What it is and what it does
The Siemens SENTRON 5SY7120-7 is a 1-pole miniature circuit breaker (MCB) rated 20 A at AC with a C tripping characteristic — the standard choice for moderate inrush loads like small motors, transformers, and lighting banks where the startup spike sits above a B-curve but below a D-curve. Breaking capacity is 15 kA per EN 60898 and 25 kA per IEC 60947-2 / UL1077, so it handles fault currents up to that level without welding or cascading failure upstream — useful for industrial panels where the available fault current is known to be in that range. It occupies one modular width unit (18 mm) on a DIN rail, installs in any orientation, and is rated IP20 with conductors connected — standard for enclosed distribution boards.
Temperature derating — the real-world current
The 20 A rating holds at 35 °C ambient. Above that, the breaker derates linearly: 18.78 A at 40 °C, 18.14 A at 45 °C, 17.48 A at 50 °C, 16.8 A at 55 °C, and 16.08 A at 60 °C. If your panel runs hot — say a sealed enclosure near a furnace line — size the load to the derated figure, not the label.
Mounting and panel fit
Snap-on DIN rail mount via the quick-assembly system; depth is 76 mm (70 mm installation depth behind the panel). The 18 mm width means it takes up one slot in a standard distribution board. Sealable with a lockable cover if you need to prevent tampering.