What it is and what it does
The Siemens 3RT1325-1AM20 is a SIRIUS S0 frame power contactor with four normally-open main contacts and no normally-closed ones. It's rated for 7.5 kW motor switching at 400 V in AC-2 duty — that's the slip-ring motor and wound-rotor load class, where the contactor makes and breaks the rotor circuit under current. The 24 VDC coil pulls in reliably; the S0 frame keeps it compact at 61 mm wide, 85 mm tall, and 91 mm deep, sized to snap onto a 35 mm DIN rail per EN 50022. The front face is IP20 — finger-safe for panel mounting — while the terminals themselves are IP00, meaning they need enclosure protection from debris or accidental contact. Operating temperature spans -25 to +60 °C, so it handles a warm cabinet or a cold warehouse start without derating.
Coordination and fault protection
For Type 2 coordination (no damage to the contactor after a short circuit), the manufacturer specifies a 25 A gL/gG fuse upstream. For Type 1 coordination (contactor may need replacement, but no fire or shock hazard), a 63 A gL/gG fuse is allowed. That's a standard choice for a motor branch circuit — the 25 A fuse limits let-through energy so the S0 contactor survives a fault, while the 63 A fuse is a simpler, cheaper option if you're willing to swap the contactor after a dead short.
Wiring and field connections
Main circuit terminals are screw-type, accepting solid or stranded wire: 2x (0.5 to 1.5 mm²), 2x (0.75 to 2.5 mm²), and up to 2x (0.75 to 4 mm²) for the finer solid sizes. For heavier runs, the main contacts take 2x (1 to 2.5 mm²) and 2x (2.5 to 6 mm²), with a single 10 mm² maximum. The auxiliary/control circuit uses 2x (0.5 to 1.5 mm²) solid or 2x (0.75 to 2.5 mm²) stranded — common sizes for a 24 VDC coil feed. AWG equivalents are 2x (20 to 16) and 2x (18 to 14) for the main, with a single 12 AWG max.