Found a lot of people having problems with adding additional length to the leads from the panels to the controller since they are only 10-12 feet long. Here is some great info I found from Marshall at All Experts.....
Hi Marshall - I recently bought a 45 watt solar panel system (harbor freight) that is actually working fairly well. My problem is that the wire leads from the panel to the regulator are only 11’ long. There’s 3 leads (2 wires from each panel + & -) and they appear to be about 18 - 22 ga. Can I extend these leads to maybe 24’ & what type & ga wire should I use for the extension. I plan to leave the original leads & use a terminal block and then run one wire each (+ & -) to the regulator. Thanks greatly for any help, ideas, & advice…..Mark
A quick look on the internet and I found a 45 watt solar power kit from Harbor Freight. The description stated: " Maximum current, 3000 mA. 15 watts max per panel. Peak voltage: 23.57 volts open current". There is a box shown in the photo which I am going to assume is what you are calling the regulator and that is can change the output voltage to 3, 6, 9, 12 volt DC. 23.57 open circuit voltage is important due to anything that is hooked up to the solar panel (not the regulator), needs to be able to handle this potential voltage or may be damaged. This is not the max power voltage (voltage under a work load). The max power voltage may be listed on the panel or can be guesstimated by dividing 15 watts by 3000mA or 3 amps equaling 5 volts. If wired in series, which they are, they will produce 15 volts with is common for a 12 volt system (wiring in series change the voltage and wiring in parallel changes the amperage). The thing with solar panels is that they grow and fall in intensity as the sun shines on or is blocked from the panel, so the odds are pretty slim that 15 volts will ever be constantly outputted. Temperature also effects the voltage output, the cooler the panel, the higher the voltage.
For wire sizing you start with the possible max amps (3amps) and find a conductor that can handle that load. A 22 gauge wire is sufficient for 5 amps, but there is also the issue of resistance in the wire can drop your voltage. This is the voltage drop. At the bottom of the following web page, you can figure out the voltage drop and the resulting voltage delivered to the work load.
So a 22 gauge wire; at 12 volts, at a distance of 24 ft, at a 3 amp current will result in a 20% loss of voltage resulting in a delivered voltage of 9.6 V. 15 volts * 80% = 12 volts.
a 18 gauge wire; at 12 volts, at a distance of 24 ft, at a 3 amp current will result in a 8% loss of voltage resulting in a delivered voltage of 11.1 V. 15 volts * 92% = 13.8 volts.
If you wanted to charge a 12 volt battery, you would want to deliver 14 v. In solar electric design, the lowest voltage drop we allow is 2% which means I would recommend a 12 gauge wire for a 24 foot run.
As far as what wire type, if it is in an NEC approved conduit, then I would recommend THHN-2. If it is going to be outdoors exposed I would recommend USE-2. You will probably need to find USE-2 on the web as most hardware stores and electrical parts houses don't carry it in-house.
Hope you found that helpful, I know I did!