Rated current and thermal derating — the real-world numbers
The 3RW4427-3BC35 is rated 161 A at 40 °C ambient — that's the headline figure for a standard panel environment. At 50 °C the continuous rating drops to 142 A, and at 60 °C it's 125 A. The derating temperature threshold is 40 °C, so any enclosure that runs hotter needs the next frame up or forced ventilation. Minimum load is 8 % of the rated current — important for lightly loaded motors or pumps that start near no-load. Below that threshold the soft starter may not trigger the bypass reliably.
Integrated bypass and inside-delta wiring
An integrated bypass contact system is built in — once the motor reaches full speed, the thyristors are shorted out, eliminating heat dissipation in the power section during run. This means the enclosure thermal budget is based on start duty only, not continuous conduction. Inside-delta circuit connection is supported, which lets the soft starter handle higher motor currents by wiring the power section inside the delta loop of a six-lead motor. This is a common retrofit trick to keep the same starter when upsizing a motor without rewiring the panel. Motor overload protection is included, and the device also provides intrinsic device protection — the thyristor junction temperature is monitored, not just the motor current. Adjustable current limitation gives you a hard cap on starting current, useful for weak supply lines or generator-fed sites.
Mounting and wiring — panel fit and clearances
Fastening is screw fixing, not DIN rail snap-on. The dimensions are 170 mm wide, 192 mm high, 270 mm deep — that's a substantial footprint, so check the backplate layout before committing. Mounting position allows vertical surface with +/-90° rotation and +/-22.5° tilt front-to-back, giving flexibility in tight enclosures. Clearance requirements: 100 mm above, 75 mm below, 5 mm at the sides. The 75 mm below is the critical one — it's needed for the main power cable bend radius and for accessing the box terminals. Main current circuit uses box terminals, accepting stranded cable 2x (4... 50 mm²) or using both clamping points 2x (10... 1/0 AWG). The auxiliary and control circuit uses spring-loaded terminals for 2x (24... 16 AWG) solid or stranded. Maximum wire length from the starter to the motor is 500 m — beyond that, voltage drop and cable capacitance can cause nuisance trips.
The SIRIUS brand is Siemens' industrial controls family, and this soft starter sits in the 3RW44 series — the high-feature line with integrated bypass, inside-delta capability, and adjustable current limit. For a panel-builder or MRO buyer, this is the part to specify when the motor is over 100 A and the panel has the vertical clearance.