Once you select the physical location of the pump, measure the distance from your panel switch (independent circuit breaker for 'just' the pump preferred); Plus, the distance to your main 12vdc supply. Then multiply by 2
to include the ground wire in the 'total' circuit length. This will give you the total 'run' of wiring; then ........
Once you have that 'total' length dimension, go to Circuit Wizard - Blue Sea Systems
and let the 'program' select the proper wire gage.
FWIW - if you plan on 'extensive' use of the wash down pump, be advised that the typical Johnson Pumps are 'thermally protected' (once the pump reaches a certain operating temperature, the pump cuts-out, automatically). So, to keep the pump motor running 'cooler', its best to oversize the wiring by approx. 10%-15% so that the overall resistance
in the wire is less and will result in less 'amps' draw for the pump. Less overall amperage used (motor, PLUS wiring losses) will allow for longer run times with the pump before the internal thermal cut-off switch activates. Typically 15-20 minutes is all you get with a Johnson Pump before the thermal cut-out switch activates. 'Oversizing' the wire a bit, will allow less amps consumed which results in less heating of the pump motor - for longer pump life, and less 'thermal cut-outs'.
If you do plan on extensive
usage of this pump, consider to add a large 'accumulator' to the 'downstream' high pressure plumbing from the pump. This will allow the pump to deliver more 'constant' water flow and with less pump 'cycling'.