Thermal derating — what the 40 A rating actually delivers at panel ambient
The 40 A rating holds at 35 °C ambient. Above that, the breaker derates linearly: 37.56 A at 40 °C, 36.28 A at 45 °C, 34.96 A at 50 °C, 33.6 A at 55 °C, and 32.16 A at 60 °C. If your panel internal temperature runs at 50 °C — common in a crowded enclosure with drives or transformers — the effective continuous current capacity drops to 34.96 A. Size the upstream conductor and load accordingly; the nameplate 40 A is only valid at or below 35 °C.
Dimensions: 90 mm high, 36 mm wide (2 MW), 76 mm deep. Installation depth is 70 mm.