If you have something like 240.4(D) (15a on 14ga copper, 20a on 12ga) you already have the 80% built into the overcurrent protection. The NFPA understands they have no control what a user will plug in. Even if you limited each receptacle to 15a configuration, that doesn't keep someone from plugging two 1440 watt heaters into receptacles on the same circuit. They set the overcurrent protection artifically low for "small conductors". 310.15 says a 14 copper is good for 20a and a 12 good for 25a, at 60c. There is your 80%.
