ABB XT4LU3040AFF00NXXX is a 3-pole molded case circuit breaker from the XT4L series, built on a 250 A frame with a TMF (thermal-magnetic fixed) trip unit adjustable from 40 to 400 A. The TMF curve gives you a thermal element for overload protection and a magnetic element for short-circuit response — no electronic adjustments, just a fixed magnetic pickup that tracks the thermal setting. Fixed front terminals and UL/CSA listing mean it lands in North American panels without a terminal conversion.
The 250 A frame provides the mechanical and thermal capacity to handle continuous currents up to that level, while the TMF trip unit lets you dial the overload protection between 40 and 400 A — meaning one breaker body covers a wide range of motor or feeder sizes. The magnetic short-circuit pickup is fixed at roughly 10× the thermal setting, so coordination with downstream breakers follows standard thermal-magnetic selectivity rules. UL/CSA listing certifies compliance with North American panel-building standards; the breaker is suitable for branch-circuit protection per NEC/CEC.
Deployment Context
The fixed front terminals accept bus bars or cable lugs; plan for 3-pole wiring with a neutral bar if the circuit requires it. The TMF trip is field-adjustable via a dial on the front face — no power-down needed for adjustment, but the breaker must be in the OFF position to change the setting.