Double batteries - before I start!

[TheBeastie]

I am almost ready to go on this one but would just like to double check I have got things right before I start.

It occurred to me I should perhaps use some mega fuses for the feeds to my proposed new distribution board - if so, what sort of size fuse should I fit? My think was along lines of spots, plus full beam plus dipped, plus lets assume fan starting and doubtless a few other odds and ends would give a draw of in the region of 350 watts so 30 amps so put in a 40 amp fuse for the line from the FIA switch and possibly a bit bigger for the one off the aux battery since the transceiver can pull quite a lot. Make sense or am I on wrong track?

My Visio skills are not high so please bear with the crappy drawing but I think it gives the drift of what I am wanting to do.

The new battery cables I have got are 40 sqmm (shown in the heavier lines) but I reckon the original ones are probable LR original fit and look closer to 16 sqmm or perhaps 25 sqmm. I thought of using these to feed back to the distribution boards.

I still have the three busbars to try and fabricate since I reckoned it would be better to feed with battery cables rather than just normal since if all lights and radiator fan are on there will be quite a current - it is split after the busbar and then fused before heading off to the front. Similarly if the radio transceiver and the inverter are going that is another quite big draw so bigger cables have to be better - don't they? Likewise all the earths will feed back to the -ve busbar so a battery cable feeding that back to the battery should help.

Thanks for any input since for once I want to do this one right first time!!

Malcy