What this contactor is and what it does
The Siemens 3RT2018-1BG42-0CC0 is a SIRIUS series contactor in the compact S00 frame, built for motor switching in control panels and machinery. Its AC-3 rating of 7.5 kW at 400 V defines the maximum three-phase induction motor it can switch under full load — a standard 10 hp (7.5 kW) motor on a 400 V line. The DC 125 V coil means this variant is intended for systems with a DC control bus at that voltage, not the more common 24 VDC or 110 VAC coils. Mounts on DIN rail via the S00 clip-in base; screw terminals accept up to 4 mm² conductors for power wiring.
What the ratings mean in practice
The AC-3 duty class covers starting and plugging of squirrel-cage motors — the contactor makes and breaks the motor's locked-rotor current (typically 6–8x rated current) during start, then carries the full-load current continuously. At 7.5 kW / 400 V, the contactor's rated operational current in AC-3 is approximately 16 A. The 1NC auxiliary contact provides one normally-closed feedback path for the control circuit, useful for signaling the contactor's open state back to a PLC or safety relay. The S00 frame size occupies roughly 45 mm width on DIN rail, leaving room for an overload relay and busbars in a standard motor starter assembly.
Panel integration note
Screw terminals on the S00 frame accept 1 x (0.75–4 mm²) or 2 x (0.75–2.5 mm²) solid/stranded copper. Torque to 1.2–1.5 Nm. The DC 125 V coil draws approximately 40 W inrush, 4 W hold — confirm the control transformer or DC supply can deliver the inrush without voltage sag. The contactor is IP20 rated for finger-safe installation inside an enclosure; no IP65 rating for external mounting.