What the 125 A rating means for fit
The XT2LU3125BFF00NXXX: The 125 A continuous current rating sets the upper limit for the protected circuit under normal load. The L-IR designation indicates a fixed thermal-magnetic trip with long-time (L) and instantaneous (I) protection, plus a rated short-circuit breaking capacity (Icu) typical of the XT2 platform. For a panel builder or site engineer, this breaker fits as a main or feeder protection device in a distribution board where the load current does not exceed 125 A and the available fault current stays within the XT2's interrupting rating.