What is the 80% rule for electrical?

The 80 percent rule says a breaker should carry no more than 80 percent of its rating as continuous load, anything running three hours or more. A 20 amp circuit is a 16 amp resource for that kind of load.

It is the field-side phrasing of the NEC's 125 percent requirement in 210.20: sizing protection at 125 percent of continuous load and loading a breaker to 80 percent are the same equation solved for different variables.

Standard breakers dissipate heat assuming intermittent household loading; hold them at 100 percent for hours and they trip from their own warmth.

EV chargers made this rule mainstream: a 48 amp charger needs a 60 amp circuit, exactly 48 over 0.8.

The branch circuit calculator applies the factor automatically to any continuous load.