That sure is an interesting discussion... I always thought the 80% rule merely applied to good design (as in not overloading a circuit right from the beginning) rather than to actual breaker tripping. Interestingly enough, breakers commonly used in the 230V part of the world (B or C trip curve DIN rail components) are designed to hold 100% inifinitely and overload tripping is specified as within less than 1 hour at 145%. The EU low voltage directive (on which to some extent most European electrical codes are based on) also only specify 100% circuits in the form of Ib<=In<=Iz (the load current must not exceed the CB rating which in turn must not exceed the current carrying capacity of the circuit conductors). As it says <= you can perfectly well load a 20 amp circuit to 20 amps, but not 20.01 amps.
From this point of view, a breaker that trips at 80% seems to be very limiting, if not pointless - after all, it could then be called a (current rating*0.8) amps 100% breaker all the same.
