Wiring a trailer is not difficult, and can be completed with minimal investments of time and expense. Trailer wiring is exposed to weather and harsh mechanical conditions, and extra care in installation will avoid problems in the future. Most trailers have a four-wire system used only for lights. However, more complicated arrangements are available for trailers with brakes and 12-volt power connectors.
There are multiple standards for trailer wiring, depending upon the configuration of the vehicle and trailer.
Begin your planning with the requirements of your trailer. Most utility trailers use a four-wire connection, which provides turn signals, brake and tail/marker lights. Depending upon your trailer, there may be additional wires for brakes, reverse lights, and 12-volt power. A ground wire may also be included, although ground is often provided by the chassis.
Examine your towing vehicle and determine the connector type required. Depending upon the vehicle manufacturer, different connector types will be used.
Numerous conversions plugs are available to convert from one type of connector to another.
A trailer is a nasty environment for electrical components. Water, heat, cold, salt and vibration are all common service conditions for a trailer, and all are detrimental to electrical components.
Purchase good-quality materials. Connectors, harnesses and light fixtures should be moisture-tight, and appear well-made and sturdy. Electrical wiring should be labeled as being compliant with SAE (Society of Automotive Engineers) specification J1128, and all lighting components should be labeled as SAE or DOT compliant.
Consider purchasing "trailer wiring" harnesses, which contain multiple strands of wire bonded together. These will simplify your wiring and assure you use a common color scheme. While you could wire an entire trailer with a single color of wire, compliance with a standard will make troubleshooting and repair much easier.
Purchase a heavier rather than lighter gauge wire (smaller numbers are heavier). The current capacity of larger wire is higher, but the real benefit is improved mechanical reliability provided by thicker, stronger wire. If you are wiring through existing holes and clearances, do not purchase wire too thick for easy clearance.
Wire your trailer in accordance to an accepted standard (see Resources).
Begin at the hitch end of your trailer, and pull the wire bundle to the first electrical component, typically the front marker lights. Continue pulling the wire bundle around the trailer.
Wire connections are reliability risks, so try to minimize the number of connections. Do not piece together small pieces of wire, but use only single runs wherever possible.
Secure all wiring with metal harnesses. Hanging or loose wiring can either rub against a chassis component and expose the conductor, or work to mechanically loosen electrical connections.
Do not secure wire with strip ties. Strip ties will break down with exposure to the elements and break.
Consider adding an small extra loop of wire into all runs to allow for expansion or repairs in the future. Secure this loop to prevent vibrations.
Wires pulled through holes drilled in chassis members are candidates for wire damage due to rubbing. Consider using rubber grommets in the holes, or vinyl sleeves around the wire where it passes through metal.
Any electrical connection between two pieces of wire is a reliability risk. Unfortunately, some connections are required to wire a trailer correctly.
Standard automotive electrical connectors are commonly used for trailer wiring connections, and they will work fine for a time. However, these connectors are not rated as being weather and waterproof, and can corrode quickly. A corroded connection does not conduct electricity.
Consider using electrical connectors designed for wet and exposed environments. One example is a motorcycle connector.
Silicone-filled connectors, or sealing standard connectors with a liquid vinyl electrical sealer, may provide enhanced reliability.
Trailers are commonly wired without a dedicated ground wire. The ground connection between the trailer chassis and the vehicle is made through the trailer hitch. This strategy works well because the wiping action between the trailer hitch and vehicle ball removes any corrosion and assures a good electrical contact.
Grounding individual lights to the trailer chassis can be less successful. Each connection is a target for corrosion of exposed metal, and grounding issues are the most common electrical problems on a trailer.
Consider running a dedicated ground wire with the other electrical wires to all points in the trailer. Connect one end of the ground wire to a screw connected to bare metal of the chassis. Seal this wire and screw with an epoxy coating to prevent moisture and corrosion between the wire and chassis.
