What it is on the BOM
The ABB ACS800-U31-0120-5 is a 125 HP drive in the ACS800 family, catalogued under Motor Drives. The line ships in the U31 enclosure form factor and is the part the buyer names on the BOM when the legacy ACS800-U31 frame is called out — not the newer ACS880-01 cabinet footprint and not the ACS580 general-purpose series. Listed weight is 45.36 kgs. That is a two-person lift and a real cabinet-mount load — plan the back-panel reinforcement and the rigging before the crate lands, not after.
Where this part actually sits on the line
An ACS800-U31 drive on a hot-strip or reversing mill earns its place because the platform holds strip tension through the roll bite and stays put next to the cabinets that bleed inverter-switching noise into everything else. Specifying the U31 frame keeps the control topology — adaptive programming, the drive-side fieldbus option slots, the DTC motor model — that the rest of the line was tuned around.