Panel fit and ratings
The Siemens 3RT1025-3BG40 is a SIRIUS power contactor in the Size S0 frame, rated for 7.5 kW motor switching at AC-2 duty on 400 V three-phase. That AC-2 rating covers wound-rotor motors or loads with moderate inrush — the contactor sees the full start current but doesn't have to break it under stall conditions. For plugging or inching duty (AC-4), it's rated 15.5 A at 400 V. Mounting is screw-fixed or snap-on onto 35 mm DIN rail per EN 50022. The 45 mm width and 100 mm depth fit standard S0 panel layouts; side-by-side mounting is permitted, so you can gang several contactors on the same rail without extra spacing. Main circuit terminals are screw-type, accepting 2x (1 to 2.5 mm²) solid or stranded, or up to 2x 10 mm². The front face carries IP20 finger protection; the terminal body itself is IP00, so keep live parts inside the enclosure.
Environmental and coordination
Operating temperature range is -25 to +60 °C, with a degree of pollution 3 rating (conductive pollution, typical for industrial control panels). Maximum operating altitude is 2 000 m without derating. For Type 2 coordination (no damage to the contactor under short circuit), the required upstream fuse is gL/gG 25 A. For Type 1 coordination (contactor may need replacement after fault), use a 63 A gL/gG fuse. The coil draws 5.4 W both on pick-up and hold — DC-operated, so no AC hum and consistent power draw.