I agree with Alex281, except that I think you will find 1/2" is perfectly adequate. I've used 1/2" for my input water with pumps as large as 1200 gpm. I probably had some restriction at that size pump, but for a 500 gpm, it's perfectly adequate - even up to 800 gpm, 1/2" is fine. Plus it's cheaper, lighter, and easier to work with too.Originally Posted by alex281