Is it legal according to the NEC to plug in a transformer to a receptacle and run the wire in the wall to a recessed control for a tanning bed?

for USA, Canada and other countries running a 60 Hz supply service.

Cords attached to power supplies/Transformers are not permitted to be installed in or through walls. You would have to surface mount the wire, or install NM cable (Romex) if you need to hide it in the wall.

There are other wiring options as well.

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As always, if you are in doubt about what to do, the best advice anyone should give you is to call a licensed electrician to advise what work is needed.

Before you do any work yourself,

on electrical circuits, equipment or appliances,

always use a test meter to ensure the circuit is, in fact, de-energized.

IF YOU ARE NOT ALREADY SURE YOU CAN DO THIS JOB

SAFELY AND COMPETENTLY

REFER THIS WORK TO QUALIFIED PROFESSIONALS.