Here is a link with the REAL ampacity limits for wire. This is from the National Electrical Code (NEC). The gauge is AWG or kcmil for the bigger stuff. This is for not more than 3 current carrying conductors in a raceway, cable or earth. There is another table for conductors in free air. Under the hood, under carpet, etc IS NOT FREE AIR so that other table is NOT the one to use. Additionally, these limits are based upon 30 degrees C ambient (86F). Needless to say underhood temps FAR exceed that even in winter months. The numbers/letters at the top of the columns are for insulation type. Those temperatures are what those insulation types are rated for. Theres a derating correction factor table at the bottom for ambient temps over/under 86F. Don't be fooled by the physical size of the wire, AWG is the amount of copper from ANY manufacturer. Just because someone's 1/0 looks physically larger than another's means NOTHING. Its still 1/0. 1/0 is defined as 105,600 circular mils of copper no matter who makes it. There is also no mention of length. This is because a two foot piece can carry as much current as a two mile piece. Current will overheat it no matter how long or short it is, a concept n2audio can't grasp. Voltage drop is a concern, but these current limits apply to all wire no matter the length. If you apply the ambient temperature correction factors you'll be surprised how little current your wire is rated for.
http://www.coastalelectric.com/Reference/NEC310-16.html
