What this MCCB carries and where it fits
The Siemens SENTRON 3VA1112-3FF46-0AA0 is a 4-pole molded case circuit breaker rated 125 A at 40 °C ambient, with a TM240 thermal-magnetic release. Breaking capacity runs from 75.6 kA at 240 V AC down to 11.9 kA at 690 V AC — that spread tells you the available fault current at the panelboard determines which voltage column you spec from. Rated insulation voltage is 800 V, so it handles 480Y/277 V and 600 V delta service without derating the insulation path. Front protection is IP40 — fine inside a dry enclosure, but not for washdown zones. Power loss runs 28.1 W max at rated load, which matters when you stack breakers in a multi-unit panel; that heat has to leave the enclosure or you'll see the thermal trip curve shift on the downstream poles.
Thermal derating — the real ampacity
This breaker holds a full 125 A from 40 °C through 50 °C ambient. At 55 °C it's 122 A; at 60 °C it's 120 A; at 65 °C it's 117 A; at 70 °C it's 114 A. No derating needed for most panel environments, but if your enclosure sits near a furnace line or a compressor discharge, you lose roughly 2-3 A per 5 °C above 50 °C. The TM240 release is calibrated for that curve — don't swap in a higher-rated frame expecting the same thermal behavior.
Panel fit — dimensions and footprint
Width is 101.6 mm (4 in), depth 70 mm (2.76 in), height 130 mm (5.12 in). That's a standard SENTRON 3VA frame size — it bolts into the same mounting footprint as other 125 A 4-pole variants in the series. The N-conductor protection is set at 50 %, meaning the neutral pole trips at half the phase-pole rating; verify that matches your distribution panel's neutral protection scheme before wiring.