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Posted by Sherman in Idaho [24.32.202.166] on Saturday, January 26, 2013 at 21:04:15 :
In Reply to: Quick look at conductivity chart, steel has 10x resistance pf cu posted by Chris Case [75.36.38.179] on Saturday, January 26, 2013 at 20:24:20 :
Let's work out the numbers so we know exactly what the laws of physics have to say about it.
By a very rough guess, the cross section of a frame might be around 2 sq in. Iron is actually only 6x as resistive as copper, so 2 sq in of iron equals 1/3 sq in of copper, which works out to a wire with a diameter of .65" or 420 MCM in electrical terms. 1/0 wire has an area of about 100 MCM (4/0 is ~200 MCM) so by using the frame you're getting about 1/4 the resistance of using a 1/0 copper wire.
Perhaps the frame is a bit less than 2 sq in in cross sectional area, and perhaps the connections from cable to frame add a bit, but then consider that we're assuming here that all the current goes through one frame rail. In reality some of it will travel through the bumper and cross-members and down the other frame rail as well, so the actual resistance of the frame could be as little as half of what was calculated here.
All in all, truck frames make excellent ground conductors, better than any reasonable gauge of copper wire (try pricing 4/0 copper these days, and then doubling it because you'd need two to give you the same resistance as you get for free with the frame). The only cautions are to make sure your connections to the frame are short and good, and that you're not inadvertently asking the current to go through something other than solidly riveted or welded heavy steel.
For what it's worth, if your winch draws 300 amps and your 1/0 cable is 10' long, it has a resistance of about 1 milli-ohm, which means the cable will drop 0.3 volts under load. This is very reasonable. The internal resistance of the battery is probably higher than that. So, realistically, using 1/0 cable out and back, for a 0.6 volt total drop, should be fine. To me, it's more a matter of saving money on precious heavy copper wire and avoiding having to deal with routing any more heavy wire than is absolutely necessary.
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