The ABB XT1NU3100AFF000XXX is a 3-pole molded case circuit breaker from the XT1N series, rated 125 A with a TMF (thermal-magnetic fixed) trip unit adjustable from 100 to 1000 A. Frame-mount construction suits panelboard or switchboard integration where the breaker is bolted directly to a bus or mounting plate — no DIN-rail clip, so plan for a dedicated mounting footprint in the enclosure layout.
For a feeder or main in a distribution panel, that figure governs the upstream cable sizing and downstream branch circuit coordination — every load downstream sums to less than this number, or the breaker becomes the weak link. The 3-pole configuration covers three-phase systems (typically 208/240/480 VAC wye or delta), so a single device disconnects all three phases simultaneously on a fault or for lockout-tagout. That adjustability is the difference between a breaker that works on a resistive heater bank and one that works on a conveyor motor start — same frame, different setting.