3. Pick a Location for the Switch
Selector switches have a four- to-six-inch-wide footprint, so pick a surface where you have enough space. ABYC standards dictate that the switch be as close to the batteries as possible and readily accessible. Some can be flush-mounted for a cleaner look abovedecks, but make sure it’s protected from spray. Hold off on final installation until after you have connected the positive cables to the posts in the back.
4. Connect the Positive Cables
Cables should have “marine cable” stamped on the insulation. Size 2/0 cable handles most outboards and gasoline inboards. Connect the positive terminals of batteries 1 and 2 to the corresponding posts on the back of the switch, and then connect the positive cable from the engine to the output post of the switch. Also connect the positive accessory wire to the output post.
5. Connect the Negative Crossover
In order for the dual-battery system to operate properly, you need a crossover cable between the negative terminals of the two batteries. Also, connect the negative cable lug from the engine/ground and the negative accessory wire to either of the negative battery posts, and make sure all terminal nuts are tight (we like to replace wing nuts with locking hex nuts).
American Boat and Yacht Council (ABYC) standards permit a 3 percent voltage drop for critical electrical systems. The length of the run (measured as the length of positive wire from the source to the device and back to the source along the negative wire) and the diameter or “gauge” of the cable determine the amount of voltage lost or dropped. To determine the cable size you need, visit boatingmag.com/cable-size.