What the ratings mean for fit
The XT1NU3125AFFBG0XXX: The 125 A frame and TMF 125-1250 trip unit cover a 125 A to 1250 A adjustable thermal-magnetic trip range — the '125-1250' in the description refers to the trip unit's continuous current rating adjustable band, not a 1250 A frame. The 125 A frame size means the breaker's physical dimensions and bus connections are sized for that continuous current; the TMF trip provides both thermal overload (inverse-time) and magnetic short-circuit (instantaneous) protection. UL/CSA listing confirms the breaker meets North American panelboard and switchgear standards, so it slots into a UL 489 or CSA C22.2 No. 5 panel without additional agency review.