Bus Bar Design for Electrical Switchboards

Introduction

To understand the bus bar as a critical element of switchboard assembly, we can draw an analogy with the human body. Just as healthy veins are vital for circulating blood throughout the body to ensure proper functioning, a properly designed bus bar system is essential for distributing electrical power safely and reliably within a switchboard.

The bus bar must be capable of carrying the continuous full-load current of the system under normal operating conditions, while also withstanding short-time fault currents that may occur during abnormalities such as short circuits. Unlike veins, however, the bus bar faces additional engineering challenges such as temperature rise, electromagnetic forces, mechanical stresses, and insulation clearances.

Therefore, bus bar design is not merely a matter of sizing copper or aluminium conductors; it is a multi-parameter engineering task. Switchboard design engineers must consider the following factors while designing the bus bars:

• Current rating & temperature rise: Ensuring the bus bar can continuously carry the rated current without exceeding permissible temperature limits.
• Short-circuit withstand capacity: Designing for high fault currents for durations typically ranging from 1 to 3 seconds to sustain high let-through energy.
• Electromechanical forces: Evaluating stresses during fault conditions to prevent deformation or failure of bus bar supports.
• Creepage & clearance distances: Maintaining safe insulation distances to avoid breakdowns or flashovers.
• Material selection & configuration: Optimising between copper/aluminium, shape, plating, and arrangement for efficiency and cost-effectiveness.

In summary, the bus bar is the backbone of the switchboard—its design directly impacts reliability, safety, and performance of the entire system.

👉 With this understanding, let us now look at the key factors that influence bus bar design in detail.

Factors Influencing Bus Bar Design

Designing a bus bar system requires balancing electrical, thermal, mechanical, and safety considerations. The following are the key factors that determine the suitability and performance of a bus bar system in a switchboard:

1. Current Carrying Capacity

• The bus bar must be sized to carry the continuous full-load current without exceeding permissible temperature rise limits.
• The current rating depends on conductor cross-sectional area, material (copper/aluminium), ambient temperature, enclosure ventilation, bus bar profile and installation method.

2. Temperature Rise

• Excessive heating can degrade insulation, reduce mechanical strength, and shorten service life.
• Standards such as IEC 61439 for “low-voltage switchgear and controlgear assemblies” define allowable temperature rise limits for bus bar systems. The said limits can be referred to from the table given in the standard.
• Proper spacing, plating (e.g., tin/silver), and ventilation improve heat dissipation.

3. Short-Circuit Withstand Strength

• During a fault, bus bars experience very high currents (tens of kA) for a short duration of typically 1–3 seconds.
• This generates both thermal stress (I²t heating) and mechanical stress (electrodynamic forces between conductors).
• Bus bar supports spacing, and bracing must be designed to withstand these stresses without permanent deformation.

4. Electromechanical Forces

• Fault currents create magnetic fields that exert strong repulsive or attractive forces on the adjacent bus bars as per Ampere’s Force Law. This force is repulsive in nature if the current is opposite, whereas it is attractive if the direction of the current is the same.
• These forces increase with the square of the current and inversely with spacing. The magnitude of the force between two long, straight, parallel running conductors per unit length will be:

F/L is the force per unit length (N/m)

𝐼1and 𝐼2 are the currents in the two conductors (A)

𝑟 is the distance between the conductors (m)

𝜇0 is the permeability of free space

• Adequate support insulators and bracing systems are necessary to prevent the displacement or collapse of bus bars.

5. Creepage & Clearance Distances

• Adequate distances between live conductors and between conductor-to-earth are critical for insulation safety.
• Clearance: Minimum air gap between conductors.
• Creepage: Minimum distance along insulating surfaces.
• These are governed by system voltage, pollution degree, and insulation level as per IEC standards.

To gain a clear understanding of creepage and clearance, refer to the diagram below.

6. Material Selection

• Copper: Higher conductivity, smaller size, better efficiency, but more expensive.
• Aluminium: Cost-effective and lighter, but requires a larger cross-section and proper joint treatment.
• Surface finishing (e.g., tin or silver plating) prevents oxidation and reduces contact resistance.

7. Bus Bar Configuration

• Arrangements like single bus, double bus, or sandwich bus impact performance, cost, and reliability.
• Compact sandwich bus bar designs offer higher short-circuit strength and reduced electromagnetic emissions, but can be more expensive to manufacture.

8. Environmental Conditions

• Ambient temperature, humidity, altitude, and pollution levels affect bus bar performance.
• For outdoor or coastal installations, special coatings or enclosures may be required to prevent corrosion.

✅ These factors collectively determine whether the bus bar system will perform reliably throughout its service life. A well-engineered bus bar design ensures not only compliance with IEC standards but also long-term safety and efficiency of the switchboard.

The biggest question is, how to validate the design? So the only option is testing in an approved laboratory. However, a prototype needs to be created to conduct the test in the laboratory. To create the prototype, calculations can be used for thermal rating, clearance, creepage, and electrodynamic force.

Let us consider a practical case to calculate bus bar cross-section and insulator spacing based on thermal rating and short circuit level.

Example

Let us calculate the busbar cross-section required for a 1250A-rated aluminium bus bar with a fault current of 36kA

Thermal Design:

• Ambient temperature 45°C and operating temperature of 90°C
• Enclosure design & ventilation < 1% (can be calculated from actual enclosure design)
• Proximity Effect & Eddy Currents for normally spaced bus bar
• Configuration of Bus Bar as standard

Table 1: Derating factors on account of higher ambient temperature

Table 2: Heat Dissipation Factor

Table 3: Derating due to the Proximity effect

Table 4: Bus Bar configurations

Derating:

• Due to ambient temperature (table 1)– 0.88
• Due to enclosure design & ventilation (table 2) – 0.95
• Proximity Effect & Eddy Currents (Table 3)- 0.95
• Configuration of Bus Bar (Table 4) – 1

Total derating due to all factors = 0.88*0.95*0.95*1=0.794

Bus bar rating required = 1250 A/0.794=1576 A

Select the bus bar from the manufacturer’s published rating chart for a standard configuration that exceeds 1576 A.

Cross-section selected from the rating chart = 80*10*2=1600 mm²

Fault Current Design:

Calculating the minimum conductor size. Refer to the formula below:  

Øt = (k/100) ( Isc/A)² (1+ α₂₀*Ø)*t  

Øt= temperature rise above operating temperature in °C

Isc= symmetrical fault current in r.m.s in Amps

A= cross sectional area of conductor in mm²

α₂₀= temperature coefficient of resistance at 20 °C

         0.00363 for Aluminium alloy & 0.00393 for pure copper

Ø= operating temperature of the conductor at which the fault occurs

k= 1.166 for aluminium and 0.52 for copper

t= duration of fault in seconds

Final temp. allowed for busbar after a short circuit is 185 deg C

By putting all the values in the above equation

100 = (1.166/100)*( 36000/A)² *(1+ 0.00363* 85)*1 

A=530 mm²

The cross-section obtained from both calculations should be compared, and the higher value is to be considered for design. Accordingly, a busbar cross-section of 1600 mm² (Aluminium) is required based on the thermal rating and short-circuit withstand requirements.

Design of Busbar Support System (Insulators):

We have calculated the cross-section of the busbar, but at the same time, they need to be installed in the switchboard supported by insulators of adequate strength to withstand the electrodynamic force during a short circuit. Refer diagram below where busbars are supported on finger-type insulators.

Busbars are supported on finger-type insulators inside the switchboard. Under short-circuit conditions, the electrodynamic interaction between adjacent phases produces a lateral line-force (force per unit length) which results in transverse loads on the busbar span between insulators. The support insulators, therefore, must be selected for the resulting shear and bending loads (and checked against the manufacturer’s mechanical ratings).

Definitions

• = electrodynamic lateral load per unit length between phases (N/m).
• = centre-to-centre distance between consecutive supports/insulators (m).
• Span considered = .

Loads on supports (simply supported span with uniform lateral load )

• Total lateral force acting on the span:

• Vertical/shear reaction at each support (equal share):

• Maximum bending moment in the span (at mid-span):

Practical checks & recommendations

1. Use manufacturer data — compare the computed shear and bending moment with the insulator datasheet (static shear, tensile and bending moment ratings).
2. Apply dynamic safety factor — short-circuit loads are dynamic: adopt a safety factor of 2.0–3.0 on shear and moment when selecting insulators/clamps (industry practice; use project/standards value where specified).
3. Check deflection & clearance — compute busbar deflection to ensure no reduction of electrical clearances or contact with adjacent parts.
4. Support spacing — reducing d reduces both  and (shear ∝ , moment ∝ ) — more supports is an effective mitigation.
5. Joint & clamp design — consider localised high stresses at clamps; ensure clamps and bolted joints have adequate mechanical strength and are specified separately.
6. Thermal/ageing considerations — higher steady losses raise operating temperatures and can reduce the mechanical strength of polymeric insulators; verify mechanical ratings at expected operating temperature.
7. Testing — include prototype short-time mechanical testing or laboratory-type tests where required by standards.

Conclusion

Designing a bus bar system is far more than a mathematical exercise — it is an engineering responsibility that directly impacts the safety, reliability, and performance of the entire switchboard. A properly designed bus bar must efficiently carry the continuous load current, safely withstand short-circuit forces, and maintain adequate electrical clearances under all operating conditions.

While analytical calculations help determine the thermal rating, mechanical strength, and insulation requirements, the final validation can only be achieved through type testing in an accredited laboratory. Prototype testing verifies that the theoretical design performs as intended under real fault and load conditions, ensuring full compliance with IEC standards.

In essence, a well-engineered and thoroughly validated bus bar system forms the backbone of a reliable power distribution network. It reflects not only sound electrical design but also the discipline, precision, and commitment to quality that define good engineering practice.

FAQ

Q- Is busbar with a lesser thickness may carry a higher current compared to a thicker busbar of the same cross-section?

A- Yes, because the wider surface area provides better heat dissipation, allowing it to handle more current before reaching the permissible temperature rise.

Q- Is the proximity effect reduces useful cross section of bus bar?

A- Yes. The proximity effect occurs when alternating current (AC) in one conductor induces eddy currents in nearby conductors. These induced currents cause the main current to concentrate near certain regions of the busbar’s cross-section — typically away from the adjacent conductor.

As a result, the current is no longer uniformly distributed, effectively reducing the useful cross-sectional area available for current flow, which in turn increases resistance and heating.

Q- Does the proximity effect also reduces the effective cross section if the busbar is thick because of the self-induced emf?

A- Yes, the proximity effect reduces the effective cross-section of a thick busbar because of the self-induced EMF generated within it.

• When AC current flows through a thick busbar, it produces a time-varying magnetic field.
• This magnetic field induces EMFs (electromotive forces) within the busbar itself — these are called self-induced EMFs.
• These EMFs oppose the flow of current in certain regions (especially the inner parts of the busbar), forcing the current to concentrate near the surface.
• As a result, the inner portion of the conductor carries less current, thereby reducing the effective cross-section available for current flow.

External Reading References

IEC 61439 – Low-Voltage Switchgear & Controlgear Assemblies
https://webstore.iec.ch/publication/6028

IEC 62271-200 – High-Voltage Switchgear & Controlgear
https://webstore.iec.ch/publication/7012

IEEE C37.20.2 – Metal-Clad Switchgear Standard
https://ieeexplore.ieee.org/document/7425739

IEEE Std 605 – Guide for Bus Design in AC Substations
https://ieeexplore.ieee.org/document/7425887

ABB Technical Application Papers – Busbar Systems
https://library.abb.com/

Schneider Electric – Electrical Installation Guide (EIG)
https://www.se.com/ww/en/work/support/electrical-installation-guide/

Eaton Power Engineering Guide – Switchgear & Busbar Design
https://www.eaton.com/us/en-us/support/product-support/power-engineering-guide.html

Copper Development Association (CDA) – Busbar Design Guide
https://www.copper.org/resources/properties/thermal/electricity/busbar/

UL 891 – Standard for Switchboards
https://standardscatalog.ul.com/standards/en/standard_891

IEC TR 60865 – Short-Circuit Forces in Busbar Systems
https://webstore.iec.ch/publication/734