How to calculate wire size
For DC wires there are two things to consider, the ampacity of the wire and the voltage drop. In low voltage DC circuits the voltage drop tables will give you the larger result in most if not all cases.
Here is a link to the tables:
Essential circuits (like VHF, Bilge Pump and Nav equipment) use the 3% voltage drop tables. Non essential circuits use the 10% voltage drop tables. When calculating use the two way run as the length, that means the round trip from battery to load and back again.
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