I have had many questions about the origin of the so-called "80% rule"
Here is my understanding of it’s history, in the form of a fairy tale.
Once upon a time, long ago and far away, when a loyal subject purchased a piece of electrical equipment, it would carry its rated current, indefinitely.
A 100 amp fused switch, or circuit breaker, would carry 100 amps.
Even when a panel was full of circuit breakers, each breaker would safely carry its rated current.
Then the wicked equipment engineers got their little heads together and said to each other "Many times the load will not be continuous." "Why can’t we cut back on the quality here and there to save on the cost of production." "We could make more money."
And so they did.
Soon, there were cases where the load actually was continuous, and the result was overheated panelboards and other equipment, resulting in fires. The engineers said "What can we do now".
Then one of them had a bright idea. "Let's get the king to make a new rule." "When the load is continuous, the load must be reduced to 80% of the circuit rating." "That way, we can continue to sell a 80 amp breaker as a 100 amp breaker."
The king did as they suggested, and they all lived happily ever after.