Proper cable in an installation is primarily a matter of sizing a cable to match its task. The table shows you the proper size of the cable according to the length, the current load and the circuit type; Non Critical & Critical. The Non Critical is based on a 10% voltage loss and the Critical is based on a 3% voltage loss. What this means is that when the circuit is fully loaded, the voltage at the receptor or appliance will be 3% or 10% below that at the battery. For example, if the battery is at 12,6volts, the receptor or appliance will be seeing 12,2 volts (3%loss) or 11,34 volts (10% loss). Many receptors will run fine with a 10% voltage loss, but other are particularly sensitive to such losses, so they will have to work below 3% volts losses. It is better to use the 3% volt drop, rather than the 10%. The cable length is total length of the positive and negative wires, so you need to multiplied the length from the battery to the receptor or appliance by 2. The negative cable as as much a part of a circuit as the positive cable, so its must be sized the same.