Integrator note on the 3RT1326-1AH20
The Siemens SIRIUS 3RT1326-1AH20 is a size S0 power contactor with four normally-open main contacts, rated for 11 kW at 400 V in AC-2 duty — that's the motor switching figure for slip-ring or wound-rotor motors, where the contactor makes and breaks under load. The operating current at AC-12 (resistive) tops out at 10 A, so for general switching of heaters or transformers, that's the number to size against. The S0 frame takes 61 mm width on the rail, 85 mm height, 91 mm depth — leaves room for auxiliary blocks alongside without crowding the next device. The main contacts accept 2x (1... 2.5 mm²) solid or 2x (2.5... 6 mm²) stranded, max 2x 10 mm². Auxiliary terminals take 2x (0.5... 1.5 mm²) solid or 2x (0.75... 2.5 mm²), max 2x 4 mm². That covers most control wiring in a mill panel — spring-cage fans might need an adapter.
Environmental and installation constraints
Above that, you'd need to account for reduced air density on cooling and dielectric strength. Front face is IP20, terminals are IP00 — so the contactor itself is touch-safe on the front but the terminal area expects enclosure protection.
Short-circuit coordination
These numbers matter when designing a motor branch circuit for selectivity with upstream breakers.