SIRIUS Size S0 power contactor — what it handles and where it fits
The Siemens 3RT1025-1AM20 is a SIRIUS Size S0 power contactor rated for motor switching up to 15.5 A in AC-4 duty at 400 V — that's the heavy reversing and plug-stop cycle where most contactors get derated. At 690 V it carries 11 kW, and at 500 V it's good for 10 kW. The 45 mm wide body snaps onto a 35 mm DIN rail per EN 50022, and the screw-type terminals on both main and auxiliary circuits accept up to 4 mm² solid or 6 mm² stranded on the control side, and up to 10 mm² on the power side.
Duty-cycle ratings and coordination
This contactor is rated for AC-2 at 7.5 kW at 400 V — that's for slip-ring motor starting where the current stays lower during acceleration. The AC-4 rating of 15.5 A at 400 V covers the toughest duty: plugging and inching of squirrel-cage motors, where the contactor makes and breaks locked-rotor current every cycle. Type 1 coordination requires a 63 A gL/gG fuse upstream; Type 2 coordination needs a 25 A gL/gG fuse. Pollution degree 3 means it's rated for industrial environments with conductive contamination — no extra conformal coating needed for most panel conditions.
Panel integration and wiring
At 45 mm wide and 85 mm tall, the S0 frame fits two units per standard 100 mm DIN-rail segment. Side-by-side mounting is permitted with no derating — the thermal design accounts for zero spacing. The screw terminals on the main circuit accept 2x (1 to 2.5 mm²) solid or stranded, or a single 10 mm² conductor. Auxiliary and control terminals take 2x (0.5 to 1.5 mm²) solid, or 2x (0.75 to 2.5 mm²) stranded. AWG equivalents are 2x 16-12 for main, 2x 20-16 for aux. Strip length is not listed here, but standard Siemens practice is 8-10 mm for screw terminals in this frame size.