Siemens 3RU2126-4NB1 is a SIRIUS thermal overload relay, S0 size, Class 10 trip. Rated for 690 V (AC-3e) and carrying motor power ratings of 15 kW at 400 V, 18.5 kW at 500 V, and 22 kW at 690 V, this relay covers a solid band of European and IEC motor loads. The current ratings at 480 V and 600 V are both 28 A, which aligns with typical North American NEMA motor frames in that power band.
S0 Footprint and Panel Fit
The S0 size means a 45 mm wide, 97 mm tall, 95 mm deep body — a compact footprint that fits alongside SIRIUS contactors on the same DIN rail without extra bridging hardware. Screw terminals accept 2× (0.5...1.5 mm²) or 2× (0.75...2.5 mm²) solid or stranded wire; a Pozidriv PZ 2 bit with 5–6 mm shaft diameter is the field tool. Mountable in any position with stand-alone installation, and temperature-compensated from -40 to +60 °C — so no derating headache for outdoor enclosures or unventilated cabinets near ovens.
What the Ratings Mean for Fit
The Class 10 trip curve is the key selection parameter: it's the standard for general-purpose induction motors (DOL starting, moderate inertia loads). If your motor has a long acceleration time (high-inertia fan or centrifuge), you'd step to Class 20 — but for the vast majority of pump, conveyor, and compressor applications, Class 10 is the right call. The 690 V rated value (AC-3e) covers 400 V three-phase systems with headroom for 480 V and 600 V installations. The MTTF of 2,280 years (high-demand rate) reflects a robust bimetal design with no electronics to fail — the failure mode is mechanical wear on the trip mechanism, not semiconductor drift.