SCV: The calculator states to NOT use round trip distance, so the TOTAL RT distance is not important, the origional 6' would be used. This is the calc i use most often as it allows for multiple conductors.
OP: My input (12.5v at 6' and 150 amps)
I would suggest you use the calculator and two 1/0 wires (0.7% loss) versus one 2/0 wire (1.14% loss).
Two 1/0 may be easier to work with than one 2/0. The caviot is that you need to know the max amps you want to pull and divide that in half for two conductore, or thirds for three and so on. Putting two 150 amp fuses on a dual conductor run is actually a ~300 amp total capacity.
