I want to use an existing 40 amp line that was originally for an electric dryer that has been removed for the hot tub. The existing wiring is 10 guage and runs very close to the hot tob. The hot tub can be hooked up 30 amp or 50 amp. The existing 40 amp circuit breaker is not GFI. I want to run the hot tub at the 30 amp setting. What is the best and safest way to hook it up using existing wires if possible, and include GFI.

Check out this site for "proper wire size" use the calculator here

http://www.cossin.com/page4.html

and here for installation help

http://spasupport.com/electrical/main/installguide.html