What it is and where it fits
The Siemens SIRIUS 3RT1025-1AU04 is a size S0 power contactor with three main poles, rated for motor switching at 7.5 kW on a 400 V AC-3 supply. That AC-3 rating covers squirrel-cage motors during starting and running — the duty cycle that matters for conveyor drives, pumps, and compressors in a control panel. Screw-type terminals on the main circuit accept up to 2x 10 mm² stranded or 1x 8 AWG, so it lands on the same lug range as most 7.5 kW motor feeds. Mounting is screw or snap-on onto a 35 mm DIN rail per EN 50022, and the 45 mm width (one standard module) means it slots into a crowded enclosure without stealing adjacent space. Side-by-side mounting is permitted, so you can gang multiple contactors without a gap — useful for grouped motor starters on a common rail.
Key ratings and what they mean for your motor load
At 400 V AC-3 the contactor switches 7.5 kW; at 500 V AC-3 it handles 10 kW, and at 690 V AC-3 it reaches 11 kW. The AC-4 rating at 400 V is 15.5 A — that's the inching/plugging duty cycle for frequent reversals, which is lower than the AC-3 continuous rating. If your application requires jogging or reversing under load, size against the AC-4 figure, not the AC-3 motor nameplate. Coil operating range is 0.8 to 1.1 x rated voltage at 50 Hz (240 V coil). The auxiliary contact block has 1 NO + 1 NC instantaneous contact.
Panel integration notes
Depth of 140 mm. Front-face protection is IP20, but the terminal area is IP00. Ambient operating range is -25 to +60 °C, with pollution degree 3. Wire sizing for the coil terminals accepts 2x 0.5 to 1.5 mm² solid or 2x 0.75 to 2.5 mm² stranded; the main power terminals take up to 2x 10 mm². Use ferrules on stranded wire to avoid whiskers under the screw clamp. For type 2 coordination (no weld under fault), the required fuse is gL/gG 25 A; for type 1 (allow weld, clear fault) use 63 A.