Hello Diyers. I am installing a Dc Dc 50 Amps but have no clue about cable size.
The manual says max cable size is 4 AWG but we don’t have them here. So the next in the range is 25mm2.
The support that I got from where I bought the Orion said take a 35 mm2 but this seems big in my opinion.
I know that length have influence on what cable that I should use so if anybody can explain how to choose the right one that would be great 
35mm2 is too large to fit the terminals. The max on the datasheet is 21.2mm2, 4awg. I would use 16mm2.!
Download the Victron Toolkit app to your phone and this will calculate voltage drop or search on line for DC voltage drop calculator.
The datasheet and manual installation section are linked below.
How Long is the Cable thats the question longer the cable so you need thicker cables
I think 5 to 6 meters
if I take 50 amps for 6 meter with 35 mm2 then there is still 3% voltage drop
When i calc with copper 48v at 50A at 6m max drop 0,2v i came to 54mm2
48 v 50 amps 6 meter then 10 mm2 is enough but this does not solve problem
6m one way distance, 12m total, 50A, 35mm2, 0.34V 2.8% loss. 3% voltage drop.is perfectly acceptable, 25mm2 would be minimum acceptable size for this length. Is your 6m the distance from batteries to the Orion or the positive plus negative cable total length. Many calculators like the one I show, you input the distance between devices and it automatically doubles this for the voltage drop calc.
