What it is and how it wires up
The Siemens 3RA2220-4DB27-0BB4 is a SIRIUS reversing starter — a pre-assembled combination of a reversing contactor pair and a motor-protective circuit-breaker in one S0-size package. It ships with the contactor 3RT2027-1BB40 and the link module 3RA2921-1BA00 already integrated, so you get the reversing interlock and overload protection without piecing together separate units on the rail.
Mounting and panel fit
Fastens via an adapter for screw mounting or snaps onto 35 mm DIN rail. Mounting position is vertical, and the envelope is 90 mm wide by 130 mm deep by 265 mm tall — that width is the critical number on a crowded DIN rail; at 90 mm it takes up two standard 45 mm-wide module slots. Clearance around the block: 50 mm upwards, 32 mm forwards, 10 mm downwards, and 10 mm at the side — no space needed behind. Main circuit terminals are screw-type, so you torque the lugs with a standard screwdriver — no special tooling. The size S0 load feeder matches the common S0 circuit-breaker footprint, keeping the wiring channel clean.