The Siemens 3RT1023-1BG40 is a SIRIUS power contactor in size S0, rated for motor loads up to 5.5 kW at 690 V AC (AC-3 duty). Screw-type terminals on the main circuit accept up to 2x 10 mm² stranded or 1x 8 AWG; the auxiliary circuit uses the smaller solid range of 2x (0.5... 1.5 mm²) per the spec.
At 45 mm wide and 85 mm tall, the S0 frame fits a standard DIN-rail enclosure. Side-by-side mounting is permitted with the 6 mm gap.
The AC-4 rating of 8.5 A at 400 V covers plugging and inching duty where the contactor makes and breaks locked-rotor current; if your application cycles through reverse or jogging frequently, size against the AC-4 number, not the AC-3. For resistive loads (AC-1), the maximum operating current is 10 A per the AC-12 rating. The mechanical life of 10 million operations means the contactor itself will outlast most panel refreshes under normal cycling.