Fusing isn't too difficult....
Fuses are used to protect "downstream" electrics.
Equipment should have its own fuse to protect itself - eg, amplifiers, PCs etc. (If not, you supply an external fuse. But some loads don't have their own fuses - eg lights etc...)
Apart from the loads (equipment, there is also distribution to get it there - ie, cables/wires & junctions for the electricity (current = Amps) to get to the load.
We connect the wires to a power source - eg, a battery.
Be place a fuse at the start of the wire as close as possible near the battery.
This fuse is simply protecting its downstream load - ie, the cable.
The cable might be rated for 30A (30 Amps) hence we might use a 30A fuse or smaller.
If we split the 30A cable into two 20A cables further down, then we should have a 20A fuse at the start of each 20A cable. Or 5A fuses at the start of 5A cables. without those fuses, the battery will supply whatever "fault" current (a high current due to overload, or short circuit etc) and melt the wires. A 10 Amp fault current will melt the 5A cables etc. So we have a fuse that melts instead of the downstream cable or equipment.
I practice we might try to avoid too many fuses. And there are practical considerations like physical security etc - eg, if a section is so well protected it can't short out, we may skip its fuse - provided there is a same-rated or smaller fuse downstream. Or we may decide that fro the battery +12V terminal to the fuse or distribution box/block is so short or safe that we don't need to fuse at the battery terminal.....
In your case, the ATX3 has a 150W peak output.
It's input power is hence at least 150W.
Power = Volts x Amps (current) - ie, P-VI, so I=P/V.
For 12V, I=150W/12V = 12.5A. So you should have at least 15A, maybe 20A.
But the you need to add ATX inefficiency etc - maybe for 150W out it uses 180W in.
And if it works down to 6V, that means I = 180W/6V = 30A.
If you will only use 100W with a battery no lower than 10V, that's 100W/10V = 10A, so then a 15A fuse and cable is fine.
FYI - a general rule of for fuses & cables to normally carry no more than 70% of its rating - eg 0.7 x 30A = 21A for a 30A cables & fuse, but 25A and 30A are okay - it's just that there is no spare overhead, and they may run quite warm.
Also to reduce voltage drops form the battery to your "max 30 Amps" ATX, you might decide to use 100A cable to the ATX or to the last section of 30A cable.
Rather than using a large fuse for the 100A cable, you could use a 30A fuse since that is all you will be using. (There are some considerations with this - like heavier cable conducting heat away from the fuse, but that shouldn't be a big issue.)
And voltage and power have nothing to do with fuses EXCEPT for their voltage handling - eg, 240A AC on a 12V fuse might arc across the fuse etc.
Fuses otherwise are simply rated for current. (Sometimes AC or DC fr BIG fuses.)