Based on the cig adapter, then 15A. But you could go 10A as discussed. (And maybe the 15A is to limit for the cig plug/socket which are normally rated at a max of 15A, though few actually handle that!)
The GM wiring for 10A should be ok, but many would run separate wiring for stuff like this, though a laptop isn't that critical. IE, it runs off its battery rather than being dependent on a good supply - not that intermittent or bad contacts would be good for it, or the converter. (If it were a cig socket, I'd feel the heat.) Many of us run our own wiring direct from the battery for PCs, audio, etc. (I do, for a normal HU and a spare cranking battery; both have dedicated + & - from the main battery. I have no PC.)
One fuse is fine. EG - if it were f.ex 50A wiring from the battery thru a 30A fuse thru a 30A relay to the dc converter with its 10A or 15A fuse, then the converters fuse could be moved to the battery instead. IE - battery with 10A or 15A fuse even though wiring and relay is 50A & 30A etc. The fuse still blows (in theory LOL) before the wire or relay melts, and it still protects the converter. (That protection is against shorts etc - not against failure.)
I'm not sure what you mean by the GM circuit supplying to activate & power the relay. In my setup, that circuit might activate the relay which switches the (fused) battery power. But if it both activates & powers, the there is no need for the relay (unless later you wanted to add some sort of off control, but then why not get the heavy power from the battery so you could activate with the GM circuit (= ACC?, hence off when cranking?), or a manual switch or a voltage switch etc?)
Glad you understood my crap. It was late last night - not that that necessarily changes anything.
But now is early morning, pre-coffee etc. And I'm up early to start a new job. Holy cow - it's still dark outside!
