The Siemens 5SY7120-8CC is a 1-pole SENTRON miniature circuit breaker with a D tripping characteristic, rated 20 A at AC and a breaking capacity of 25 kA per IEC 60947-2. It occupies a single 18 mm modular width on the DIN rail — one slot in a standard distribution board. The D-curve means it tolerates inrush currents typical of motor and transformer loads without nuisance tripping; the 20 A rating is the continuous thermal current at 35 °C, derating to 17.78 A at 40 °C and 15.26 A at 50 °C.
Mounting and panel fit
Snaps onto a 35 mm DIN rail via the quick-assembly system. The 18 mm width and 76 mm depth mean it occupies one module slot; installation depth is 70 mm. Mounting position is any, and the combined top and bottom terminals accept wiring from either direction. IP20 with connected conductors — standard for enclosed distribution panels, not for wet or outdoor locations.
Thermal derating and environmental limits
The 20 A rating holds at 35 °C. Above that, the continuous current must be reduced: 17.78 A at 40 °C, 16.56 A at 45 °C, 15.26 A at 50 °C, 13.82 A at 55 °C (–). Ambient range is -40 °C to 75 °C. Humidity limits apply: max 95% RH up to 55 °C, then 55% RH up to 70 °C, then 35% RH up to 75 °C. Overvoltage category III and pollution degree 3 suit it for fixed-installation mains distribution.
Selectivity and coordination
The 25 kA breaking capacity at 440 V AC per IEC 60947-2 gives headroom for high-fault-level industrial panels. The D-curve provides magnetic trip at 10–20× In, so it coordinates with upstream SENTRON selective main breakers. Rated frequency is 50/60 Hz. Sealable for lockout/tagout compliance.