What it is and where it lands
The Siemens SIRIUS 3RT1015-1BB42-ZW96 is a Size S00 power contactor built for switching motor loads in control panels. It snaps onto a 35 mm DIN rail per EN 50022 — screw-mount also works if the rail is full. The 24 V DC coil pulls 3.3 W on close and holds at the same power, so the supply just needs to be sized for a steady 3.3 W per contactor.
What the ratings mean for fit
The main contacts are rated 10 A in AC-12 duty — that's the resistive-load ceiling, fine for heater banks or transformer primaries. For motor reversing (AC-4) at 400 V, it's rated 6.5 A, which translates to about 3 kW at 400 V. At 500 V it handles 3.5 kW motor loads; at 690 V, 4 kW. The AC-2 rating at 400 V is 3 kW — wound-rotor motors or slip-ring applications. Those are the numbers you use for sizing, not the AC-12 figure. The coil pickup threshold is 0.85 x rated voltage (20.4 V DC), and it drops out at 1.1 x rated (26.4 V DC). That 0.85 pickup means a saggy 24 V bus still pulls it in — useful on long cable runs. The auxiliary contact ratings cover 10 A at 24 V, 6 A at 230 V, 3 A at 400 V, and 0.3 A at 220 V DC. If you're switching DC loads, the 220 V DC / 0.3 A line is the one to watch.
Panel fit and wiring
It's 45 mm wide, 57.5 mm tall, 72 mm deep — compact enough for dense S00 layouts. Side-by-side mounting is allowed with 6 mm spacing to adjacent devices. Screw terminals on the main circuit accept 2x 0.5–1.5 mm² solid, 2x 0.75–2.5 mm² stranded, or a single 4 mm². AWG equivalents are 2x 20–16, 2x 18–14, or 1x 12. The enclosure is IP20 on the front and at the terminals — finger-safe for panel interiors, not for washdown.