I am adding a 240 volt dedicated circuit from main 200 amp surface mounted outdoor service panel. It will be used to power outdoor 50 amp subpanel. Run is 50 feet and plan to use #6 stranded copper for hot and neutral and #8 for ground. Plan to use liquidtight flex conduit (gray) exiting main panel into crawl space. It it permissible to use this to continue run exiting house under wooden deck to terminate in subpanel? Must I use other type of conduit with junction box under home to continue run to terminate in outdoor mounted subpanel. Subpanel is for hot tub which rigid PVC and liquidtight whip will be used from subpanel.
The use of flex in general is usually for short extensions called whips. From a junction box to a motor per se. When ran in that manner I believe the Code says "no longer than necessary" but I will need to double check.
In your case it's not the same, plus it will be under a deck so I think you will be fine. Just FYI, it will probably be easier to run the wires through PVC conduit instead of flex as flex will bow and twist on long runs.
Tip: Run conduit then a short piece of flex at the end. Inside the house transition over to NM-B with a small 6x6 junction box
Code requires nonmetalic flex to be supported at intervals of 3 ft either by straps or wood members and within 12" of the termination points if it is not fished or if the length is <=6ft,
For example under a deck that is accessible would require straps. Through a wall or under a deck not accessible would be ok to not strap. But as recommend, PVC conduit would be a better choice for longer runs.