What is the 80% rule for sub panels?

The 80 percent rule applied to sub panels means the feeder breaker should carry no more than 80 percent of its rating in continuous load. A 100 amp feeder is treated as an 80 amp resource for anything running three hours or more.

It is the practical inversion of the NEC's 125 percent continuous-load requirement; same math, opposite direction.

Momentary loads can use the full rating, which is why a sub panel's breakers can total far past its feeder without a problem.

The rule that actually gets enforced is the load calculation: everything on the sub plus everything on the main must fit the service. One more habit worth keeping: separate neutrals and grounds in every sub panel, always.

The load center sizing calculator checks feeder, breakers, and calculated load in one pass.