The ABB XT1NU3125AAA000XXX is a Tmax XT1 molded case circuit breaker frame rated 125 A continuous, 3-pole, with a fixed thermal-magnetic trip unit. The 125 A rating governs the continuous current the breaker can carry without tripping under normal conditions — size the upstream conductor and downstream load to this figure. The 480 VAC rating is the maximum line-to-line voltage the breaker can interrupt; for 480 V delta or 277/480 Y systems this frame covers the standard distribution voltage.
Panel-mount construction — the breaker frame bolts into a switchboard or panel enclosure, not a DIN rail. The front flat terminals accept standard bus bar or cable lugs. In a retrofit or panel build, verify the mounting footprint matches the existing cutout; the Tmax XT1 frame has a specific bolt-hole pattern that differs from older ABB SACE or other manufacturer frames.
The 3-pole configuration covers three-phase loads — motors, pumps, compressors, or distribution feeders. The fixed 125 A thermal-magnetic trip provides short-circuit and overload protection in one device. For selective coordination studies, the Tmax XT1 series has published I²t let-through curves; request those curves from the manufacturer or distributor when engineering a coordinated distribution system.