Originally Posted by DrCharles
Originally Posted by CMcAllister
Length? An electric guy once told me the length is more important than the gage in determining amp capacity. I believe him.


I'd find a new electric guy. Both are important. The wire gauge determines resistance per unit length. Obviously you then multiply by length to get total resistance! Then you figure out voltage drop by multiplying the resistance by the current. (Ohm's law).

A very short piece of small gauge wire is known as a "fuse" and it works by melting whistling

In response to Cab's question: at 77F, 4 ga. wire has a resistance of 0.25 milliohms per foot. (At 122F it's about 0.28).
So... how much voltage drop are you willing to tolerate, how long is the wire and how much current is passing through it? work

Incidentally a "12 volt" system with an alternator is around 13.8 volts which also varies with temperature (higher voltage in colder weather). If you're not running an alternator then your battery voltage will be dropping with time also

Edited to add: 14.5 volts, take a SWAG (Scientific Wild-Ass Guess) that you are OK with 0.5 volt drop, and the cable is six feet long? That's 0.25 * 6 = 1.5 milliohm.
1.5 milliohm will drop 0.5 volt if 333 amps are flowing. (0.5/0.0015 = 333).
Unless you're using it for starter cable, you don't even need #4 most likely.
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I had a incident on my S/P bracket car when I used the alternator output to feed 12V to my timing light, I hooked it up with the motor not running and then started it up. The timing light lead vibrated over sideways and grounded out the alternator output lug burning up the 10 gauge feed wire all the way back to the trunk where the two batteries are, big stinking smoking mess whiney That ended my day of racing in Sacramento, CA at the Mopar race their.
I rewired it with 10 gauge and had a little lower output reading on my Volt gauge and I ended up calling Power Master about them rebuilding it or selling me parts for me to fix it, they wanted to know what gauge feed wire I was using and insisted that I needed a #4 gauge feed wire and to ground the alternator with a # 4 wire to their grounding lug to a threaded connection on the block confused
I was going to use two #6 wires to feed the batteries, one to each battery but he insisted on one #4 instead. I've grounded and am ready to install the #4 wire now back to the batteries today, probably less than 10 Ft total length.
Thanks to all for the help, up I should have paid more attention in basic electricity math class back in 1975 whistling That or dug out one of my old text books on basic electricity blush

Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)