You can approximate wire gauge size with this calculator (copper wire only).

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1. Solar Array Optimum Operating Voltage (Vmp)

Volts

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2. Solar Array Optimum Operating Current (Imp)

Amps

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3. Wire length from Solar Array to Charge Controller (ft.)

Feet

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4. Acceptable Vmp Percent Loss (2 to 5% recommended)

%

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Gauge Size:

AWG

NEC maximum current for different wire sizes

AWG	16	14	12	10	8	6	4	2	0
Max. Current	10A	15A	20A	30A	55A	75A	95A	130A	170A

Resistance increases with length

The reason different wire lengths bear different ratings is because the electrical resistance builds up as the cable gets longer. At that point, up-sizing the power cable will restore the voltage to its intended level.

Acceptable Power Loss

Undersizing the wire size will result in excessive power (watts) being lost in the wires rather than delivered to the load (battery bank, inverter. Typically we recommend the power loss below 5%.

Voltage Drop

A 5% voltage drop causes an approximate 10% loss in light output. Voltage drop causes a nearly proportional drop in light output. A voltage drop greater than 5% will reduce this necessary voltage difference, and can reduce charge current to the battery by a much greater percentage. Our general recommendation here is to size for a 2-3% voltage drop. If you think that the PV array may be expanded in the future, size the wire for future expansion.

Don't choke the flow of power

On the other hand, installing too large a wire gauge doesn't really have a downside, but there is the potential for better performance. Obviously, there's no need to buy 2-gauge wiring when 10-gauge will do. That kind of overkill would be a waste of money. But if the calculator could lean either way between two sizes, going with the larger wire size would be the smart choice.

Select Inverter Voltage 12V 24V 48V

Select Battery Voltage 6V 12V

Number of batteries in series 0 2 4 6 8

Number of strings in parallel 0 2 4 6 8

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Number of interconnect cables: