What This Motor Starter Is and Who Specifies It
The Siemens 3RA2125-0GD23-0AP6 is a SIRIUS non-fused motor starter — a direct starter in size S0 that combines a contactor, a thermal bimetallic overload relay, and a magnetic short-circuit release in one assembly. It is typically specified by panel builders and OEMs who need a coordinated motor feeder for a fixed-speed load on a 50/60 Hz line, with the short-circuit protection built in rather than relying on a separate fuse.
Rated for AC-3 switching duty up to 690 V, this starter handles the inductive load of a squirrel-cage motor — starting and disconnecting it under load — not just resistive heating. The coil voltage is the deciding factor for control-circuit compatibility: the 3RA2125-0GD23-0AP6 accepts 176–242 V at 50 Hz and 192–264 V at 60 Hz, so it works on a standard 208–240 V control transformer. At 400 V the motor power rating is 180 W; at 500 V it is also 180 W; at 690 V it reaches 250 W — these are the maximum continuous motor loads the starter can switch at those line voltages. Coordination type 2 (per IEC 60947-4-1) is the assignment here — meaning after a short-circuit fault at the rated conditional current, the starter remains serviceable with only minor contact welding that can be separated without impairing function. This is the standard for most industrial motor control centers where downtime for replacement after a fault is unacceptable.
Mounting and Integration in the Panel
Mounts vertically by snapping onto 60 mm busbar systems — the standard for SIRIUS S0 feeders. The overall dimensions are 155 mm deep, 45 mm wide, and 260 mm tall, fitting a single 45 mm slot on the busbar. Clearance requirements: 30 mm upward, 10 mm forward, 10 mm downward, 9 mm at the side, and 0 mm backward — so the back can sit flush against the panel backplate. Main contact wiring accepts stranded conductors from 1 to 10 mm², or two conductors of 2.5 to 6 mm² each (AWG equivalents: 2x 16–12 or 2x 14–8). The supplied components are a 3RT2023-1AP60 contactor, a 3RA2921-1AA00 link module, a 8US1251-5NT10 busbar adapter, and a 3RV2011-0GA15 circuit breaker — everything needed for a complete feeder except the busbar itself.