A Basic Guide for Understanding Trailer Wiring

By law all trailers must be wired to provide rear and side boundary lighting, tail lights, break lights, turn signals, and license plate lights. Before you hit the road with your trailer, you will want to make sure that it meets these minimum standards. Not only is proper working wiring required by law, it is important for safety. The following tips are a basic guide to trailer wiring to help you gain a basic level of familiarity with the wiring of your trailer.

Different Types of Trailer Plugs

It is important to make sure the trailer and tow vehicle have compatible plugs. The most basic plug is the four-pronged plug, which meets the minimum legal lighting standards for trailers. A five-pronged plug permits electric trailer brakes, plus basic trailer lights. Both the four and five-pronged plugs come in round and flat connector styles. Six-pronged plugs add a 12-volt “hot” lead that can provide power to a breakaway battery and interior lights. This style of plug is often found on horse trailers and campers and can be a round or flat style plug. The seven-pronged plug can be found in the flat pin or round pin style, with the flat pin style being most common. The additional wire typically provides back up lights for RV trailers.

Trailer Plug Compatibility

With eight different styles of plugs available, you may find that your tow vehicle and trailer plugs do not match. Most tow vehicles are wired with six or seven pin outlets. Many outlets are round, rather than the flat style, since the round outlets have covers on them that protect your vehicle wiring from the elements when it is not in use. With six or seven pin outlets, your vehicle will be able to tow most trailers. If your trailer has a different style plug or fewer prongs than your tow vehicle, you can remedy that by purchasing an adapter. If your tow vehicle has fewer prongs than your trailer, you will need to have it professionally rewired to match your trailer.

Understanding Wire Colors

Trailer wiring is generally standardized by color. In a four-pronged plug, you will find green, yellow, brown, and white wires. The green wire is the right turn signal and brake light while the yellow wire is the trailer’s left turn signal and brake light. The brown wire provides power to your marker lights, which are the license plate lights, the taillights, and any other marker lights on the trailer. The white wire serves as the ground wire. Five-pronged plugs also include a blue wire that provides power to a trailer’s electric brakes. The red or black wire in a six-pronged plug is the 12-volt “hot” lead that provides power to a breakaway battery and interior lights on a trailer. The seven-pronged plug adds a purple wire that permits back up lights to function on a trailer.

Trailer Wire Colors are not Standard

The wires on your trailer or tow vehicle may not match the standardized colors cited in this article. This may be particularly true if one or both of the vehicles are older. Before you do any work on your trailer, you may need to trace the circuits to determine which wire matches up to which function. There are many trailer wiring diagrams online that can help you understand the wiring in your trailer and tow vehicle if you want to perform this work yourself.

Making Sure Your Trailer Lights Work

One of the most common problems with trailer wiring is that some, but not all of the trailer lights will function. This is often caused by a weak ground. Grounding problems can occur in both the trailer and the tow vehicle. On your tow vehicle, check to make sure the ground wire is secured to a clean surface on the vehicle’s frame. If the wire is attached to the body of the vehicle or the frame has rust or a coating, it can affect the ground. On your trailer, the ground should also be attached to a clean surface. If these do not work, you will want to look at the trailer’s wiring design or the ground wires in the tail light assemblies.

Trailer wiring is not complex and is fairly standardized. Basic trailer wiring is required to help you meet minimum trailer safety standards by law. The tips in this article are designed to provide you with a basic guide to the wiring in your trailer and tow vehicle, to help you understand any basic issues that may occur. If you want to work on the wiring yourself, there are many online resources to provide diagrams of trailer wiring.


Trailers must have sidelights to denote the length of the trailer. Rear sidelights must be red and placed as far back on the trailer as possible, at a height of 15 to 60 inches. Front sidelights must be yellow and placed as close to the front of the trailer as possible, at a height of at lest 15 inches. Trailers more than 30 feet long must have yellow lights in the middle of the trailer, at lest 15 inches high. Trailers wider than 80 inches must also have additional lighting to indicate width.