Replacing cable 120 W folding <span class="highlight">solar</span> <span class="highlight">panel</span>

Sure you can upgrade.
8 B&S cable is more than sufficient to do the job.
This cable can be bought at Battery World or most Auto Electricians. Just ask for heavy duty twin flat cable suitable for Anderson Connector termination. This may be slight overkill as the 120w panel would be putting out less than 10A, but the insulation is rugged and the diameter of the cable ensures good fit into the Anderson lugs. This also ensures no voltage drop will occur.

I have a 120w folding solar panel and using the cable as above, added a second circuit to the panel junction box, bypassing the on-board regulator in favor of a Ctek D250S dc-dc charger with built-in MPPT solar controller. I have a 5 metre lead and another 5 metre extension lead, all teminating with Anderson connectors.

Where is the reg ??

5M of cable to the reg would ideally be approx 6mm2, and if the reg to battery is longer than 2 or 3 M, then this would be 10mm2.

If the Topaz is wired correctly, then the cable already installed should be at least 13 or 16mm2.

Anderson power connections

This link will probably save you a lot of running around. Even phone them, they are very helpful.

I don't know the topaz but if it has a built in MMPT circuit then you should take your panel feed from before the glued on panel reg.

Your biggest loss in your system will be volts dropped due to cable length.
You are not really trying to pump massive current so the need for massive cable is not really needed.
Think power lines. How do you get zillions of amps through them to feed suburbs when they are only a finger width in size. The answer is in have high volts which negates some of the transmission loss when you process it at the regulator end. I'm simplifying this as you don't need pages of theory.

So, sensible sized cable but thicker if have long runs.
I run a 120w panel wired with 8B&S over a 15 meter length. Theoretically this is only good for about 8-9 amps and I should pump it up to 6B&S.
However 6B&S is bulkier PLUS you are very unlikely to ever get 100% output from your panels so massive cable in this application is usually overkill.

That said, if you already have heavy duty Anderson extension leads then use them as its silly to carry heaps of different sized cables if one can do multiple jobs.