The Siemens SIRIUS 3RT1945-5AC21 is a magnet coil for Size S3 contactors, wound for 24 V AC at 50/60 Hz and terminated with a screw terminal block. This is a phased-out product; the manufacturer has designated the SIRIUS 3RT2 series as the official successor.
What the specs mean for fit
Rated for 24 V AC control voltage at both 50 and 60 Hz, so it drops into any existing Size S3 contactor frame that expects that coil voltage — no rewiring of the control transformer needed. The screw terminal matches legacy panel wiring practices; no crimp tool required for the coil connection. Because it is a Size S3 coil, it physically fits only the S3 contactor frame (the 3RT104/3RT105 class). It will not mechanically seat in a Size S0 or S00 contactor — the magnet yoke and armature dimensions are different.