the short answer is 5HP needs 10 gauge wire, and a 30A breaker. There really is no way to be legal with 12ga 20A breaker... sure you can do it, and a ton of other bogus wiring stuff that would never be permitted or pass inspection, and it will work just fine until your house burns down
I've got 3 5HP machines that I want soooo much to just run on my existing 20A circuits. After 15 hours with the NEC, there's just no way to do it, unless I just choose to ignore the code and wish it true...
Good luck