The current flowing in the wires of a solar system depends on the power that is being transferred. The relevant formula is Watts = Volts x Amps. When calculating wiring sizes there are two separate circuits operating at different voltages and so they each get calculated separately, The goal in the wiring is 1) to make sure the wire is capable of carrying the current and 2) to minimiize voltage drop while still keeping the wire to a reasonable size.

The wiring between the charge controller and the battery will be at battery voltage. In a 12 volt battery system you can it would be fair to assume that it will be around 13.3 - 13.5 volts for much of the charge cycle. Because the voltage is low the current will be high. With 500 Watts of solar panel the formula Watts = Volts x Amps gives you about 37 Amps of current.

You can use the voltage drop tables to calculate the wire size based on the round trip distance between the batteries and the controller. You are aiming to be between 3% and 10% voltage drop, preferrably nearer the 3% end. Its usually becomes very impractical to do better than 3% drop.

You can do the same calculation at the solar end of the circuit. The distance will be longer, the voltage will be higher and the current will be less. The voltage depends on the voltage of the panels, I would go with the "maximum output" voltage figure. Then it depends on if you put the panels in series or parallel.

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