SIRIUS power contactor, 24 V DC coil, Size S0 frame
The Siemens 3RT1025-1DB44 is a SIRIUS power contactor in the Size S0 frame, built for switching motor loads and resistive circuits in industrial control panels. The 24 V DC coil pulls 5.4 W during both closing and holding, so the control transformer or DC supply needs to deliver that continuously — no inrush spike to size for, but the steady draw matters for multi-contactor banks. Mounts via screw and snap-on onto 35 mm DIN rail per DIN EN 50022. The IP20 front enclosure protects against finger contact; the terminal area is IP00, so the panel builder must maintain clearance around live parts. Side-by-side mounting is allowed, which saves rail space in dense cabinets. Rated operational current in AC-3 duty at 400 V is 7.5 kW — that's the motor-switching figure to use for sizing. For AC-4 (plugging / inching) at 400 V the contactor is rated 15.5 A. The coil picks up at 0.8 × rated voltage and drops out at 1.1 × rated voltage, so a 24 V supply that sags below about 19.2 V will cause chatter or dropout.
Termination and wiring
Main current circuit uses screw-type terminals accepting 2x (1... 2.5 mm²) solid/stranded, 2x (2.5... 6 mm²), or max. 2x 10 mm². Auxiliary and control circuit terminals accept 2x (0.5... 1.5 mm²), 2x (0.75... 2.5 mm²), or max. 2x (0.75... 4 mm²). AWG equivalents for main: 2x (16... 12), 2x (14... 10), 1x 8. For aux: 2x (20... 16), 2x (18... 14), 1x 12. The contactor ships with a built-in varistor surge suppressor across the coil — no external diode or RC snubber needed for DC switching. Pollution degree 3 (conductive pollution, typical for industrial enclosures without climate control).
Fault coordination and protection
For Type 2 coordination (no damage to the contactor after a short circuit) the required upstream fuse is gL/gG 25 A. For Type 1 coordination (contactor may need replacement) the fuse can be 63 A gL/gG. The contactor itself is rated for 10 million mechanical operations typical.