SIRIUS S0 direct starter — what it is
The Siemens 3RA2120-4NA27-0FB4 is a SIRIUS-brand direct-on-line starter in the S0 size frame. It ships as a pre-assembled unit: a 3RT2027-1FB40 contactor and a 3RA2921-1BA00 link module, ready to snap onto 35 mm DIN rail or screw-mount. The 24 V DC coil and screw-type main-circuit terminals make it a straightforward drop-in for a panel that already runs on 24 V control.
AC-3 duty and trip class — what the ratings mean
The 28 A at 400 V translates to about 15 kW (20 hp) motor full-load current, which lines up with the horsepower ratings Siemens publishes for this unit at 460/480 V. That's the standard for general-purpose motor starting — fast enough to protect the motor windings during a stall, but slow enough to let the motor accelerate a normal load. If your application has a long acceleration time (high-inertia fan or conveyor), you might need Class 20 or 30, but for most pump, compressor, and light conveyor work, Class 10 is the right call.
Mounting and panel fit
Mounts vertically on 35 mm DIN rail or with screws. The S0 footprint is 45 mm wide, 193 mm tall, 107 mm deep. Clearance requirements: 50 mm above, 20 mm below, 20 mm to the side, and 20 mm forward. Zero clearance needed behind the unit.