I am wiring a new Imperial Winchester Spa (mfg by Jacuzzi) that I bought at Costco. It is being wired 50 amp. I understand that they are a little different in that they run everything off 240V inside the spa and only require 3 wires (red, black, green) instead of 4. So I have run 3 wires from my GFI breaker/disconnect I am mounting outside to the spa. I am all set with the wiring from the GFI to the spa. No questions there.

My question is regarding the use of 3 wire instead of 4 wire from my main breaker box to the GFI breaker/disconnect outside. I have #6 with red + black + green already run from the main box in the garage to the crawlspace that was being used for an electric dryer. I now have a gas dryer and have this #6 wire available to me without the hastle of running new wire to my main breaker box. Can I use this 3 wire or do I have to run 4 wire from the main box to my GFI/disconnect?

Most GFI disconnects require 4-wire setup to be sent to the GFI but not to the spa, check the GFI manual to determine if the requirements are different, but more than likely you will need to run 4 wires to the GFI (Black, Red, White, Green).