Post a link to the specs for your hot tub.
I have a 50amp 220v line (3 wire plus ground) in my master bath that is hard-wired to a hot tub. Remodeling the bathroom and replacing the hot tub with a 110v air bath. Can I use the 220v line to feed a sub-panel (I would hide it in a closet in the bathroom) and then branch two 110v circuits out of the sub-panel?
It's 18 years old, I can't even find it on the manufacturers website.
You can not put a panel in the bathroom or in a closet.
Yes you can use the branch circuit to feed a panel as long as the conductors are protect at or below the ampacity of the conductors.
The panel can not face in the bathroom or a closet but can be in the same wall as long as it faces out of these places.
It is an electrical code issue (NEC). Panels are not allowed in closets or bathrooms.
Bummer, thanks for the clarification. Guess I need to go to plan B
Besides all this it is a violation of 240.24(D)&(E) of the NEC. (D) says no overcurrent devices in a closet and (E) says no overcurrent in a bathroom. Now if you wanted to set a panel in either location it would be okay just as long as you didn't install any overcurrent devices such as fuses or breakers.
I think you would need to run another cable. While neutral is connected to ground in the main panel, a subpanel needs the neutral and ground to be separate, along with the two legs (i.e. a 4-wire cable, not three). The neutral is supposed to carry the return current, not the ground wire. The ground is for safety, not current for normal operation. A 240vac device does not need the neutral lead, but a 120vac circuit does.
Important note - I'm not a pro
Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013
Seen MANY panels in closets. Never saw a fire. They get a lot less water vapor than the feeder panel outside on the pole or wall. Those code panels exposed in a hall usually get a picture hung over them so they cant be located anyway.
By the way, you already have at least one 120v circuit by using that big cable and one of the hot wires. Get rid of the 50 amp breaker and feed one wire with a 20 amp breaker at the main panel. Put a J box in the bath and hook up your 12g romex to that one wire and the ground and neutral.
You also have the second 120v circuit by duplicating that with another 20 amp breaker at the panel and sharing the neutral, but the guys here will probably scream.
PS: why  120 v circuits for a bathtub air blower? You would also need GFCI breakers unless the tub has its own.
What is your total amp draw expected? Blower will be off most of the time anyway. Likely one 20A circuit will suffice.
The tub install instructions say: "The pump and blower are designed for use with an independent 15 amp, 120V electrical power supply protected by an over current protection device rated not more than 15 Amp."
I am sure the NFPA could quote case histories of fires or shock hazards caused by such location. It wasn't a good idea when it was done, and now it is against code.
Actually, besides the fire/moisture issues, I believe part of the reason for the restriction is that they like the panels to be visible and readily accessible.