What this MCCB is and what it carries
The Siemens 3VA1225-4EF42-0AA0 is a SENTRON molded case circuit breaker (MCCB) sized for line protection — meaning it sits at the incoming feeder or main distribution point, not downstream on a motor branch. It is a 4-pole unit rated for 250 A continuous current at 40 °C ambient, with a TM240 thermal-magnetic overcurrent release that gives an adjustable trip range from 1 250 A to 2 500 A (the magnetic instantaneous setting). Breaking capacity is the headline selector here: 121 kA at 240 V, 75.6 kA at 415 V, 52.5 kA at 440 V, 17 kA at 500 V, and 11.9 kA at 690 V. That 121 kA at 240 V tells you this breaker is built for high-fault infrastructure — transformer secondaries or large busway feeds where prospective short-circuit current is substantial. The 690 V figure (11.9 kA) is the ceiling for a 690 V line-to-line system; if your panel runs 400/415 V, you have 75.6 kA of interrupting capacity, which covers most industrial service entrances. Rated insulation voltage is 800 V, so the internal clearance and creepage are designed for 690 V systems with margin. The breaker dissipates up to 57 W at full load — not negligible in a sealed enclosure; factor that into your thermal budget.
Thermal derating and panel fit
The 250 A rating holds flat from 40 °C through 50 °C. Above that it derates: 243 A at 55 °C, 237 A at 60 °C, 230 A at 65 °C, and 223 A at 70 °C. If your panel ambient runs 55 °C or higher, size the upstream conductor and downstream bus for the derated value, not the nameplate. The operating temperature range is -25 °C to 70 °C; storage range is -40 °C to 80 °C.