Copper Clad Wire

Only real problem I know of using stuff like THHN "house" wire is that it's fairly stiff, as in hardened. Makes it a bit more difficult to "lay" into some places. You CAN buy automotive wire with better insulation. Years ago I bought some kind of Belden heavy duty truck wire to rewire the old Landcruiser. You could take a short piece of say, no14 and lay it across the battery, and most of the time it would "blow" the wire apart without melting through the insulation.

Much better than thermoplastic. Sorry, don't remember the name of it, and this was in the 70's