SIRIUS non-fused direct starter — S0 frame, busbar snap-on
The Siemens 3RA2120-1CD23-0AK6 is a SIRIUS non-fused direct motor starter in size S0, designed to snap onto 60 mm busbar systems — no separate mounting plate needed. It ships as a pre-assembled unit: a 3RT2023-1AK60 contactor, a thermal bimetallic overload relay, a link module 3RA2921-1AA00, a circuit-breaker size S00, and a busbar adapter 8US1251-5NT10. That means the BOM line is one order code, not five separate picks.
Ratings that define the load envelope
Rated AC-3 switching up to 690 V, with a motor output of 750 W at 400 V — that covers a standard 1 hp (0.75 kW) three-phase motor at 400 V line. The coil accepts two voltage ranges selectable by assignment: 88–121 V at 50 Hz and 96–132 V at 60 Hz, so it works on both 100–120 V control transformers without a separate coil change.
Mounting dimensions and clearances
Dimensions are 260 mm height × 45 mm width × 155 mm depth. The 45 mm width matches the S0 busbar slot pitch — it occupies one 45 mm module on a 60 mm busbar system. Required free space around the unit: 30 mm upwards, 10 mm forwards, 10 mm downwards, 9 mm at the sides, and 0 mm backwards. That 0 mm backwards clearance means it can sit flush against a back panel or enclosure wall. Mounting position is vertical only — horizontal mounting would affect the thermal overload trip curve.