The ABYC voltage drop tables give figures for wire size for 3% and 10% voltage drop. I am only showing the tables for 12 and 24 volt systems here but there is also a table for 32 volt systems.  A formula for incremental distances, voltages and amperages not covered by the tables in included at the end.  The ABYC standard calls for the 3% table to be used for essential items and the 10% table for non essential items.  It leaves it up to the user to determine what is essential or non-essential.  Maybe for some people having cold beer in the fridge would be essential !  However what they really had in mind for essential items would be things like bilge pumps, navigation equipment, communication equipment and navigation lights.  On boats with an enclosed bridge maybe windscreen wipers might be considered essential.  In most cases the wire feeding the circuit breaker panel would also be serving essential items and so it too would count as an essential circuit.

When measuring the distance run for circuits it is the round trip run from the power source (battery) to the item and back again that counts.  The distance is measured along the wire.

Sizing wires for 10% voltage drop comes up with some pretty conventional wire sizes but sizing wires for 3% voltage drop gives you some quite chunky wires and for these circuits paying attention to minimizing the distance of the run is quite worthwhile.

10% Voltage Drop Table

The formula for non-standard calculations goes like this:

CM= 10.75 x I x L/E

CM is the conductor circular mils (see table below)
I is the current in Amps
L is the round trip distance in feet
E is the voltage drop in volts

Table of Circular Mils

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