How to size wire for solar panels
First Step: First of all, you have to decide the required voltage for your system. Typically, it is 12, 24 and 48 volts. Solar panels need the required size of wire which can last long. The simple equation is that if the voltage is higher, then solar panels need the smaller size of wire in order to carry the current smoothly and safely. In the power equation (P=V x I) of a circuit, it has been observed that the Power (Wattage) "P" is equal to the Voltage "V" times the current I. Therefore, it is realized that if the voltage increases the current decreases, because V x I always equal to P. If, the total amount of current is very small. then you will need the wire which is small in size. Therefore, a higher system voltage is chosen as a thumb rule. You have to remember that all your equipment must run with your specified system. For example, if you choose the system voltage 24 volts, then your solar panels, battery bank, inverter and solar charge controller will require be 24 volts.
Second Step: In this step, you have to determine the maximum currents (amps) are produced by your solar panels. This can be determine by multiplying the rating of one panel with the quantity of panels in your array. For example, if two 12 volt panels are connected in series to increase the voltage to 24 volts, then two panels should be count as one. This is done in this way, because in a series circuit, the current remains the same, but the voltage increases. For example, we can say about 12 solar panels rated at 12 volts and 6 amps. If you need the system voltage of 24 volt, then you should wire 2 panels in series to create required 24 volts for your system. You should do this 6 times. When 6 pairs are wired in parallel, 6 times 6 amps currents are added which provides you total current of 36 amps. This is the maximum amps your wires will carry.
Third Step: Now, you need to determine the distance in feet from your solar panels to the solar charge controller and battery bank location. Never double the distance, even though indeed you will be running two wires, one negative and one positive.
Fourth Step: Due to the resistance of the wire, there will be a transmission loss of the electrical power from your solar panels to your equipment location. You can't avoid this. Typically 3, 4 and 5 percent floss factors are considered for 12, 24, and 48 volt systems respectively. Based on copper wire using the standard AWG (American Wire Gauge) sizes, 00, 000, and 0000 gauges are generally referred as 2/0, 3/0 and 4/0. There are comparatively larger in size. A 4/0 size wire is fairly large. If you use this in a 48 volt system with a 5% loss factor, then you can expect the flow of current of 100 amps over 250 feet. This is considered as a very large system. Whatever gauge wire you use, you have to ensure that it is capable to carry the required amount of current produced by the system.