Wire is sized based on the load and taking into consideration voltage drop, temperature & bundling, which might require an increase in wire size.
Depending on the location of the protection, the fuse size will be sized to the wire, but most of the time will be sized to the load as you'll see.
As you said above, the wire size
is based on the load. Short circuit protection is based on the wire size. Obviously they are related, but there is a difference.
The fuse for the battery is chosen based on the wire size..
Agreed, as should all other branch protection.
My example called for 5 Amps of load, on a #18 wire, which allows 20 Amps. If I were to size my fuse to the wire (20 Amps), then I will not be protecting the load at all.
80% of 20A
I agree you are not protecting the load with this fuse, nor is that fuse intended to protect the load.
If my short circuit happens to draw 18 Amps, the 20 Amp fuse will never blow and the equipment connected to the circuit will be damaged
If you have a short circuit, it is either in the wiring run to the device, in which case the potential will find an alternate path to ground prior to reaching the load and the only thing that needs protecting is the wire between the source and the fault, or it is in the device itself, which should have it's own protection (ie the small fuses on the back of a VHF, stereo, chart plotter, etc).
The fuse size cannot be larger than the current carrying capabilities of the wire, you are correct on that. The fuse for any particular circuit on the panel will have to be at least equal to the sum of all loads on that particular circuit.
Agreed, or else you'll wind up with nuisance trips/fuse failures.
The 80% rule of thumb is a design standpoint type of thing and it always applies to the wire, and in the end to the fuse/breaker if it is sized to the wire.
It takes into account that you do not want to run things constantly at 100% capacity...sort of like you do not run your engine at 100% power. The thing is that as current passes through the fuse, heat builds up and degrades the fuse over time, which is why you have to replace them every 5-10-15 years. Most of the time you have to replace it not because of a short circuit, it's that mechanically, it was no longer good. It also takes into account that you might add future loads to a circuit and that there will probably be multiple loads on a circuit. It's just the accepted design philosophy.
I agree that mechanical failure is much more common in fuses than most realize, particularly on boats. My personal and professional design philosophy, from this standpoint, is that the wire size is chosen to account for future additions to the circuit, not the fuse size.
Once again, My recommendation is to read the manufacturer's recommendations to wire & fuse size. Their recommendation already have in place all the 80% bull^%$, etc, and you just have to forget about all of it. This is especially true for load that have electric motors on them, where things change big time.
I agree that the manufacturer is your best source of information on wire (assuming it is a dedicated circuit) and fuse sizes, with the caveat that the fuse size they will recommend will be to protect their device, and as such will be located at said device, not at the source.