WTF... NO-No-No...
Use #6...
Use a 30 amp breaker...
Don't worry about it...
You can, and should always on a long run, just for safety up size your wire..
When you upsize, you do not also need to up size your breaker...
And No.... Your breaker will not blow at 80% of load... that's bull shit....
Breakers are engineered to with stand 125% of load for a short time...
If your load will always be less than 24 amps like snaps says 80 % of max...
You want 30 amps... use a 30 amp breaker...
or
You want 40 amps... use a 40 amp breaker...
or...
You want 50 amps... use a 50 amp breaker...
Use bigger wire then your options will be better...
You can hook up 500 MCM to a 20 amp breaker [ok adapt it]....
Run it a couple of thousand feet... and still have 20 amps out on the end of your wire....
Never hook up a smaller sized wire to a breaker than you should...
But a larger sized wire to a breaker... totally OK..Legal, and very safe...
Good Luck man....