You can shuck it if and only if the internal wires have markings, and the markings are appropriate for this use. For instance if the interior wires said "THWN-2" that would be good enough. But they won't. Partly, THHN has a magical nylon outer coating which makes it slippery for pulling through conduit. Without that, good luck!
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You're not allowed to assemble the conduit around the wire. So you can't slide the conduit on one stick at a time. You must finish the conduit pipe, then pull the wire through it in one shot.
Aside from the complete horrorshow of trying to drag stiff NM cable through conduit, it simply will not fit. You would need bare minimum 0.85" inside diameter, and that is not available from any 3/4" conduit of any kind.
And as ThreePhaseEel says, you can't use NM cable anywhere outdoors, so that's the end of that.
The outdoor rated version of NM is called UF-B cable. Aside from the horrorshow of pulling that through conduit, UF really, really won't fit. It needs a pipe minimum 1.68" inside diameter, and that means we need to kick up to 2" conduit. Are you kidding me?
Enough with the masochism. Back to the store the cable goes. Get yourself THWN-2 wire.
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If you're none too thrilled about returning already-cut NM cable, then it's a simple matter. (but next time, buy the wire last!)
You can transition from one wiring method to another in any (large enough) junction box. Run the NM cable as cable inside the house. When you get near the outdoor transition, go ahead and install a junction box indoors. Continue in conduit from that junction box. In that conduit (junction box through disconnect through tub), run THWN-2 type individual wires. The neutral must be white or gray. The ground must be bare, green, or yellow/green. The hot wires can be any other color(s) including black-black.
You will neeed 35 cubic inches in this box, so use a 4-11/16" deep square box. The junction box must remain accessible forever, so it'll need to be in an attic, basement or just an unsightly blank cover somewhere. You could put a recep in that cover, but you can't feed it from this #6 cable so you'd need to bring in a separate feed for the recep.
There simply is no such thing as water-tight conduit outdoors. All outdoor conduit pipe is presumed to be 100% full of water 100% of the time. So liquidtight doesn't buy you anything unless Code requires it, but it still is not counted on to keep out water. The moisture defense is in the skin of the wire itself. Further, by being a flexible conduit, you don't get to use it as a ground wire as you could with regular metal conduit.
There is no requirement to have the shutoff switch also be a GFCI. These are totally separate functions: the shutoff needs to be near the hot tub, and the GFCI needs to be anywhere it can protect the circuit. I for one am a fan of putting GFCI indoors (e.g. as a GFCI breaker in the panel) then use a plain disconnect switch. Being indoors is easier on the electronics.
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