Trailer wiring is its own small world of connector standards. A trailer cord links the tow vehicle's electrical system to the trailer's lights and, on larger rigs, its brakes and battery. The number of functions the trailer needs determines which connector configuration you use — from a simple 4-way flat to the 7-way RV blade found on most modern tow vehicles.
The Common Configurations
- 4-way flat — the basic set: ground, tail/running lights, left turn/brake, right turn/brake. Standard on small utility trailers and boat trailers.
- 5-way flat — adds a fifth wire, typically for hydraulic surge-brake reverse lockout or an auxiliary function.
- 6-way round — adds circuits for electric brakes and a 12 V auxiliary/battery feed; common on livestock and mid-size trailers.
- 7-way RV blade — the full set: running lights, left turn, right turn, electric brakes, reverse, 12 V battery charge and ground. Standard on RVs and larger utility/car trailers.
Wire Color Conventions
North American trailer wiring follows a widely used color code that makes connections predictable:
- White — ground
- Brown — tail/running lights
- Yellow — left turn and brake
- Green — right turn and brake
- Blue — electric brakes
- Black — 12 V battery/auxiliary
These are conventions rather than a single legal mandate, so verifying with a circuit tester before towing is always wise.
Construction and Durability
Trailer cords live outdoors, so jacket and connector choice matter. Cords use weather- and oil-resistant insulation (see cord types such as SOOW), and quality connectors add sealed, corrosion-resistant terminals with spring-loaded covers. A robust strain relief at the plug keeps road vibration and yanking from breaking the conductors. Coiled cords are popular because they retract out of the way and resist dragging.
Safety and Compliance
Trailer lighting exists for safety, and functional turn, brake and tail signals are required for road-legal towing. Federal requirements for vehicle lighting are administered by the National Highway Traffic Safety Administration, and connector/lighting practices are informed by SAE standards. When wiring a trailer, match the connector to the tow vehicle's factory harness and confirm every circuit before the first tow.