Good information, but this guy sure does overkill in sizing some of his cables. Though, that is better then undersizing therm.
Jim is correct, the cables are oversized if the panels were to remain only on the trailer all the time. I do however have a 50 foot 10 gauge extension for the panels that may be used at times. ie:trailer is parked in the shade while panels are mounted on truck which is parked in the sun. The wire run was sized to give me a 3% line loss over the entire section of the run, including the extension cord.
I am using a 24 volt panel array(two 12v panels each with an Imp of 4.8 amps, wired in series) to charge a 12 volt battery
bank. Originally I had purchased a different controller and was going to wire the panels in parallel. However, when wired in parallel the size of the wiring must be doubled to handle the extra current at the lesser voltage. A 50 foot extension run of 8 or 4 ga. would be too cumbersome and expensive.
As it is configured right now the run of wire is as follows:
50 ft. of 10 ga. extension
25 ft of interior 10 ga run to charge controller
1.5 ft run of 8 ga. from controller to busbars
4 ft run of 4 ga from busbars to battery
(remember to double these values when using the voltage drop calculator as you must account for the total length of the positive and negative runs)
In the above run the line loss from the array to controller is approx 3.05% while the line loss from the array to battery
If the panels remain on the roof and the extension is not used then the line loss from array to controller is 1.04% while the line loss from array to battery is 1.16% (this is "overkill" if this is your only wiring setup for the solar
Here is a link to a useful tool. It is a voltage drop calculator designed by members of the Northern Arizona Wind and Sun discussion forum. It has a few quirks to it but it will point you in the right direction if you wish to calculate wire sizes for your own installation.
If you don't have Excel then you can download a free version of OpenOffice at
In small applications such as our trailers I don't think the line voltage loss is that critical, so don't sweat it if your numbers don't add up to exactly 3%.