The Siemens 3RA1145-4JB44-1BB4 is a SIRIUS non-fused motor starter, Size S3, configured as a direct starter. It combines a contactor and a thermal bimetallic overload relay in a single assembly, ready to snap onto a 35 mm standard DIN rail in a vertical mounting position. Rated for AC-3 duty up to 690 V, it delivers 63 A at 480 V — enough to handle a 50 hp motor at 460/480 V or a 30 000 W load at 400 V. The control coil is 24 V, so it integrates directly into a 24 VDC control circuit without an extra power supply step-down. That is the standard for standard-duty motor starting — it protects the motor during a stall or locked-rotor condition without nuisance-tripping on a normal start.
The 63 A at 480 V AC-3 rating is the headline number for motor switching. AC-3 is the duty cycle for starting and disconnecting squirrel-cage motors under load. The same part carries 61.5 A at 600 V and 37 000 W at 500 V, so it covers the common North American and European motor voltages in one package. The supplied contactor is the 3RT1044-1BB40, and the link module is 3RA1941-1BA00 — these are the internal subassemblies that make up the starter. If you are replacing only the contactor or overload block on an existing SIRIUS S3 feeder, those are the order codes to cross-reference. Wire range for the main contacts is 2.5 to 70 mm² stranded, or 2× (10 to 50 mm²) — that is a heavy lug range, sized for the 63 A feed. The enclosure clearances are specified: 90 mm upward, 10 mm forward, 10 mm downward, 12 mm at the side, and 0 mm backward, so panel layout must respect those ventilation and arc-flash gaps.
For a BOM line that calls out the 3RA1145-4JB44-1BB4, the fit is confirmed: Size S3 footprint, 24 V control, Trip Class 10, and the full AC-3 current range up to 63 A at 480 V.