For instance, your new electric cockpit lights may well come with installation instructions that specify a wire size at a certain voltage. But you’d still need to know, as would a certified marine electrician, whether, because of the length of the wire run between the power source and the lights, a voltage drop would occur. As the length of the wire run increases, resistance increases. With an increase in resistance comes, among other things, a drop in voltage. Larger wire than the directions state may be required to alleviate the problem. The installation must be built safely around such facts.