In our vehicles there are lots of different types of wire used depending on where it is to be used and also budget.
The current buzz words in the after market wiring are ‘mil-spec’ and ‘Tefzel® wire’. What people don’t realise is that Tefzel® only refers to the insulation used on the wire, another name for it is spec 55 referring to the specifications that the wire is made to. The wire inside is a silver plated copper core with ethylene-tetrafluoroehtylene (ETFE) insulation. Spec 55 is light, thin, resistant to abrasion, most chemicals, fuel and oil. It’s designed for aerospace applications where dimensional tolerance, weight, and mechanical durability are paramount, which makes it excellent for a non-budget limited project. Spec 55 wire is rated -65°C to 200°C (-85°F to 392°F) making it an excellent choice for wiring inside an engine bay.
Another wire that is commonly used in aftermarket wiring of vehicles is *XL, normally TXL, (SXL, GXL and TXL which refers to the wall thickness of the insulation). *XL wiring uses just a stranded copper or tinned copper core and is insulated using cross-linked polyethylene (XLPE), TXL wiring is the thinnest wall of the of the *XL specification wires making it best suited to a light weight small cross section wiring loom where budget cannot stretch to a spec 55 loom. *XL wire is rated -40°C to 125°C (-40°F to 257°F) making it a good choice for wiring for an engine bay, just not as good as spec 55.
In areas of a vehicle that don’t get as hot as the engine bay, PVC (Polyvinyl-chloride) wire is commonly used. PVC wiring uses a stranded copper or tinned copper core and PVC insulation. PVC wire is rated to -40°C to 80-105°C (40°F to 176-221°F) depending on specification making it not suitable for use in the engine bay.
There are lots of other wires available and used but these are the regular after market choices.