The Siemens 3RT1017-1AR21 is a SIRIUS power contactor in frame size S00 — the smallest footprint in the series, at 45 mm wide, 57.5 mm tall, and 72 mm deep. It switches motor loads up to 5.5 kW in AC-2 duty at 400 V and handles 8.5 A in AC-4 reversing or plugging service. The coil is a 24 VDC type, with a rated operating range of 0.8 to 1.1 times nominal at 50 Hz and 0.85 to 1.1 at 60 Hz. That means the coil holds in reliably down to about 19.2 VDC and up to 26.4 VDC — useful for control circuits with voltage drop on long cable runs.
The screw-type main terminals accept 2x (0.5 to 1.5 mm²) solid or stranded, 2x (0.75 to 2.5 mm²), or up to 2x (0.75 to 4 mm²) — enough for the motor leads this contactor handles. AWG equivalent: 2x (20 to 16), 2x (18 to 14), or 1x 12.
Ratings that govern the real choice
The headline motor ratings are 5.5 kW at 400 V for AC-2 (slip-ring motor, start/run) and 8.5 A at 400 V for AC-4 (inching, plugging, reversing). For resistive loads (AC-12), the contactor is rated 10 A maximum. The auxiliary contact block is a single NO instantaneous contact.