SIRIUS S0 power contactor, 110 VDC coil
The Siemens 3RT1025-1BF40 is a SIRIUS size S0 power contactor with three normally-open main contacts and a 110 V DC coil. It's rated for 7.5 kW motor switching at 400 V AC-3 duty, which covers the common 400 V three-phase induction motor range up to that power. The DC coil pulls 5.4 W on closing and holds at 5.4 W — no economizer circuit needed, but verify your DC supply can deliver the inrush without droop. Mounting is screw-and-snap-on to 35 mm DIN rail per EN 50022. The S0 frame is 45 mm wide, 85 mm tall, 101 mm deep — standard footprint for this class. Side-by-side mounting is allowed with 6 mm spacing at the side for heat dissipation.
What the ratings mean for your panel
The AC-3 rating of 7.5 kW at 400 V is the motor-switching current for squirrel-cage motors — starting and disconnecting under load. That's the number that governs real motor duty. The AC-4 rating of 15.5 A at 400 V covers plugging and inching, which is harder on the contacts; if your application does frequent reversing or jogging, size on the AC-4 figure, not AC-3. For coordination, the spec gives fuse requirements: Type 2 coordination (no damage to the contactor on a short circuit) needs a gL/gG fuse rated 25 A. Type 1 coordination (contactor may need replacement after a fault) allows up to 63 A gL/gG. This matters for selectivity studies — if you're protecting downstream devices, the 25 A limit constrains your fuse choice. Pollution degree 3 means the contactor is rated for industrial environments where conductive pollution occurs or dry non-conductive pollution becomes conductive due to condensation. That's standard for unsealed panels in factory floors, not for clean rooms or sealed enclosures without condensation management. The IP20 front / IP00 terminal rating means the front of the contactor is finger-safe, but the terminals are exposed — they need to be inside a panel or guarded. Not a standalone device for open mounting.
Wiring and terminal details
Main circuit terminals are screw-type, accepting solid wire: 2x (0.5 to 1.5 mm²), 2x (0.75 to 2.5 mm²), max 2x (0.75 to 4 mm²). Stranded: 2x (1 to 2.5 mm²), 2x (2.5 to 6 mm²), max 2x 10 mm². AWG equivalents: 2x (16 to 12), 2x (14 to 10), 1x 8. That covers most panel wiring up to 4 mm² solid or 10 mm² stranded — enough for the 25 A fuse coordination path. Operating temperature range is -25 to +60 °C during operation. That's typical for a panel-mounted contactor — no special derating needed for most indoor industrial environments, but if your panel ambient runs above 60 °C, you'll need to check the thermal curve or ventilate.