Traditionally, 14 gauge has been the most popular size of primary wire used. However today¿s Tectran tractor and trailer 7-way wiring systems use 12 gauge wiring for lighting, 10 gauge wiring for the stop lamp (red) and ABS (blue) circuits and an 8 gauge wire to provide an adequate ground return. All new tractors and trailers must be equipped with these sizes of wiring to provide the necessary power to support lighting & ABS systems. To make matters worse, the SAE J1128 passenger car standard to which GPT primary wire is made, calls for up to 10% less copper than that used in heavy trucking applications. When you¿re wiring using primary wire, it is important to at least match the wire sizes used in the rest of the circuits. Most wiring in newer tractors and trailers will be 12 gauge or higher.