up to 500 watts rms 8 gauge is acceptable
over 500 watts rms 4 gauge is required
over 1000 watts rms 2 gauge
over 1500 watts rms 1/0 gauge
if using 4 gauge and above you it is a wise decision to upgrade the ground from the battery. run a ground the same gauge from the neg bat terminal to the frame rail below it
in DC voltage current actually flows from the negative terminal to the positive terminal. if you add a 4 gauge power wire, the current is still trying to flow through the 6 gauge wire these cars come with from the factory. what goes in must come out!
do not use factory bolts or nuts to attach the ground for the amp- ground it directly to the sheet metal with a self tapping screw and a star washer, remove all the paint at the grounding point. the ground cable should be very tightly screwed to the sheet metal- use more than one self tapper if you have to- if you can move the ring terminal its not good enough. after you have grounded the wire, hit the ground with some spray paint to prevent corrosion
always fuse power wire!! if there is a short to ground anywhere in the wire and theres no fuse, it will pretty much begin to glow like a lightbulb filament- you WILL burn your car up! the fuse should always be located within 18 inches of the battery- no more!
dont run power wire through door jambs- looks ghetto and it will get shorted out. there is always a factory grommet you can go through- take a box cutter, make a small slit in the grommet, and poke the wire through- this gives a water tight seal.
some cars have a difficult to acess grommet- no problem! use an old car antenna, and use e-tape to attach it to the antenna, and poke the antenna through. get inside and pull it though.
if you do end up drilling a hole through the firewall to run wires- ALWAYS USE A SNAP GROMMET!!!!! they usually come with the power wire kits or can be had at any car audio shop or radio shack.
look before you drill- double check and make sure you arent drilling into a big wire harness, brake booster or gas tank.
a little bit of patience and common sense will save a lot of headaches and $$$$ down the road