Posted by David Sherman on Wednesday, January 03, 2007 at 16:05:05 :
In Reply to: Advice needed-Mounting a small winch posted by Mark in NJ on Wednesday, January 03, 2007 at 15:17:13 :
He way to calculate it is like this. First figure out how much voltage drop you can live with. Say you decide 1 volt is okay. Most starter systems have more drop than that if you check the voltage at the starter terminal during cold cranking. I don't quite understand your setup, especially the part about the gage of the wires going to the handheld control unit, because the motor current should not be going to any handheld control unit. The wires with the 130 amps in them should be as short as possible and the switching should be done with contactors (1 for one-direction, 4 for 2-direction). The handheld unit only switches enough current to actuate the contactors. That's how most electric winches are set up.
Assuming for now that you have 30 feet of total wire, including the ground wires from battery to frame and from the winch or winch contactor pack to the frame, and assuming the framee and the connections have no resistance, your wire needs to have a resistance of less than 1 volt divided by 130 amps equals .0077 ohms. Since it's 30 feet long, you need wire with less than .0077 divided by 30 times 1000 equals .25 ohms per thousand feet. Looking in a wire table, #4 copper has a resistance of .25 ohms per thousand feet at 68 degrees, so it would be just barely adequate. As wire heats up, its resistance increases, causing it to heat even more. 30 feet of #4 wire with 130 amps going through it and a 1 volt drop would dissipate 130 watts over the length of the wire, which is a safe value if the wire isn't coiled up, and is right on the edge of what the national electrical code allows. #2 would obviously be better, but is considerably harder to work with and of course more expensive. I would probably go with #4, paying very good attention to all the connections and soldering everything that can be soldered.
If you really have 130 amps going out and back to the handheld control box through 4 wires (for a 2-way winch), that's where all your voltage drop is going to be and nothing you do between there and the battery will help much.