What it is and what it does
The Siemens 5SU9404-0KK25 is a 1P+N RCD-operated circuit breaker (RCBO) from the SENTRON 5SU9 family, combining overcurrent and residual-current protection in a single 18 mm-wide module. It's rated 25 A at 30 °C with a B tripping curve — meaning it trips between 3 and 5 times rated current, which is the standard choice for general-purpose residential and commercial final circuits where no high inrush from motors or transformers is expected. The breaking capacity is 10 kA per both EN 60898 and IEC 60947-2, so it can safely interrupt fault currents up to that level without upstream cascading. That's the figure that governs coordination with the upstream breaker.
Thermal derating — the real-world rating
The 25 A rating holds at 30 °C ambient. At 40 °C it's 23.5 A; at 45 °C it's 22.75 A; at 50 °C it's 21.75 A; at 55 °C it's 20.25 A. If this breaker lives in a crowded enclosure with other heat sources — typical for a DIN-rail panel — use the 40 °C or 45 °C column for sizing. The ambient operating range is -25 to +45 °C, so the 45 °C derated figure is the one that applies at the top of the spec window. Storage range is wider: -40 to +75 °C.
Panel fit and wiring
The 5SU9404-0KK25 occupies one width unit (18 mm) on a DIN rail, with an installation depth of 70 mm and overall depth of 77 mm. It accepts solid or stranded conductors from 0.75 mm² up to 35 mm², which covers everything from lighting final circuits up to submain feeds within its rating. Supply can land on either top or bottom terminals — no orientation restriction. The IP20 rating applies once the distribution board is installed with conductors connected; it's a panel-interior device, not a standalone enclosure. Touch protection is built in.