The NEC does not apply to auto's and also anticipates more than one current carrying conductor grouped together. It's a good place to start if you have no other info. You can only go wrong under sizing wiring. 10 AWG is a bigger diameter than 14 AWG. You can also increase the temperature rating of the wire and make the wire Ga size smaller within reason. 60, 90 and 105C are very common insulation ratings. You also want insulation that is oil and gas resistant.