The Siemens 3RT1024-1BF40 is a SIRIUS power contactor in the S0 frame size, rated for 7.5 kW at both 400 V and 690 V AC-3 duty — meaning it handles motor loads up to that power in standard three-phase induction motor applications. The 110 VAC coil pulls 5.4 W on close and hold, so the control transformer sizing is straightforward.
AC-3 switching current is 7.5 kW at 400 V and the same at 690 V — that's the motor-rating curve you care about for pump, fan, and compressor duty. If the load is reversing or plugging (AC-4), the contactor is rated 12.5 A at 400 V, which is lower than the AC-3 figure; derate accordingly for high-inertia or frequent jogging applications. The coil holds at 110 VAC nominal. Closing and holding power is 5.4 W DC, which keeps the thermal load on the control transformer low. Short-circuit coordination is specified: type 2 coordination requires a 25 A gL/gG fuse; type 1 coordination allows a 63 A gL/gG fuse. That difference matters for selectivity with upstream breakers — if you need the contactor to survive a fault without replacement, spec the type 2 fuse. The front of the contactor is IP20 (finger-safe), but the terminals are IP00 — keep that in mind for clearance inside the panel; no touch-safe protection on the power connections.
Main current circuit terminals are screw-type, accepting 2x (1 to 2.5 mm²) solid or 2x (2.5 to 6 mm²) stranded, with a maximum of 2x 10 mm². The auxiliary/control terminals are smaller: 2x (0.5 to 1.5 mm²) solid and 2x (0.75 to 2.5 mm²) stranded, max 2x (0.75 to 4 mm²). AWG equivalents are 2x (16 to 12) for the smaller terminals and 2x (14 to 10) for the main, with 1x 8 AWG maximum on the main. Side-by-side mounting is allowed, with a 6 mm spacing required at the side for heat dissipation.