What size cables do I need

[pwfarnell]

There are some pointers in this FAQ.

Smart alternators often drop the voltage to 12.5V and are not suitable for direct charging of a house battery bank. The alternator charges the engine battery and then you connect an Orion DC to DC charger between the starter battery and the house battery. This boost the low voltage from the Smart alternator to a suitable voltage for charging the house bank. Look at the Victron Orion XS.

What fuse capacity or wiring size or bolt torque should I use for the DC connection to my Victron Energy Product

Often the wire size is dependent not only on the rating of the device, but also the distance between the device and the batteries as voltage drop is the limiting factor. Devices such as inverters also have to deal with ripple current due to the cyclic nature of the ac loads being supported by the inverter. Both of these may require that a larger cable size or fuse rating is required compared to that expected based solely on the simple current rating of the device (Amps = Watts/Volts) and the pub…

https://www.victronenergy.com/dc-dc-converters/orion-xs-dc-dc-battery-chargers