The Siemens 3VM1112-4GE42-0AA0 is a 4-pole SENTRON molded case circuit breaker with a TM220 thermal-magnetic release, rated 125 A continuous at 40 °C and delivering 121 kA interrupting capacity at 240 VAC. That 121 kA at 240 V means it handles high-fault utility feeds or transformer secondaries without needing an upstream current-limiting fuse — it's sized for main or feeder duty where the available fault current is serious. The interrupting curve drops to 76 kA at 415 V and 53 kA at 440 V, so it still holds its own on 400 V-class distribution.
Thermal derating and panel fit
The 125 A rating holds flat from 40 °C up through 50 °C — no derating needed in a typical warm panel. Above that it steps down gradually: 122 A at 55 °C, 120 A at 60 °C, 117 A at 65 °C, and 114 A at 70 °C. If your enclosure sits near a heat source or in a non-conditioned space, that 114 A floor at 70 °C tells you the breaker still carries a solid margin on a 100 A feeder. The LI protection curve (long-time and instantaneous) with adjustable li max to 1,250 A gives you some tuning room for motor inrush or transformer magnetizing current without nuisance tripping.
Installation notes
IP40 on the front. 23 W maximum power loss at full load. Rated insulation voltage 690 V. Operating range -25 °C to 70 °C, storage -40 °C to 80 °C.