What the XT1NU4050AAA000XXX is and what it does
The ABB XT1NU4050AAA000XXX is a molded case circuit breaker from the XT1N series, built on a 125 A frame with a TMF (thermal-magnetic fixed) trip unit adjustable from 50 to 500 A. The 4-pole configuration with front terminals and UL/CSA listing makes it a drop-in for North American panelboards and switchgear assemblies where a 4-pole MCCB is required for line-to-neutral or dual-fed applications.
What the ratings mean for fit
The 125 A frame with a TMF trip adjustable 50–500 A covers motor branch circuits and feeder protection where the continuous current falls within that band. The thermal-magnetic element provides instantaneous magnetic pickup for short-circuit protection and a thermal inverse-time curve for overloads — standard for general-purpose distribution and motor control center (MCC) feeders. The 4-pole variant switches the neutral, which is required in North American separately derived systems (per NEC 230.95) or where 4-wire feeders need full isolation. The UL/CSA listing confirms acceptance in the U.S. and Canadian markets without additional agency review.