S0 direct starter for motor branch circuits
The Siemens 3RA2125-1FA24-0AK6 is a SIRIUS non-fused motor starter in the S0 frame size, combining a contactor and a thermal bimetallic overload relay in one assembly. It is configured as a direct starter, meaning it connects the motor directly across the line without reduced-voltage starting — the simplest and most common starting method for fixed-speed pumps, fans, and conveyors up to about 3 hp at 460/480 V. This is the standard trip class for general-purpose induction motors that accelerate to full speed in under 10 seconds — if the motor has a high-inertia load (centrifuge, large fan), a CLASS 20 or CLASS 30 overload might be needed, but for most pump and compressor duty this is the correct fit. Rated for AC-3 switching duty up to 690 V, the starter handles the inrush and breaking of squirrel-cage motors within its power band. At 400 V it delivers 1 500 W (about 2 hp), and at 500 V it delivers 2 200 W (about 3 hp). The thermal overload uses a bimetallic element — no electronics to fail, but ambient temperature compensation is built into the design.
Panel fit and clearance envelope
Mounting position is vertical — horizontal mounting would require a derating check against the operating temperature range of -20 to +60 °C. Dimensions are 45 mm wide, 193.1 mm high, and 97.1 mm deep — the S0 footprint is taller than the S00 size, so check the backpanel layout before committing. Clearance requirements: 30 mm upwards, 10 mm forwards, 10 mm downwards, 9 mm at the side, and 0 mm backwards. The zero backward clearance means it can sit flush against a mounting plate or busbar support without additional air gap. The supplied contactor is a 3RT2024-1AK60, and the link module is a 3RA2921-1AA00 — these are the factory-matched components inside the starter assembly. If either fails in the field, the entire starter is replaced as a unit rather than swapping individual contactor or overload parts.
Current production — sourcing posture
For volume or project pricing, submit an RFQ with the target quantity and delivery window.