I was typing and didn't see your replies, Duncan and DHoladay!
Very good point on the extension of an existing run of wire. That must be calculated into the total. That's also a place where one may accidently choke a circuit by patching in a little piece of light gauge wire, not knowing that the rest is one or two gauges larger! Just as bad, a circuit run in 18 gauge that then has a device added that pulls a lot of amps and is wired in 12 gauge is going to stress the whole run of light gauge.
Just like everything else, there are premium grades of wire and junk stuff out there too. In general Marine spec'd wire is going to be good stuff, soft pliable copper instead of cheap brittle alloys. Of course the coating type and quality matters too.
Though slightly off topic: wherever possible except in emergency repairs, solder and seal ALL connections.