ECN Forum Forums General Topics of the Trade General Discussion Area Derating

Where in the code does it say you can only load a breaker to 80%?

I think you are thinking of 210.20(A). The rule only applies for continuous loads. Note that requiring the OCPD to be not less than 125% of the continuous load is another way of saying that the continuous load may not be more than 80% of the OCPD rating.