Trailer Wiring Data

    How to Select the Correct Size of Electrical Cable

    It is important to note that all electric-components have a good ground. In a combination of vehicles (tractor-trailer) it is important that a good and adequate size ground wire be provided between the two vehicles to carry the current.

    Wire has certain resistance to current flow, the smaller the wire, the greater the resistance.

    Also as the amount of current (amps) increases, the resistance (ohms) increases.

    For a 12v system there are two factors to be considered:

    1. Voltage drop is the primary consideration in selecting the proper electrical wire size. The length of wire in a circuit is the major contributing factor to voltage drop.
    2. The Current carrying capacity of the wire. The heavy load in an undersized wire may increase the temperature of the PVC insulation above a safe level of 180º F.
    7 conductor cable example

    7-Conductor Cable: (SAE J1067 and J560)

     
    • White – Ground (10 ga.)
    • Brown – Tail and license lamp, and clearance lamps (12 ga.)
    • Black – Clearance, side marker, and identification lamps (12 ga.)
    • Yellow – Left turn signal lamp (12 ga.)
    • Green – Right turn signal lamp (12 ga.)
    • Red – Stop lamps (12 ga.)
    • Blue – Auxiliary circuit (12 ga.)
    7 conductor cable example

    7-Conductor Cable: (SAE J1067 and J560)

     
    •  White – Ground (8 ga.)
    • Brown – Tail and license lamp, and clearance lamps (12 ga.)
    • Black – Clearance, side marker, and identification lamps (12 ga.)
    • Yellow – Left turn signal lamp (12 ga.)
    • Green – Right turn signal lamp (12 ga.)
    • Red – Stop lamps (10 ga.)
    • Blue – Auxiliary circuit (10 ga.)

     

    5 conductor cable example

    5-Conductor Cable: (chassis ground and without auxiliary circuit)

     
    •  Brown – Tail and license lamp, and clearance lamps
    • Black – Clearance, side marker, and
      identification lamps
    • Yellow – Left turn signal lamp
    • Green – Right turn signal lamp
    • Red – Stop lamps

     

    4-Conductor Cable: (boat trailer harness)*

     
    •  White – Ground
    • Brown – Tail and license lamp, and side marker lamps
    • Yellow – Left turn signal lamp
      and stop lamp
    • Green – Right turn signal lamp
      and stop lamp

     

    *Color coding applies to flat ribbon wire also.