I am looking to install a 12V DC bow thruster in a boat. The device draws up to 500A but is only used on and off for a few seconds at a time. I have an 800AH 12V battery stack but it's 8 metres away. Can this be used? If so, what size copper cable should I use?
There are two problems with sizing a cable when powering a remote load. The first is cable heating, the second is voltage drop.
As you will only be operating for a few seconds at a time, cable heating (which gives rise to the 10A/mm2 figure for small currents, less for larger currents, in mains applications) will not be your problem.
How much voltage does your thruster need to work, or to work well enough?
An easy to remember rule of thumb for room temperature copper cables is 1m of 1mm2 cable has a resistance of 17mohm. It's a convenient size figure, you can work everything else out by scaling, and it's a lot easier than making sure that you have all the right factors of 10 if you look up the ohms for one cubic metre!
Anyhows, if we start with a current density of 5A/mm2 (just seat of the pants, let's see where we get to) 500A would need 100mm2 cable, which is available as welding cable. 16m (8m there and back) of this stuff will have a resistance of 17mohm x 16 / 100 = 2.7mohm. At 500A, that's a voltage drop of 1.36v.
That doesn't look too bad from where I'm sitting. Will your thruster work well with 1.36 volts less than whatever your battery bank puts out at 500A, less some further drop for a switch?
If you double your cable area to 200mm2, that's fairly expensive, and only saves you 0.7v drop. Probably not worth doing.
You could halve your cable area to 50mm2, but your drop would increase to 2.7v, possibly prohibitive, I don't know?
If your thruster draws 500A in normal operation, it will probably require more to start, which will lower the voltage even further. However, I suspect that a thruster is easy to start, the static load is very low, so this additional voltage drop should not be a problem.
Your call for cost of cable versus performance of thruster tradeoff. Of course moving your battery or installing a another closer one will not need as much cable.
The thickest wires you can install. It would help if you install one additonal batery near bow thruster, and wire parallel wires to the existing bank. In this way you would require some 150mm^2. The best way is to hire some experinced boat electrician. The very important thing is to use hydraulic clamp tofor mounting the cable terminals and to cover the space between terminal and cable insulation with thermo strech tube. This prevens moisture from entering into copper wire, also the hydraulic clamp binds the terminal so strong, that no moisture can find the way in. Look the bow thruster connection, and you can determine the max. wire thickness, you can't attach any larger conductor.