What this part is and what it fits
The Siemens SIRIUS 3RT1945-5AL21 is a replacement magnet coil for Size S3 contactors in the SIRIUS family, terminated with screw terminals and wound for 230 V AC control supply at both 50 Hz and 60 Hz. The coil hold-voltage range is 0.8 to 1.1 times the rated voltage at 50 Hz, meaning the contactor will reliably stay sealed down to about 184 V and up to 253 V — useful to know if your control transformer is lightly loaded or your line sags under a large motor start.
Lifecycle and sourcing reality
This part is a phased-out product. The manufacturer lists the successor as the SIRIUS 3RT2 series — a different mechanical platform, so a direct drop-in without panel rework is not guaranteed. For a BOM line that must stay on the original Size S3 footprint, this coil is sourced through surplus and broker channels against an RFQ.
What the coil ratings mean for fit
The 230 V AC coil draws its pick-up and holding power from the control transformer; the Size S3 platform typically requires around 64 VA inrush and 8.4 VA sealed. If your panel's control transformer is already near its kVA budget, verify headroom before swapping in this coil. Screw terminals on the coil match the existing Size S3 contactor base — no crimp or spring-cage adapter needed. The coil mounts directly onto the Size S3 magnet stack; no additional bracketry.