Gauging the wire....

Chris Bryant wrote:
Guys, he's adding resistance. Why not minimize that added loss?

One foot # 6 vs #2 is 20 millivolts (0.020) at 40 amps.

Chris Bryant wrote:
Guys, he's adding resistance. Why not minimize that added loss?

We don't know his usage. If he's adding resistance in that little short wire he's going to have bigger problems on the feed wires.

Wire size is determined by current carrying requirements (and acceptable voltage drop), regardless of where the wire is. For a short jumper between batteries, the voltage drop is rarely much of a great concern as the length of the wire is short and so the overall resistance low.

If you go with the same size as your power and ground wires, as others have said, you will be fine assuming they have been appropriately sized. If you have more than one of either of them, it's the aggregate cross-sectional area that probably should be considered. Alternately, if you can readily determine the maximum current you'll see, that can also be a good guide. One example of this might be if there's a large fuse or circuit breaker inline with the battery; you can be pretty sure that the maximum current draw will not exceed that fuse or breaker's rating, and size based on that.

If the cost is not excessive and you have the room, there's nothing at all wrong with choosing a larger size than necessary.

Guys, he's adding resistance. Why not minimize that added loss?

There is no point in going any bigger then your positive and negative wires are, that would be like adding a 2" fire hose to your 5/8" garden hose. Now if you are going to add a big inverter you may need to upgrade all the wire.

Depends on your use. It doesn't need to be any bigger than the supply wires. The amperage is the same everywhere.

Even for a short jumper, you really cannot get too big- I would go with #2, even if the main cables are #4.