The 80% thing is mostly a myth.
You can run a single 20A non-continuous load on a 20A circuit. If it's a continuous load, it would be limited to 80% or 16A.
For 15A and 20A circuits with multiple outlets, then you are limited to 80% for any one cord-and plug connected load not fastened in place. But you could use multiple such (non-continuous) loads up to the full circuit rating.
Also, for a 15A or 20A circuit that serves equipment fastened in place, as well as lights or equipment not fastened in place, the total load of the equipment fastened in place is limited to 50% of the circuit rating. So a single 20A circuit could supply both a 12A dishwasher and an 8A garbage disposal, but not a 12A dishwasher and a general purpose receptacle.
Cheers, Wayne