Solar panel and cable length.

use the largest dia cable you can afford
Terry

Opinion please here from the experts. If the solar panel is putting out 20V and the regulator knocks it back to 12V then the main consideration here is the distance from the regulator to the battery. So, yes, use the biggest cable you can but as long as the regulator remains close to the battery it is not as much of a problem as the situation where the regulator is on the panel, meaning there will then be a drop between the regulator and the battery.
cheers
Kevin

Lux,
You said somewhere above that your reg is close to the battery - good!!

Assuming you have the same size cable all the way from reg to panels, and if your panels put out 10 amps, then if you use

3mm twin core the voltage drop will be 3.07V. Power loss 30 watts
4mm twin core the voltage drop will be 1.87V Power loss 19 watts
5mm twin core the voltage drop will be 1.2V Power loss 12 watts
6mm twin core the voltage drop will be 0.76V Power loss 7.6 watts

Your panels may not output 10 amps. Say it's 7 amps. The voltage drop and power loss will be 7/10, = 0.7 of the above figures. If they put out 15 amps (!) then it would be 15/10 = 1.5 times the above. And so on.

If you double the cable length you will double the voltage drop and power loss. If you halve the length you will halve them.

As someone said, the bigger the cable the better, but there are practical limits so use the above to work out your choice.

Cheers

Forget the 20 V referred to above. The only time you will see 20 V across the panel is when it is lightly loaded. When you drawing more current the voltage drops quite a bit. See the following diagram. There are 36 cells in the "12 V" panels so multiply the voltage readings across the bottom of the curve by 26.

The open circuit voltage of a panel is in the region of 21 - 22 V. The maximum power is generated when the panel is working at 17 V or so. If you load the panel a bit more you will draw slightly more current but the voltage (and thus the generated power) drop significantly. To find the actual values of your panel you need to look at the specifications for your panel. The voltages vary between manufacturers.

LAZYLUX16, you did not give me much to go on. We really need to know the Vmp (maximum power point voltage) and the maximum power of your array. However it is safe to use 17 V as your Vmp. Another assumption I will make is your regulator has a 1 V drop across it and your absorption voltage is set at 14.3. V. This will mean that you should design your system with a maximum of 1.7 V drop between the panel and the regulator. Work on the conservative side as the Vmp drops a little as the panel heats up on a hot day.

I would not recommend you make up an extension lead for your system. Replace the whole cable between the panel and the regulator with heavier cable for better results. When you are looking at cable remember that 6 mm auto cable is not contain 6 square mm of coper, it's only 4.8 mm2 or less. Read about that here. There is also instructions on that page for determining the minimum size cable you need.

Give it a try you have nothing to loose, if it works fine, if adding a few metres tips the bucket then your set was most likely very marginal to start with and maybe an additional panel would be a good idea.

I have several different cables in my setup and use whatever is needed at the time, you may loose a few watts but for convenience that may no be of a concern to you, like if your camped under shade and need to run a long length of cable to the panel, you'll put up with the loses for the convenience factor.

Your original cable is likely to be around 3 or 4mm twin. I'd follow Peter's response above and simply make up a new lead of the required total length. If you use 6mm tinned twin sheathed marine cable, you'll have everything covered.

This is my ‘go to’ resource for sizing solar cables.
http://hobohome.com/news/?p=581