Rick,
Sorry to over-complicate things.
The take-away from my comments is only that it's better to err on the side of a larger supply cable. For a moderate to long run, I'd go for 16 or 14 gauge to supply an SSR4 (though 18 might be fine for a reasonably short run).
But if you start having erratic behavior, try a heavier supply cable first to see if that fixes the problem.
--Dave