How to Select the Correct Size of Electrical Cable Wire is used to carry current from the battery through the vehicle to various electrical components: lights, instruments, etc. In automotive use, the grounded return system is used almost exclusively.  In this system, a single wire is run from the battery through switches to the load and the vehicle itself provides the ground or return to the battery. 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: 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. The Current carrying capacity of the wire.  The heavy load in an undersize wire may increase the temperature of the PVC insulation above a safe level of 180º F.