What it is – and what that means for your panel
The Siemens 3RT2028-1AN20 is a current-production SIRIUS power contactor in the S0 frame – 45 mm wide, screw-type magnet coil terminals, with a built-in auxiliary switch. Industrial gear runs forever, and this one's still rolling off the line, so you're getting the same guts that have pulled decades in thousands of panels. It switches motor loads at AC-3 duty up to 750 cycles per hour, and can handle AC-4 jogging at 250 cycles per hour. For straight resistive loads (AC-1) it does 1 000 ops/h.
Mounting and mechanical fit
Fastens by screw and snap-on onto 35 mm DIN rail (DIN EN 60715). The mounting position is forgiving: +/-180° rotation on a vertical surface, plus +/-22.5° tilt forward/backward – helps in tight retrofit bays or odd panel orientations you inherit from a previous builder. Physical envelope: 85 mm high x 45 mm wide x 97 mm deep. When ganging contactors side by side, maintain 6 mm clearance on the sides. For vertical spacing above/below, 10 mm each way. That's enough for airflow and wiring access with the screw terminals. Main contact wiring accepts 1 x 1 … 10 mm² solid or stranded (fine strand same range). Auxiliary terminals take 2 x (0.5 … 1.5 mm²) or 2 x (0.75 … 2.5 mm²) – typical for control wiring.