Most of the 12-volt electric outlets are installed in the stern of the boat, near the transom. Start the process by measuring the distance from the batteries, then doubles that figure to come up with required wire lengths for both positive and negative leads. Before selecting the gauge of the wire, check the product specifications of the accessory to determine the amperage. Deep-drop rigs, for example, draw more amps because they must have the strength to pull heavy swordfish weights off the bottom. Kite reels draw less. The wire gauge needs to be heavy enough to handle the load.