400W, 400/12 = 33A.
I'd say u need fatter than 12 gauge, quite a bit fatter.
As for a fuse, anything above 33A will be fine, go for a 40 to allow for spikes.
depending on its max wattage of course. if its 400 steady then the above is fine. It also depends on your load if your only using 100W (make adjustments for innacuracy) then 12gauge will be fine.