What this MCCB carries and where it fits
The Siemens 3VA1225-4GF42-0AG0 is a SENTRON molded case circuit breaker rated 250 A continuous, 4-pole, with a TM240 thermal-magnetic overcurrent release. It is designed for line protection — meaning it sits at the incoming feeder or main distribution point, not on a motor branch. The 121 kA breaking capacity at 240 V and 75.6 kA at 415 V tell you it can interrupt high fault currents without upstream fuses, which simplifies coordination in a panel. Rated insulation voltage is 800 V, so it handles 480/600 V systems comfortably. The 70 mm depth and 140 mm width are standard for this frame size — it bolts into a SENTRON 3VA switchboard or a standalone enclosure without a footprint surprise.
Thermal derating — what the numbers mean for a hot panel
This breaker carries its full 250 A from 40 °C up to 50 °C without derating. At 55 °C it drops to 243 A, at 60 °C to 237 A, at 65 °C to 230 A, and at 70 °C to 223 A. If your panel ambient runs above 50 °C — say a non-vented enclosure near a furnace line — size the feeder for the derated value, not the nameplate. The 57 W maximum power loss is the heat it dumps into the enclosure; that matters for thermal rise calculations in a sealed cabinet.