A circuit breaker including an adjustable instantaneous trip level. Adjustable instantaneous trip level can be applied to a one-pole and/or two-pole circuit breakers including a thermal and magnetic mechanism, including AFCI, CAFCI, and/or GFCI constructions. The circuit breaker includes a magnet position adjustment mechanism allowing an operator to adjust the instantaneous trip level to a desired setting, such as between about 5 to 10 times the handle rating. The design also allows for alternate magnets to be used for either an increased or decreased instantaneous settings as desired. In two-pole circuit breakers, the instantaneous trip level can be set independently for each mechanism pole. Multi-pole circuit breakers and methods of adjusting instantaneous trip level are provided, as are other aspects.