The circuit is designed with that spec in mind but the actual breakers you find in use don't trip until 15-20% over their rating. No they do not trip at 80% unless the breaker is fucked.

-source me, EE, work with this shit quite literally every day and trust me, you are not tripping at 1400w and no one is assuming that we only have 1400w available when designing products. And yes I do work on things that pull that much or more power continuously and we are using residential breakers for testing. The reason a device might trip a breaker when it's continuous rating is 1400W is if it pulls an instantaneous load over ~2-2.5kW. but under normal operation a 15A breaker will never even sneeze at a 1400w continuous load.