
- Using a connector such as weatherproof Deutsch connectors can be handy. If a light is damaged in future, you can simply disconnect and replace without needing to cut and solder.
- Often when you need to make multiple joins or splices into the main trailer cable, it is best to use a junction box. This allows you to keep all your joints in one secure location and can make tracking down an electrical fault simple.
Trailer Cable
Typically, there are two variants, 5 core (small trailers without electric brakes or additional power requirements) and 7 core. Cable consisting of more cores, such as 12 core, can be sourced where auxiliary power sources are needed such as for caravans.
- Lights such as markers and work lights that only require earth and power can be spliced into your trailer cable using 2 core to save on using 5 or 7 core where the additional wires are not required.
- Most trailer cable comes double insulated (individual cores insulated plus outer insulation). If the outer insulation is removed, for example, when you make a joint, the individual cores need to be shrink wrapped, then all cores can be either shrink wrapped or wound in electrical tape. Finally, protect the joint with flexible electrical conduit or a similar product.
- When passing cable through a chassis member, use a rubber grommet to protect cable from chafing.
Trailer Plugs
Typically, there are two main variants, 7 or 12 pin, and they come in two shapes; round or flat.
Which should you choose?
This depends on how many wires you require, what type of plug is already fitted to the tow vehicle and your personal preference.
The tricky part…
A good wiring loom leaves no wire unprotected. Where this becomes a problem is when the cable connects to the vehicle.
From our experience this approximately 1m length of cable is exposed to rain, sunlight, road debris and operator error! (check that your lights are connected for legal reasons and to prevent damage from dragging the lead). Electrical conduit isn’t the best protection as it typically can’t handle the constant exposure to the elements.
Some better options include:
- Heavy duty cable lead with the plug moulded to the cable (problematic if you need to change the plug).
- Cover cable with UV rated rubber piping (difficult to get a watertight connection at the plug).
- Protect cable with ‘spiral cable wrap’ like what is commonly used to protect hydraulic hoses.
