You'll find 30 amps isn't going to be enough. 50 or 60 is more in line with what you'll need.
That said, how many wires are there? 4 is an excellent answer although there are tubs that will run using 3.
Is it Romex or individual wires? If it's Romex, it's not good enough. You need individual conductors.
What gauge of wire is there? 6 gauge is an excellent answer; 4 gauge an even better one. But it depends on long the run is from the panel to the tub.
How big is the conduit? 1" is a good answer.
I'm not an electrician so please do consult with one regarding the tub you choose.
Jim