Is the engine loom made up of generic copper centred wire? Or is it of a higher spec?
it'll have higher-temp insulation but nothing out of the ordinary. Multi-strand copper wire. Somewhere like vehicle wiring products will have wire specifically for engine looms.
Cheers, does anyone know what the current carrying capacity of the wire needs to be?
Highest rated fuse is 20A so that answers my question
20A fuse will need to have well over 20A pulled through it to blow, so you should size the conductor with some headroom over this figure to ensure the wire is capable of carrying the fuse blow current. If you don't, your fuse might sit happily at 25A during a short-circuit, and the wire gets hot and ends up acting as the fuse instead.
Yeah looking at the thinwall cable rated at 33A
Thin wall automotive wire is what you'd want, I've found some tri rated in my original internal looms.
You want copper multistrand cores, size wise depends where on the car and for what as the seem to range from 0.33mm (7/0.2 strands) 24 AWG to 2.5mm (35/0.3 strands) 13 AWG. Best looking what was there before if that is possible...
This is one supplier we use at work http://www.autotechnik.co.uk/Products/Automotive-Cable