Shorter cables vs thicker cables
Shorter cables are recommended because longer cables have higher resistances. A cable with a higher resistance will dissipate more power, reducing the total power output of your system (a minor annoyance), and causing the cables to heat up (a possible fire hazard).
The formula for the resistance R of the cable is R = (ρ x L) / A, where ρ is resistivity (a property of the conductor material), L is length, and A is the cross-sectional area of the cable. Two important things to note from this formula:
- As you double the length
L, the total resistance will double
- As you double the area
A, the total resistance will halve
Because it is cheaper to use shorter cables than thicker cables, the manufacturer always recommends keeping the cables as short as possible. In your case, a possible "off-label" recommendation would be to double the cable area each time you double the length.
The kit includes 5m of cable with 6mm^2 area for the connection from panel to regulator. To get to 30m (a 6x increase), you'd theoretically need 36mm^2 cable. You can use this calculator to play around with different cable lengths, materials, and diameters.
Higher voltage, lower current
Another option is to reduce the amount of power that the cable is dissipating. If we combine the formula for power (P = V x I) with Ohm's Law (V = I x R) we can establish two relationships for power dissipated by the cables:
Note that in this case the entire current I generated by the panels will be passing through the cable -- so if we double the system current, the cable will have to dissipate four times as much power.
However, the voltage V in this case is the voltage measured from one end of the cable to the other -- it is not the total system voltage. So, doubling the system voltage will not have the same quadrupling effect.
Therefore, we want to minimize the current output. Incidentally, for a system with the same total power output, this can be accomplished by increasing the system voltage:
80A x 12.5V = 1000W, or40A x 25.0V = 1000W
The kit you linked includes two 12V solar panels, one 12V battery, and one 12V regulator. These voltages are all nominal values. As specified, the two panels are to be wired in parallel. With a 24V battery (or two 12V batteries) and 24V charger, you could wire the panels in series, after confirming that the maximum charger input was greater than twice the maximum voltage output of the two panels (in this case, 22.97V x 2 = 45.94V). The maximum voltage output is given in the technical specs as VOC, the open-circuit voltage. Note that you may still need to increase the cable thickness with such an arrangement, but not to the same extent.
Typically for these higher voltage systems, an MPPT charger would be a better choice, as it will have a higher efficiency over a broader range of input voltages. For a 12V system with panels in parallel, it's usually not worth the cost -- but for systems at 24V and higher, with panels wired in series, the broader range of voltage input means that the additional cost of an MPPT system makes more sense, as the charging efficiency gains will be significant.
Another option for reducing current: Assuming you will be using the AC power output from the inverter, you could install all system components outside the camp (meaning less distance between panels and regulator) and use an AC extension cord to bring power to the camp. A single AC extension cord of sufficient gauge will likely be cheaper than the larger DC power cables you'd need to buy.
