What it is and what it does
The Siemens SENTRON 3VM1112-4EE32-0AA0 is a 3-pole molded case circuit breaker (MCCB) designed for line protection in distribution panels and motor control centers. It carries a continuous current rating of 125 A at 40 °C ambient, with the TM220 thermal-magnetic overcurrent release providing both overload and short-circuit protection. The interrupting rating hits 121 kA at 240 VAC — enough to clear high-fault conditions without cascading upstream, which is what you need when the transformer is right around the corner.
Interrupting capacity — what the numbers mean
This MCCB's interrupting capacity drops off as system voltage climbs: 121 kA at 240 V, 76 kA at 415 V, 53 kA at 440 V, and 12 kA at 500 V. That 12 kA at 500 V is the figure to check if you're feeding a 480 V panel — it's well within typical utility fault levels for most industrial services, but verify against your transformer impedance and available fault current study. The insulation voltage is rated 800 V, so the breaker itself is comfortable on 480/277 V and 600 V systems.
Thermal derating and panel fit
The 125 A rating holds steady from 40 °C up through 50 °C, then begins to taper: 122 A at 55 °C, 120 A at 60 °C, 117 A at 65 °C, and 114 A at 70 °C. If your panel ambient runs above 50 °C — say, a sealed enclosure near a furnace line — you'll need to account for that 8–9 % drop at the top end. The breaker measures 130 mm high, 76.2 mm wide, and 70 mm deep, with an IP40 front (tools and fingers stay out, but it's not washdown-rated). Mounts on a standard DIN rail or directly to the panel backplate via the screw terminals.