What size soft starter should be used in an MCC feeder?

The soft starter should be sized primarily from the motor full-load current, starting duty, ambient temperature, and overload profile, not just horsepower. In an IEC 61439 MCC, the selected unit must also fit the enclosure thermal budget and the assembly’s declared current rating. As a rule, manufacturers such as ABB, Siemens, Schneider Electric, and Eaton publish frame sizes and duty curves that indicate continuous current, overload class, and number of starts per hour. If the motor starts frequently or under heavy inertia, oversizing or using an integrated bypass contactor may be necessary to reduce losses and increase thermal margin.

Do soft starters in MCCs need bypass contactors?

Bypass contactors are not mandatory, but they are very common in MCCs because they reduce steady-state power dissipation inside the bucket after the motor reaches full speed. This is especially useful in compact IEC 61439-2 assemblies with multiple feeders and limited ventilation. A bypass configuration improves thermal performance, lowers component stress, and can increase panel density. For process-critical applications, bypass also helps preserve the soft starter’s electronics by removing them from the load current once the start sequence is complete. The final choice depends on heat rise calculations, maintenance philosophy, and the manufacturer’s coordination data.

How is short-circuit protection coordinated with a soft starter in an MCC?

Short-circuit protection is typically provided by an MCCB, motor circuit breaker, or fuses upstream of the soft starter. The selected protective device must coordinate with the soft starter’s semiconductors and the MCC busbar system so the assembly meets the declared Icw and Ipk withstand ratings under IEC 61439-1/2. The soft starter manufacturer’s coordination tables should confirm the maximum prospective fault current, the required backup protective device, and whether type 1 or type 2 coordination is achieved in accordance with IEC 60947-4-2. Proper coordination protects both the starter and the downstream motor circuit while maintaining selective tripping where required.

Which IEC standard applies to soft starters inside a Motor Control Center?

The MCC assembly itself is governed by IEC 61439-1 and IEC 61439-2, which cover low-voltage switchgear and controlgear assemblies. The soft starter as a motor-controller device falls under IEC 60947-4-2. If the panel is installed in an explosive atmosphere, surrounding equipment and installation practices may also need to comply with IEC 60079. Where arc-flash containment or arc-resistant design is required, IEC 61641 is often referenced for internal arc testing. In a compliant project, the panel builder must demonstrate that the complete assembly, including the soft starter feeder, meets all applicable ratings and verification requirements.

Can soft starters in MCCs communicate with PLCs and SCADA systems?

Yes. Most modern soft starters offer communication modules or встроенные interfaces for Modbus RTU/TCP, Profinet, Profibus, EtherNet/IP, and other industrial protocols. This allows the MCC to send motor status, current, fault codes, thermal capacity, and start/stop commands to a PLC or SCADA system. For facility management, this is useful for predictive maintenance and alarm handling. In engineered assemblies, communication wiring should be segregated from power circuits and implemented with proper EMC practices to preserve signal integrity. Products from Siemens SIRIUS, Schneider Altistart, ABB PSTX, and Eaton S811 are common examples in digital MCC architectures.

What forms of separation are common for soft starter buckets in an MCC?

Soft starter feeders in MCCs commonly use Forms 2b, 3b, or 4 depending on access, service continuity, and maintenance requirements. Higher forms of separation provide better compartmentalization of busbars, functional units, and terminals, limiting the impact of a fault or maintenance intervention. In an IEC 61439 MCC, the chosen form must be consistent with the builder’s design verification and the site’s operational needs. For plants with high availability requirements, withdrawable buckets and higher separation are often preferred because they improve replacement speed and reduce exposure to live parts.

How do soft starters affect MCC thermal management?

Soft starters add heat during both starting and running, with the highest dissipation usually occurring when the semiconductor path is carrying load current. In dense MCC lineups, this heat can reduce the allowable current of adjacent feeders and may require derating, forced ventilation, or heat-exchange measures. Bypass contactors significantly reduce steady-state losses. Thermal verification under IEC 61439 should consider ambient temperature, enclosure IP rating, internal spacing, cable heat contribution, and the number of simultaneously running feeders. If the thermal model is not respected, nuisance trips, premature aging, and reduced service life can result.

What is the difference between a soft starter and a VFD in an MCC?

A soft starter reduces starting voltage and current but generally runs the motor at fixed speed once starting is complete, often with an internal or external bypass. A VFD controls both starting and running speed by varying frequency and voltage, offering process control and energy savings on variable-torque loads. In an MCC, soft starters are usually chosen for constant-speed loads that only need reduced inrush and mechanical stress, such as pumps and conveyors. VFDs are better where speed regulation is required. Both devices must be integrated in compliance with IEC 61439 and coordinated with upstream protection devices and the MCC busbar system.

Panel + Component

Soft Starters in Motor Control Center (MCC)

Soft Starters selection, integration, and best practices for Motor Control Center (MCC) assemblies compliant with IEC 61439.

Overview

Soft starters in a Motor Control Center (MCC) are used to limit inrush current, reduce mechanical stress, and improve process control for fixed-speed motors driving pumps, fans, compressors, conveyors, and mixers. In an IEC 61439-2 MCC assembly, the soft starter must be selected not only for motor nameplate current but also for the application duty, starting frequency, overload class, ambient temperature, and the thermal profile of the cubicle. For typical industrial use, soft starters are commonly specified in the 3 to 1250 A range, with coordinated motor protection provided by MCCBs or fuses upstream, and with optional bypass contactors to minimize losses after ramp-up. In heavy-duty service, integrated bypass is often preferred to reduce heat dissipation within the enclosure and preserve adjacent feeder performance. Engineering an MCC around soft starters requires verification of the assembly’s rated current, short-circuit withstand rating, and internal segregation. Under IEC 61439-1 and IEC 61439-2, the builder must demonstrate that the busbar system, feeder devices, terminals, and wiring can withstand declared Icw/Ipk values and that temperature rise remains within limits at the rated assembly current. In practice, this means validating the soft starter’s contribution to losses, especially when multiple starters are mounted in adjacent functional units. Forms of separation such as Form 2b, Form 3b, and Form 4 are commonly used in MCCs to improve maintainability and limit fault propagation between feeders. Where process continuity is critical, higher segregation and withdrawable starter buckets are preferred. Modern soft starters frequently integrate fieldbus communication, including Modbus RTU/TCP, Profibus, Profinet, EtherNet/IP, and DeviceNet, enabling diagnostics to be captured by PLCs, SCADA, and BMS platforms. This is valuable for monitoring motor current, start time, stall events, phase loss, overload trips, and thermal model status. Device choices often include ABB PSTX, Siemens SIRIUS 3RW5, Schneider Electric Altistart 22/48, and Eaton S811, depending on network requirements and installation footprint. For demanding applications, soft starters may include pump-cleaning logic, torque control, kick-start, and current-limit modes to match load inertia and process constraints. Coordination with upstream and downstream protection devices is essential. The soft starter must be matched with an MCCB, motor protection circuit breaker, or fuse set that provides both short-circuit protection and discrimination with downstream motor circuits, consistent with IEC 60947-4-2 and IEC 60947-2. Where the MCC is installed in harsh environments, the enclosure thermal design, ventilation strategy, and pollution degree must be considered, and in hazardous areas the surrounding installation may also require conformity with IEC 60079. For industrial installations with arc-fault concerns, panels may also be assessed against IEC 61641 for arc resistance. In all cases, the correct coordination package, cable sizing, component derating, and environmental limits determine whether the soft starter performs reliably as a robust MCC feeder solution. A well-designed MCC with soft starters provides compact motor control, lower starting stress, better energy utilization, and clear integration with digital asset monitoring. The result is a maintainable, standards-compliant assembly that supports continuous production while simplifying startup, protection, and diagnostics for plant operators and EPC contractors.

Key Features

Soft Starters rated for Motor Control Center (MCC) operating conditions
IEC 61439 compliant integration and coordination
Thermal management within panel enclosure limits
Communication-ready for SCADA/BMS integration
Coordination with upstream and downstream protection devices

Specifications

Panel Type	Motor Control Center (MCC)
Component	Soft Starters
Standard	IEC 61439-2
Integration	Type-tested coordination

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Motor Control Center (MCC)

Soft Starters

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