50 A D-curve for motor and transformer circuits
The Siemens 5SY4450-8 is a 4-pole miniature circuit breaker rated 50 A with a D tripping characteristic, meaning it tolerates the high inrush currents of motor starters, transformers, and welding equipment without nuisance tripping — the D curve trips at 10–20× rated current, so a 50 A breaker holds a 500–1000 A inrush for the brief start-up period. Breaking capacity is 10 kA per EN 60898-1 and 15 kA per IEC 60947-2 / UL 1077 — the higher figure applies when the breaker is specified as a supplementary protector under UL 1077, giving headroom for industrial panel fault currents.
At 40 °C ambient the 50 A rating derates to 48 A; at 50 °C it drops to 45.95 A, and at 60 °C to 43.8 A — if the panel runs warm, size the breaker for the actual ambient, not the nameplate 50 A. The breaker is rated for ambient temperatures from -40 °C to 75 °C, with humidity limits of 95 % up to 55 °C, tapering to 35 % at 75 °C — this covers outdoor enclosures in cold climates and warm indoor switchgear alike.