I just used14 gauge for everything. What confuses me is the tiny wire used in the PIAA loom.
If you look at the ST's wiring harness there is very little #10 wire or even #12. The manufacturers are planning that NOTHING else will be added to their circuit, they might be using higher temp insulation (tho I doubt it is very much higher than (some) primary wire which is rated a little higher than 100deg C), and they are very cost conscious. They are also designing their wiring w/ a very low safety factor - look at all the problems we report with terminations. I cannot remember ever seeing a problem with a car's wiring. Sure moisture has a role to play, and yes, I am ignoring corrosion caused by road salt, but mc' seem to have more electrical gremlins.
That said, I'm a retired electrician, etc. like Larry, and I echo everything he said. Since I don't load my circuits up very much at all, and tend to run few items per circuit, I use fuses closer to the actual load (load +20%, then the closest fuse to this), and run almost exclusively #14 wire - mostly because I don't want to stock multiple colors in several different sizes. When I was working and running my own company, things were different - and I stocked a variety of sized wire (stranded only) in several colors.
Uncle Phil, I'd suggest you use the chart. And check the full load of your heated gear, add 20% of that (using watts, if you wish) divide by 12 and that will give you the current flow. Then pick your wire size. I don't know about you, but I find I very rarely run my Warm n Safe above half way, so there will be extra room above the actual
load. Rules of thumb are fine, but when you can, run the calculations to make sure. We are talking simple math here.