40 A, 2-Pole — the rating that decides the branch circuit
The Siemens 5SJ4240-7HG42 is a 2-pole miniature circuit breaker from the SENTRON 5SJ4 series, rated 40 A at 30 °C and 40 °C — the same 40 A holds at both temperatures, so no derating is needed for a panel running at typical ambient. Above 40 °C the continuous current steps down: 38.8 A at 45 °C, 38 A at 50 °C, 37 A at 55 °C, and 36 A at 60 °C. That derating curve is the one to check if the breaker sits near a heat source or in a sealed enclosure.
Width is 36 mm (2 modular units), height 121 mm, depth 70 mm.