What it is and what it does
The Siemens SIRIUS 3RT1044-1AL20 is a Size S3 power contactor for switching three-phase motor loads and resistive loads in control panels. It is rated 37 kW at 500 V for AC-3 duty, which is the standard category for squirrel-cage motors — so this contactor handles motor starting and running currents up to that power level without welding the main contacts during inrush.
Mounting and integration
Mounts via screw or snap-on onto 35 mm and 75 mm standard mounting rail — the snap-on feature speeds panel assembly and replacement. The S3 frame footprint (146 mm height, 70 mm width) fits standard DIN-rail enclosures; its 139 mm depth clears most gland plates and back-panel wiring channels. Side-by-side mounting is permitted, so you can gang multiple contactors without spacing gaps.
Termination and wiring
Main current circuit uses screw-type terminals. Solid wire accepts 2x (0.5…1.5 mm²), 2x (0.75…2.5 mm²), or max 2x (0.75…4 mm²); stranded accepts 2x (10…50 mm²). The range covers both control-circuit pilot wiring and power-circuit cable up to 50 mm² — common for a 37 kW motor feed. The terminal enclosure is IP00 (open), but the front with cover or box terminal achieves IP20, which is finger-safe for panel-side access.