I am installing a hot tub outdoors. It requires 220-240 volt service. My main is 200amps. I have plenty of room to physically add the 50-amp double-pole gfci breaker that I'm going to use to feed a 50-amp gfci disconnect at the outdoor spa location. The panel is located within 30 wire-feet of the spa location. The spa is being installed on an elevated wood deck 15 feet off the ground. The panel is just inside the wall nearest the spa location. I'm planning on routing the cable through the wall, along the underside of the deck and terminating in the disconnect mounted to one of the 8x8 deck support posts. The cable I selected is Cerrowire 250 ft. 6/3 UF-B Wire Model # 138-4203T.

Do I need conduit for this installation, or can I staple the cable to the deck joists?

If I need conduit, what diameter do I need without violating conduit fill rules?

I would use 3/4" PVC, 2 #6's for the hots,an 8 for the neutral, and a #10 for ground.

You do not need a GFI breaker in the panel and at the tub. You can use a standard breaker in the panel, and the GFI at the tub.

UF cable is acceptable since you are not burying it. It will be exposed underneath the deck, and your plan to staple it underneath is fine.

If the cable is exposed top physical damage, for example, where it may rise through the deck floor up to the disconnect, it should be sleeved with conduit at that point.

Thank you for the replies. I used Stratmando's suggestion of using individual conductors (Southwire rated for indooor/outdoor applications) instead of the UF-B cable - savings of about $20. I also used the 3/4" conduit, but opted for the more expensive flexible LiquidTite to ease the conduit installation. I used tkrussell's suggestion to eliminate the GFCI at the main panel and saved $80 on that alone. Other than needing to paint the overly-obvious gray box mounted on my deck, I'm very pleased with the final installation.