Sizing a soft starter for a motor load
The 3RW4425-1BC45 is a SIRIUS soft starter rated 99 A at 40 °C, which means it handles a standard 55 kW (75 hp) motor at 400 V in normal starting duty — the current rating is the starting current ceiling, not the motor FLA, so size it for the motor's locked-rotor current, not the run current. It carries an integrated bypass contact system, so once the motor is up to speed the thyristors are shorted out — no external bypass contactor needed, and the bypass carries the full 99 A continuously without thyristor heat dissipation. The adjustable current limitation lets you set the starting current between 8 % and roughly 400 % of the soft starter's rated current, so you can tune the start to the load inertia — a high-inertia fan gets a longer ramp, a conveyor gets a quicker pull-up.
Panel fit and wiring terminals
Dimensions are 170 mm wide, 192 mm high, 270 mm deep — the 270 mm depth is the biggest fit constraint; check the enclosure depth before committing. Main circuit terminals are box terminals accepting 2x (4... 50 mm²) stranded or 2x (10... 1/0) AWG using both clamping points. Clearance around the unit: 100 mm upwards, 75 mm downwards, 5 mm at the sides — the side clearance is tight, so plan for airflow and cable bending radius.
Thermal derating and operating limits
The derating temperature threshold is 40 °C — above that, the current drops linearly. Maximum operating ambient is 60 °C; storage range is -25 to +80 °C. Pollution degree 3 per IEC 60947-4-2 means it tolerates conductive dust and humidity typical in industrial panels. Insulation voltage rated 690 V — suitable for 400 V and 480 V systems with margin.
The inside-delta circuit option reduces the line current to 58 % of the motor winding current — useful when the supply transformer or cable is undersized for a standard-line start. Motor overload protection is integrated, along with intrinsic device protection — no separate overload relay needed in most installations.