Siemens SENTRON 5SL4320-8 — 3-pole D-curve MCB at 20 A
The Siemens 5SL4320-8 is a SENTRON miniature circuit breaker rated 20 A at 400 V AC, 3-pole, with a D tripping characteristic and a 10 kA breaking capacity per both EN 60898 and IEC 60947-2. The D-curve means it tolerates the high inrush currents typical of motor and transformer loads — the magnetic trip threshold sits between 10× and 20× rated current, so it won't nuisance-trip on a compressor or conveyor start. That 10 kA SCCR at 400 V AC is the fault-clearing capability the breaker guarantees; if the available fault current at the panel exceeds that, you need an upstream current-limiting device or a higher-rated breaker.
Thermal derating and panel fit
The 20 A rating holds at 30 °C ambient. At 40 °C it derates to 18.48 A; at 45 °C to 17.6 A; at 55 °C to 15.92 A. If the enclosure runs warm — say a packed panel with drives — size the branch circuit for the actual ambient, not the nameplate. The breaker occupies 3 modular width units (54 mm total) and installs on a DIN rail; mounting position is any, so vertical or horizontal busbars are fine. Depth is 76 mm (70 mm installation depth), which fits standard 80 mm deep enclosures. IP20 with connected conductors — adequate for inside a locked panel, not for washdown.
Power loss and environmental specs
Per-pole power loss at rated current in hot operating state is 1.6 W — three poles total 4.8 W dissipated inside the enclosure. That is low enough that no forced cooling is needed in a standard steel enclosure, but worth noting if the panel is densely packed. The breaker is halogen-free and silicon-free, which matters for outgassing-sensitive environments (clean rooms, semiconductor fabs). Operating temperature range is -40 °C to 75 °C; storage range is the same. Overvoltage category III, pollution degree 2 — standard for fixed-installation distribution boards.