12 cable size confusion!

[Freshtrax]

Trying to order all my electrical stuff for this setup:
30amp victron DC-DC (already ordered 16mm and fuses etc)

12 way 12 v fuse box To power:

Some down lights and map lights, usb chargers, a fridge, and eventually a diesel heater. From what I can see, the fridge is max 5amps, a diesel heater will pull up to 10 on start up maybe. I know about voltage drop, but what type of wire for all of this in a LWB, with map lights on the back pillars?
1.5 or 2mm (squared) twin core? Siamese?

 

[Dellmassive]

Fatter cable is always best.

 

[DXX]

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[Matt-T68]

My fridge is very susceptible to voltage drop. I ended up running everything but the lights in 2.5mm to get around this as I have 12v feeds in the rear of the van, mainly to power the fridge when it's in the rear under the bed, and the battery is under the passenger seat. Had no issues fingers crossed.

 

[Dellmassive]

2.5mm sub circuits

4mm2, 16mm2, 25mm2 main feed s