Bailz, its not about the cable size that is the reason for running it outside the cab. Its purely a safety issue. If you get a short, the cable will cheese insde the cab and potentially cause a fire, and a short doesent have to be caused by a cable being too small. (sure it can contibute, but not the only answer) Since battery cables need to be directly connected to the starter motor (ie no fuse) there is no protection on the circut. Again, we could discuss the liklyhood of a short occuring in the cab for ever, but it seems thats a moot point.

An engineer has said it must be outside the cab, so thats it reguardless of reasons.

As for minimum cable sizes, well thats another conversation altogether. Cable sizes are govened by an Australia standard ( as I am sure you know bailz), and even though you are not directly leagally required to follow these, the consequences of not following them could be nasty.

If you ignore the standard, you can look at determining cable size on a sliding scale as a ratio of total amp draw vs total time that a cable of given length will experiance that draw. You can use a smaller cable to draw the same amount of amps accross it if the time is proportinally less. Just look at the cheap and nasty jumper lead sets, usually only a 10 mm diamater cable. They can handle short jumps, but begin to get very hot if you attempt to use them for anthing over that. If you then throw in enviromental factors such as ambient temp, then that changes things too.

In the end, bigger is always better for battery cables.