Electrical Design and Build References
This calculator from BlueSea calculates the wire gauge you need based on the current flow, allowed voltage drop, and the type of wire.
It is intended for use on boat electrical systems, which seems like a good match for van conversions.
The calculator accounts for both the ampacity of the wire and also for voltage drop, so you don't have to use two separate tables to get to a wire size.
Cerrowire Ampacity Table
The table gives the maximum current a given size copper wire can carry based on its insulation type.
Do not exceed these amperages, but also check that the voltage drop is not excessive -- see next ref.
Voltage Drop Tables
These voltage drop tables give the voltage drops for wire runs as a function of wire gauge, amperage, and length of run for a 2% voltage drop.
Always do a check to make sure your voltage drop is not excessive -- especially for long runs and high currents.
Water Systems and Plumbing