In distribution of power at 12-Volts, it will almost always be necessary to select the wire gauge based on consideration of the voltage drop rather than consideration of maximum current rating of the conductor. Selection based on voltage drop will result in a larger wire gauge being used than dictated by selection based on current. This will be typical for any length of conductor more than a few feet long.
The tolerance for voltage drop is typically figured at a maximum of 10-percent for non-critical circuits, and a maximum of 3-percent voltage drop for critical circuits.
Voltage drop is entirely dependent on current flow and resistance of the total stretch (from and to the battery) of wire. Even 22ga wire drops zero voltage with zero current flow.
Either max current load or calculated voltage drop at that current are satisfactory criteria for selecting minimum wire gauge.
It is important that the gauge of the ground wire from each load to the battery or ground terminal be considered as well as the gauge of wire from the battery to the main terminal for +12V distribution.
Considerations of individual loads include each item of electronic hardware, lighting and auxiliary outlets. Each line should be fused or breakered for 50% above the anticipated maximum load current.
The mathematical gymnastics needed to select the minimum size for each wire and fuse can be a nightmare. In practical reality it is more realistic to sum currents and select wire well oversize for each. Remember that the ground wire from the distribution panel carries the same current as the positive supply wire and must be included in the length of wire selected for the entire circuit.
I agree with NYBo. For 1 each SONAR and GPS plus standard nav lighting and a 3 amp aux outlet use 10ga to and from the distribution terminals. If you add more lighting, audio, a spotlight or a bigger aux outlet, use 8ga.
