6 volt systems suffer voltage drop and should use this heavier cable.
general guide to auto cable applications:
14/.030 8amp 1.00mm cross section area used side and rearlamps
28/.030 16amp 2.00mm cross section area used head and spot lamps
44/.030 27amp 3.00mm cross section area used battery feed and ammeter circuits
37/.090 160amp 25mm cross section area used on starter circuit
it is sometimes beneficial to use a heavier cable than stated on 6 volt systems. never replace any cable with one of smaller cross section area, if unsure of application always seek expert advice.