Article 430 is the section that deals with motor circuits. Motor circuits are wired VERY differently than regular branch circuits. First of all, take that 21 FLA that's printed on the motor and disregard it entirely. Everything is wired according to HP rating. And according to the NEC, a 5 HP motor has an FLA of 28 amps. So you need to assume your motor has an FLA of 28 amps, regardless of what the motor says.
Second of all, the wiring needs to have an ampacity of 125% of the FLA. So, 28 x 1.25 = 35A. That means #8 Romex, or #10 THHN if you're running conduit.
The breaker can be sized UP TO 250% of the FLA. That means you could use a 70A breaker if you wanted to. (28 x 2.5 = 70A) The reason you can use such a large breaker on relatively small wire is because the motor already has its own overload protection built in ( the little red button/breaker on the motor). The breaker only protects against a dead short in the wiring (which would result in current way above 70A and would still trip the breaker immediately). You want a large breaker to prevent nuisance trips from the large in-rush starting current. That being said, I have a 40A breaker on my 60 gallon, 5 HP Quincy and it works just fine.
Lastly, you can't use a cord and plug with this compressor; it needs to be hardwired in (there aren't really any plugs and receptacles rated for more than 3 HP). There also needs to be a disconnect right next to the compressor IF the breaker panel is in another room, or over 50 ft away, or not within sight when standing at the compressor. The disconnect must also be rated for at least 5 HP.
