What this contactor is and what it does
The Siemens 3RT1025-3BW44 is a SIRIUS power contactor in frame size S0, designed for switching motor loads in industrial control panels. It mounts via screw and snap-on onto a 35 mm DIN rail per EN 50022. Its motor-switching capacity is rated at 10 kW at 500 V AC-3 and 11 kW at 690 V AC-3 — the AC-3 duty class covers starting and stopping squirrel-cage motors, which is the majority of conveyor, pump, and fan applications. For less demanding loads, AC-2 at 400 V is rated at 7.5 kW, and for reversing or inching duty (AC-4), it handles 15.5 A at 400 V. The contactor is also rated for AC-12 switching at 10 A maximum. The DC magnet coil draws 5.4 W both on closing and holding — a steady-state power that simplifies the control transformer sizing.
Mounting and integration into the panel
The S0 frame occupies 45 mm width, 85 mm height, and 150 mm depth — a standard footprint that fits alongside other SIRIUS S0 components on the same DIN rail. Side-by-side mounting is permitted, so multiple contactors can pack tightly in the enclosure. Main circuit wiring uses screw-type terminals accepting solid or stranded conductors: 2x (0.25 to 2.5 mm²), 2x (1 to 2.5 mm²), 2x (2.5 to 6 mm²), or up to 2x 10 mm². AWG equivalents are 2x (16 to 12), 2x (14 to 10), and 1x 8. The front face is rated IP20, protecting against finger contact; the terminal area is IP00, so live parts are exposed during installation — standard for panel-mounted contactors. Pollution degree 3 rating means it tolerates conductive dust in industrial environments. Operating temperature range is -25 to +60 °C, and the unit is rated for installation at altitudes up to 2,000 m.
Lifecycle and sourcing
For short-circuit coordination, the contactor requires a gL/gG fuse: 25 A for Type 2 coordination (minimal damage to the contactor) and 63 A for Type 1 coordination (contactor may need replacement after a fault).