I'm finally getting to tidying up the dual battery and accessory wiring on my County, so thought I'd follow Drivesafe's suggestion from a while back and post my plans here so I don't inadvertently do something stupid and burn it to the ground.

Its a tight squeeze, but so far in the normal battery box I've got:
- N70EX main battery
- N87 size 130Ah AGM second battery

Other power supply info:
- Alternator is currently the original Isuzu 40A unit, but will soon be fitting a 70A one.
- Currently have 50mm2 welding cable going to the starter
- Replaced the original (tiny) earth lead with one from a D1 300tdi, which also seems to be 50mm2 by my calcs.
- Main feed from the alternator goes directly to starting battery and is un-fused.

I've built a new center cubby box, with a lower section that will house some electrical stuff including:
- Traxide SC80 controller
- Marine battery switch (MBS) to allow connecting the 2 batteries for winching or starting from Aux battery if the main is flat
- Accessory fuse panel
- 2 extra cigarette lighter style 12v outlets

At the moment "accessories" consists of:
- compressor clutch for onboard air
- fridge
- water pump
- interior and exterior LED lighting

My plan is to:
- run a +ve feed from each battery out the inboard side of the battery box and up through the top of the seatbox into the electrical cubby box.
- connect those cables to each side of the MBS
- connect the SC80 to each side of the the MBS
- take a feed from the Aux battery side of the MBS via a fuse to the accessory fuse block, which will then power each accessory via individual fused circuits.

So, some questions for starters:
- What size cable to use for the battery to MBS cables? The Traxide diagram I'm following suggests 33.5mm2, but I'm thinking I should probably just go 50mm2 the same as the the starter and earth cables. Overkill??
- What size cable should I run from alternator to battery. I'd like to allow for the possibility of a 120A alternator in the future without rewiring.
- Should the alternator to battery wire be fused, and if so where should the fuse be located (battery or alternator end).
- Whats a reasonable fuse size in the feed to the accessory fuse block?

Any other suggestions as to things to allow for at this stage would be great, and if you think the whole idea sucks then tell me why

Steve