Motor rating and what drives the selection
The 3RT2317-1BG40: The 12 A at 400 V AC-3 rating (5.5 kW motor) is the load-bearing figure for this SIRIUS size S00 contactor — it's the current the main poles can switch under the inductive motor load of a standard squirrel-cage machine starting and running. For reversing or inching duty (AC-4), the same contactor is rated 4 kW at 400 V, reflecting the higher electrical stress of plugging or jogging where the motor is reconnected while still spinning. The 125 V DC coil means this is a DC-operated contactor — the magnet coil holds in with DC, not AC, so the pickup and dropout timing (7...13 ms operating time) is consistent regardless of line frequency, which matters for sequenced starter schemes where the contactor must close within a defined window.
Terminal wiring and auxiliary contacts
An auxiliary switch is included (20...12 contacts for auxiliary circuits), which saves a separate add-on block for basic status feedback — the auxiliary contacts share the same 6 kV rated impulse withstand voltage as the main circuit, so they can be wired into the same control transformer secondary without extra isolation.