The Siemens 3RU2126-4BC0 is a SIRIUS thermal overload relay in the Size S0 frame, designed for contactor mounting. The thermal bimetallic release provides Class 10 trip protection. Rated insulation voltage is 690 V, and the auxiliary contacts are integrated.
Ratings and what they mean for fit
The relay carries motor power ratings at several common line voltages: 7.5 kW at 400 V, 11 kW at 500 V, and 15 kW at 690 V. At 480 V and 600 V it's rated 20 A. That tells you the current range this S0 frame covers — it's sized for the smaller end of the SIRIUS contactor family, typically up to around 32 A motor FLC. The auxiliary contact ratings are given per voltage: 2 A at 24 V, 3 A at 120 V, 2 A at 230 V, 1 A at 400 V, and 0.75 A at 690 V — these are the make/break capacities for the trip-indication circuit, so if you're feeding a 24 VDC PLC input, the 2 A rating is plenty; if you're switching a 690 V control transformer, you're limited to 0.75 A. The spring-loaded terminals (cage-clamp style) accept 2× (0.5 to 2.5 mm²) solid or stranded — no screw torque to worry about, and no periodic re-torquing on a vibrating panel. Temperature compensation is active from -40 to +60 °C, so the trip curve stays accurate across the typical enclosure ambient range. The relay can be mounted in any position, which helps when you're packing components into a tight gland-plate layout.
Panel integration notes
Dimensions: 102 mm high, 45 mm wide, 84 mm deep. The 45 mm width matches the S0 contactor footprint, so when you gang them on a DIN rail, the relay doesn't steal extra width — it sits flush. The depth of 84 mm includes the spring terminals; allow clearance for wire bending radius below the relay. The screwdriver slot for the terminal release is a 3 mm diameter shaft, tip size 3.0 × 0.5 mm — a standard slotted screwdriver, no special tool. Storage and transport temperature range is -55 to +80 °C; operating ambient is -40 to +70 °C, so it's fine for unheated enclosures in cold climates as long as the motor current is within the relay's adjustment range.