there have been numerous threads on this topic. suggest you search for them.
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In response to Avguygeorge's suggestion, I first used 12 gauge unstranded and didn't notice much change, then went to 10 gauge unstranded and got a BIG change. I think most would advise no smaller than 10 gauge, though of course I could be wrong about this. Whether stranded or not, again I'd seek expert opinion. I was told to use unstranded, and it certainly worked for me. I also use an 8' copper-plated brass rod all-but-completely buried in the ground.