MCC Panels

Busbar Systems in Soft Starter Panel

Busbar Systems selection, integration, and best practices for Soft Starter Panel assemblies compliant with IEC 61439.

Busbar Systems in Soft Starter Panel

Overview

Busbar systems in a soft starter panel are the main power distribution backbone that transfers incoming feeder energy to the soft starter, bypass contactor, motor feeder, control transformers, and auxiliary circuits with low impedance and predictable thermal behavior. In IEC 61439-2 assemblies, the busbar arrangement must be designed and verified for rated current, short-circuit withstand, dielectric performance, and temperature-rise limits under the actual operating profile of the panel. For typical industrial motor applications, incomer and main distribution busbars may range from 160 A up to 3200 A or higher, depending on the number of outgoing starters, duty cycle, and ambient conditions. Copper busbars are generally preferred for compact soft starter panels because they offer higher conductivity, better short-circuit performance, and easier thermal control, while aluminum busbars may be selected for cost-sensitive projects with adequate cross-section and joint design. A properly engineered busbar system for soft starter panels must coordinate with the soft starter’s current rating, the bypass contactor arrangement, and upstream protection devices such as MCCBs, ACBs, or fused switches. For example, a 55 kW motor feeder may use a soft starter with a current-adjusted semiconductor rating, a bypass contactor sized to motor full-load current, and busbars selected for continuous current plus harmonic and ambient derating. The busbar supports, phase spacing, insulation system, and enclosure ventilation must be verified so that temperature-rise stays within IEC 61439-1/-2 limits without compromising adjacent VFDs, PLC power supplies, or protection relays. In multi-feeder motor control centers, form of separation to IEC 61439-2, such as Form 2, Form 3, or Form 4, is often applied to improve service continuity and segregation between incoming, starter, and control compartments. Short-circuit rating is a critical selection criterion. The busbar system must have a verified rated short-time withstand current Icw and peak withstand current Ipk matching or exceeding the prospective fault level at the installation point. In industrial plants with fault levels of 25 kA, 36 kA, 50 kA, or 65 kA, the busbar supports, joints, and bolted connections must be designed and tested accordingly. Joint surfaces, contact pressure, torque values, plating type, and thermal cycling performance are especially important in soft starter panels because frequent motor starts can generate repeated thermal stress even when starting current is limited by the soft starter. Busbar systems should also be coordinated with outgoing motor protection devices, protection relays, and monitoring options such as power meters and SCADA/BMS communication gateways for load visibility and preventive maintenance. For harsh environments, additional requirements may apply. In hazardous areas, related equipment may need compliance considerations with IEC 60079, while arc fault mitigation and internal arc containment practices may reference IEC 61641 where applicable to adjacent low-voltage assemblies. In real-world applications, these panels are used for pumps, fans, compressors, crushers, conveyors, and HVAC chillers, where soft starting reduces mechanical stress and busbar stability ensures reliable power transfer during repetitive starts. Patrion engineers design busbar systems for soft starter panels as part of complete IEC 61439 assemblies, balancing thermal performance, fault withstand, maintainability, and future expansion capacity for EPC and industrial customers.

Key Features

  • Busbar Systems rated for Soft Starter Panel 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

PropertyValue
Panel TypeSoft Starter Panel
ComponentBusbar Systems
StandardIEC 61439-2
IntegrationType-tested coordination

Other Components for Soft Starter Panel

Soft Starters

Reduced voltage motor starting, torque control, bypass options

Moulded Case Circuit Breakers (MCCB)

Branch protection 16A–1600A, thermal-magnetic or electronic trip

Contactors & Motor Starters

DOL/star-delta/reversing starters, overload relays, Type 2 coordination

Protection Relays

Overcurrent, earth fault, differential, generator protection relays

Other Panels Using Busbar Systems

Main Distribution Board (MDB)

Primary power distribution from transformer to sub-circuits. Rated up to 6300A. Houses main incoming breaker, bus-section, and outgoing feeders.

Power Control Center (PCC)

High-capacity power distribution for industrial facilities. Controls and distributes incoming power to MCC, APFC, and downstream loads.

Motor Control Center (MCC)

Centralized motor control with starters, contactors, overloads, and VFDs in standardized withdrawable/fixed functional units.

Variable Frequency Drive (VFD) Panel

Enclosed VFD assemblies with input protection, line reactors, EMC filters, output reactors, and bypass options.

Metering & Monitoring Panel

Energy metering, power quality analysis, and multi-circuit monitoring with communication gateways.

Lighting Distribution Board

Final distribution for lighting and small power. MCB/RCBO-based with DALI or KNX integration options.

Busbar Trunking System (BTS)

Prefabricated busbar distribution per IEC 61439-6. Sandwich or air-insulated, aluminum or copper.

Custom Engineered Panel

Bespoke panel assemblies for non-standard requirements — special ratings, unusual form factors, multi-function combinations.

DC Distribution Panel

DC power distribution for battery systems, solar installations, telecom, and UPS applications. MCCB/fuse-based DC protection.

Capacitor Bank Panel

Fixed or automatic capacitor bank assemblies for bulk reactive power compensation in industrial and utility applications.

Frequently Asked Questions

Busbar sizing in a soft starter panel should be based on the assembly’s rated current, installation method, ambient temperature, enclosure ventilation, and diversity of starts. Under IEC 61439-1 and IEC 61439-2, the busbar system must be verified for temperature rise and short-circuit withstand at the declared rating. In practice, copper busbars are often selected for compact MCC and soft starter panels because they handle higher current density and better fault currents than aluminum. The panel designer must also account for continuous feeder current, bypass contactor heat, and any harmonic or auxiliary loads. Final sizing should be validated by design verification or test data, not by cross-section alone.
The busbar system must match or exceed the prospective short-circuit level at the point of installation. Typical industrial ratings are 25 kA, 36 kA, 50 kA, or 65 kA for 1 second, with peak withstand current Ipk also verified under IEC 61439-1/-2. The incomer device, busbar supports, joints, and outgoing protection devices such as MCCBs or ACBs must all be coordinated to the same fault level. For motor control applications, the soft starter itself is usually protected by upstream devices, but the busbar must survive a fault without deformation or loss of insulation. Always confirm the declared Icw and Ipk values from the panel builder’s verified design.
Copper is generally preferred in soft starter panels because it provides higher conductivity, smaller cross-section for the same current, and stronger short-circuit performance. This is especially useful in compact IEC 61439 assemblies where heat from soft starters, bypass contactors, and control power supplies must be managed within tight enclosure limits. Aluminum can be a viable option for larger, cost-sensitive panels if joints are properly engineered, surface treatment is suitable, and thermal verification is completed. The choice should consider available space, corrosion environment, maintenance policy, and required short-circuit level. In many EPC projects, copper is selected for critical motor feeders and aluminum for main distribution where space allows.
Busbars must be coordinated with the soft starter’s rated current, the bypass contactor current rating, and the feeder protection device. The soft starter carries the starting current only during ramp-up, while the bypass contactor usually carries the motor current in steady state. That means the busbar must support both thermal loading during acceleration and continuous full-load current afterward. IEC 60947 devices such as contactors, MCCBs, and overload relays must be selected so their coordination does not create nuisance tripping or localized heating at connection points. In multi-feeder panels, phase spacing and tap-off arrangement should also minimize hotspots and voltage drop.
Soft starter panels commonly use IEC 61439 forms of internal separation to improve safety, maintainability, and fault containment. Form 2 provides separation between busbars and functional units, while Form 3 adds segregation between functional units, and Form 4 further separates outgoing terminals from adjacent units. The selection depends on required service continuity, maintenance access, and project specification. For panels with multiple motor feeders, Form 3 or Form 4 is often preferred because a fault or maintenance activity on one starter is less likely to affect other feeders. Proper barriers, insulated busbar sleeves, and partition design are essential to maintain the declared form.
Yes. Thermal verification is mandatory under IEC 61439-1/-2 for the complete assembly, including busbars, connections, devices, and enclosure ventilation. Soft starter panels are especially sensitive because the semiconductor section, bypass contactor, and control transformers can add concentrated heat near the busbar route. The panel builder must verify that temperature rise remains within permissible limits at the declared current, ambient temperature, and duty cycle. This may be done by test, comparison with a verified reference design, or calculation methods accepted by the standard. For higher-density panels, forced ventilation or increased busbar spacing may be required.
Yes, but the busbar system itself is the power path; SCADA or BMS integration is achieved through metering and monitoring devices installed on or near the busbar. Typical additions include multifunction power meters, current transformers, communication gateways, and protection relays with Modbus RTU/TCP or Ethernet interfaces. These devices allow real-time measurement of bus current, voltage, power factor, and energy consumption, which is useful for pumps, HVAC, conveyors, and process motors. While IEC 61439 governs the assembly, communication architecture is usually implemented alongside the power system to support predictive maintenance and energy management.
Busbar systems in soft starter panels are widely used for industrial motor loads that benefit from reduced inrush current and controlled acceleration. Common applications include pumps, fans, compressors, crushers, conveyors, mixers, and HVAC chillers. In these installations, the busbar must handle repeated starting cycles while maintaining thermal stability and low voltage drop. For larger sites, soft starter panels may be integrated into MCC lineups with ACB or MCCB incomers, feeder protection relays, and metering for energy analysis. The busbar design must therefore support both operational reliability and future expansion capacity in line with the project’s IEC 61439 requirements.

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