I was in the believe that -12V cable should have same size of +12V cable so i used 16qmm or 6 AWG for all 3 cables even if each cable length to batteries is not longer than 3ft. At 50A output cables get warm but not too hot to touch.
@ Lunatic How long have to be your cables?
Have a look at this tread, there is a link to the Victron cable length calculator.