Busbar Sizing

Busbar sizing is two independent checks, not one calculation: a temperature-rise check for the continuous current, and a short-circuit check for the fault duty. Area = Current ÷ Current Density gives a starting cross section in seconds, but it proves neither. A busbar comfortable at 2500 A continuously can still fail at 65 kA for 1 second if its supports, spacing or joints were never verified for that force.

So a busbar is rated for a current only in the context of a specific assembly — a defined bar geometry, spacing, support pitch, enclosure, IP rating and ambient temperature. Change any one of those and the rating has to be re-established. This is why IEC 61439-1:2020 and IEC 61439-2:2020 treat temperature rise and short-circuit withstand as assembly characteristics rather than conductor properties, and why "we use thick copper" is not an answer to either question.

The Question Behind the Specification

A customer asks for:

2500 A PCC Panel

and two panel manufacturers submit different busbar designs.

Manufacturer A proposes:

2 × 100 mm × 10 mm copper bars per phase

Manufacturer B proposes:

3 × 100 mm × 10 mm copper bars per phase

Both may say:

“Busbar is suitable for 2500 A.”

So which one is correct?

The answer cannot be determined from busbar cross-sectional area alone.

Busbar sizing in an LT panel depends on much more than:

Ampere ÷ Current Density

A properly engineered busbar system must consider:

• Continuous current
• Copper or aluminium material
• Ambient temperature
• Temperature rise
• Number of bars per phase
• Bar thickness and width
• Busbar spacing
• Orientation
• Enclosure ventilation
• Adjacent heat-producing devices
• Skin and proximity effects
• Busbar joints
• Short-circuit withstand
• Support spacing
• Neutral loading
• Harmonics
• Protective earth requirements
• Applicable IEC/IS assembly verification

IEC 61439-1:2020 establishes the general construction, technical-characteristic and verification requirements for LV assemblies, while IEC 61439-2:2020 applies specifically to power switchgear and controlgear assemblies. Temperature-rise and short-circuit withstand are therefore assembly-level engineering requirements, not simply busbar-area calculations.

This guide explains a practical method for selecting and verifying busbars for PCC, MCC, PMCC, APFC, VFD and other LT panels.

The Biggest Busbar-Sizing Mistake

One of the most common approaches is:

Copper busbar = X A/mm²

Aluminium busbar = Y A/mm²

Then:

Required area = Current ÷ Current density

This method can be useful for a first-pass estimate.

But it should not be treated as the final IEC busbar design.

Why?

Because two busbars with the same cross-sectional area can operate at different temperatures depending on:

• Their shape
• Number of parallel bars
• Spacing
• Enclosure
• Ventilation
• Ambient temperature
• Connection arrangement

Published IEC 61439 busbar-system data shows this directly: rated current is stated for a particular bar size and a particular set of assembly conditions, and the current capability moves with ambient temperature.

So the correct workflow is:

Calculate → Select preliminary busbar → Check thermal design → Check short circuit → Check joints/supports → Verify against the applicable assembly design.

Step 1 — Determine the Required Panel Current

Before sizing the busbar, determine the actual rated current required.

For a three-phase load:

I = P / (√3 × V × PF × η)

where applicable.

If transformer or system apparent power is already known:

I = kVA × 1000 / (√3 × V)

Example: 1000 kVA Transformer at 415 V

For:

Transformer = 1000 kVA

Voltage = 415 V

Full-load current is approximately:

1391 A

Therefore, an LT incomer associated with that transformer may commonly be engineered around a standard rating above this current, subject to the complete project design.

But this does not automatically mean:

1600 A breaker = any 1600 A busbar is acceptable.

The busbar arrangement must still meet its thermal and short-circuit requirements.

Example: 2000 kVA Transformer at 415 V

For:

2000 kVA

415 V

the transformer full-load current is approximately:

2782 A

Depending on the project, an engineer may therefore evaluate a switchboard system around a standard current rating such as:

3200 A

But again, transformer current is only the starting point.

The final busbar design must account for actual assembly conditions.

Step 2 — Use Current Density Only as a Preliminary Estimate

For preliminary engineering:

Busbar Area = Current / Assumed Current Density

or:

A = I / J

where:

• A = conductor cross-sectional area in mm²
• I = current in amperes
• J = assumed current density in A/mm²

The important word is:

Assumed.

IEC 61439 does not provide one universal statement such as:

“Every copper busbar shall be designed at exactly 1.2 A/mm².”

The allowable current depends on the complete assembly and its temperature performance. IEC 61439 focuses on verification of the assembly's temperature-rise performance rather than prescribing one universal current-density number.

Practical Example — 2500 A Busbar

Consider two possible copper arrangements.

Option A

2 × 100 × 10 mm per phase

Cross-sectional area:

2 × 100 × 10 = 2000 mm²

At 2500 A, the arithmetic current density is:

2500 / 2000 = 1.25 A/mm²

Option B

3 × 100 × 10 mm per phase

Cross-sectional area:

3000 mm²

Current density:

2500 / 3000 ≈ 0.83 A/mm²

Is Option B automatically correct and Option A automatically wrong?

No.

Option B has more conductor material and lower arithmetic current density.

But the final decision still depends on:

• Verified thermal capability
• Bar spacing
• Support arrangement
• Enclosure
• Connection arrangement
• Short-circuit rating
• Applicable tested/verified assembly design

Similarly, Option A should not be approved merely because somebody's internal thumb rule permits 1.25 A/mm².

Why A/mm² Is Not Enough

Current density considers conductor area.

It does not directly describe how effectively the heat leaves the busbar.

Consider two conductors with equal total copper area.

One arrangement could be:

1 very thick bar

Another:

2 thinner parallel bars

Their:

• Surface area
• Cooling
• AC current distribution
• Proximity to adjacent phases

can be different.

That changes the thermal behaviour.

Therefore:

Cross-sectional area tells us how much metal exists. Temperature-rise performance tells us whether the complete design can actually carry the required current in its enclosure.

Temperature Rise Is the Real Continuous-Current Test

Every ampere flowing through a busbar produces heat because the conductor has resistance.

The approximate loss follows:

P = I²R

Therefore:

• Higher current increases losses rapidly
• Poor joints add localized resistance
• High ambient temperature reduces thermal margin
• Restricted airflow increases internal temperature

IEC 61439 requires temperature-rise performance to be verified as part of the assembly design. IEC's 2020 edition specifically refocused temperature-rise verification around rated current characteristics of loaded assemblies.

IEC 61439-based switchboard guidance consistently emphasises busbar and assembly temperature limits rather than treating current density alone as proof of compliance.

Can Copper Busbar Reach 140°C?

This statement needs to be understood correctly.

IEC 61439-based manufacturer guidance identifies 140°C as a reference maximum temperature for bare copper busbars under the applicable conditions, corresponding in a 35°C reference ambient to a 105 K rise.

But that does not mean:

“Design every panel so the busbar normally operates at 140°C.”

Practical switchboard engineering may deliberately use lower internal design temperatures depending on:

• Connected breakers
• Cables
• Insulation
• terminals
• enclosure components
• required service life
• project specification

The thermal limit of the complete assembly is often governed by more than the copper bar itself.

Ambient Temperature Changes Busbar Capability

A busbar that operates safely at:

35°C ambient

may not have the same continuous-current capability at:

50°C ambient

without derating or design changes.

This is particularly important in India for panels installed in:

• Outdoor substations
• Hot industrial sheds
• Boiler areas
• Steel plants
• Desert environments
• Non-air-conditioned electrical rooms

Therefore an RFQ should specify:

Maximum ambient temperature

rather than allowing every panel builder to assume standard laboratory conditions.

This is the item we push back on at clarification stage, because ambient temperature is the single most frequently omitted line in an Indian LT panel enquiry — and it is the one that changes the busbar. If the reference data assumes 35°C and the panel actually sits in a non-air-conditioned electrical room in Gujarat that reaches 50°C in May, the available temperature margin has shrunk by 15 K before a single ampere flows. In our experience, two bidders quoting the same "2500 A copper busbar" for the same enquiry have often simply assumed different ambients, and the cheaper offer is usually the one that assumed the more comfortable figure. State the design ambient and the IP rating together, because they act on the same margin.

Enclosure Size Also Affects Busbar Temperature

Consider exactly the same:

2500 A copper busbar

installed in two panels.

Panel A

Large enclosure with good natural airflow.

Panel B

Compact enclosure containing:

• ACB
• multiple MCCBs
• VFDs
• control transformer
• communication devices

The thermal environment is completely different.

Therefore, busbar selection cannot be separated from:

panel architecture.

IEC 61439 is an assembly standard for exactly this reason: incorporated components, conductors and enclosure conditions interact thermally.

Width vs Thickness — Does Busbar Shape Matter?

Yes.

Compare:

100 × 10 mm

with another bar having a similar area but different width/thickness.

At AC frequency, current is not always distributed perfectly uniformly throughout a large conductor.

The design can be influenced by:

• Skin effect
• Proximity effect
• Phase spacing
• Parallel-bar arrangement

Therefore, simply creating one very thick bar is not automatically the best high-current design.

In many high-current assemblies, manufacturers use multiple parallel bars with defined spacing because current distribution, cooling and mechanical design can be optimized as a complete busbar system.

Again, use the verified assembly arrangement rather than inventing a new geometry purely from area.

Why Busbar Spacing Matters

Spacing affects both:

Electrical behaviour

and:

Thermal/mechanical behaviour.

Busbars placed too close can experience:

• Greater proximity effects
• Reduced cooling
• Different electromagnetic forces

Spacing also interacts with:

• Clearance
• Creepage
• Insulation
• Support design
• Short-circuit forces

Therefore, if an assembly's short-circuit and temperature-rise verification was performed using a particular busbar/support arrangement, changing the spacing should not be treated as a cosmetic modification.

Copper vs Aluminium Busbar

Both copper and aluminium can be used successfully in LV switchboards when properly engineered.

Copper generally allows a more compact conductor arrangement for the same electrical duty, while aluminium can offer weight and commercial advantages.

But direct comparison should not be based only on:

₹ per kg

or:

A/mm².

The complete comparison should consider:

• Required cross section
• Panel size
• Joint design
• Surface preparation
• Thermal expansion
• Mechanical strength
• weight
• support system
• long-term connection behaviour
• verified assembly design

A detailed Copper vs Aluminium Busbar comparison is best treated as a separate engineering topic.

Main Busbar vs Vertical Distribution Busbar

Not every busbar inside an LT panel necessarily needs the same current rating.

Main Horizontal Busbar

Carries current between major sections.

Example:

3200 A main bus

Vertical Distribution Busbar

Supplies feeders within a particular panel section.

Its required rating depends on:

• Connected feeders
• Diversity
• assembly design
• group rated-current considerations
• project requirements

IEC 61439-1:2020 introduced/refined concepts around rated currents of circuits within loaded assemblies, reinforcing that an assembly is not always appropriately represented by one single current number.

Do not automatically make every internal distribution bar equal to the full incomer busbar without engineering need.

At the same time, never reduce it purely for cost without a valid design basis.

Neutral Busbar Sizing

Neutral sizing deserves separate engineering attention.

A common old approach is:

Phase busbar = 100%

Neutral = 50%

That may be suitable in some traditional balanced three-phase systems.

But it is not a universal rule.

Modern installations can contain significant single-phase and nonlinear loads such as:

• IT equipment
• UPS systems
• LED lighting
• switch-mode power supplies
• data-center loads
• electronic equipment

Harmonic currents can materially affect neutral loading. IEC 60364-5-52 includes specific considerations for conductor sizing where harmonic currents are present.

Therefore, neutral busbar sizing should consider:

• Expected load imbalance
• Single-phase load percentage
• Harmonic spectrum
• Third-harmonic/triplen content
• source arrangement
• project specification

Never reduce neutral simply because:

“Three-phase currents cancel.”

They cancel only under the relevant balanced fundamental-frequency conditions.

Can Neutral Busbar Be Common for Multiple Feeders?

A common neutral busbar can be technically workable in an appropriate distribution architecture.

But the design must still consider:

• Neutral rating
• Segregation
• identification
• fault-current path
• maintenance/isolation philosophy
• individual feeder arrangement
• neutral switching where required
• earthing system

A common busbar should not compromise the ability to safely isolate or maintain circuits according to the project design.

The question is therefore not merely:

“Can we install one common neutral?”

but:

“Does the common neutral arrangement satisfy the electrical loading, protection, isolation and maintenance philosophy of the complete assembly?”

Earth Busbar Sizing Is Different From Phase Busbar Sizing

The protective earth busbar does not normally carry continuous load current.

Its primary duty is related to:

• Protective bonding
• Earth-fault current
• Fault clearing
• equipotential connection

Therefore it should not be sized using the same continuous-current-density rule as the phase busbar.

The protective circuit must be capable of withstanding the thermal and mechanical stresses produced by applicable earth-fault/short-circuit conditions.

IEC 61439 includes verification requirements related to protective circuits and assembly short-circuit withstand.

Short-Circuit Withstand Can Be More Critical Than Continuous Current

A busbar may comfortably carry:

2500 A continuously

but still be inadequate for:

65 kA for 1 second

if:

• conductor section
• busbar supports
• joint design
• spacing
• structural arrangement

cannot withstand the fault.

Short-circuit current creates both:

Thermal stress

and:

Electromagnetic mechanical force.

IEC 60865-1:2011 provides standardized procedures for calculating the thermal effects on bare conductors and electromagnetic effects on rigid/flexible conductors during short-circuit conditions.

Therefore:

Continuous-current sizing and short-circuit sizing are two separate checks. The final busbar must satisfy both.

Where Does the Fault Current Come From?

Busbar short-circuit rating should come from the electrical-system short-circuit study.

The current IEC standard for calculating short-circuit currents in three-phase AC systems is IEC 60909-0:2016 (Edition 2.0). It applies to short-circuit-current calculations in LV and HV three-phase 50/60 Hz AC systems.

The prospective fault level can depend on:

• Transformer rating
• Transformer impedance
• Utility contribution
• Parallel transformers
• DG sets
• large motors
• cable/busduct impedance
• operating configuration

Therefore the panel manufacturer should ideally receive:

Calculated prospective short-circuit current at the panel location

rather than be asked to guess it from transformer kVA.

Example — Continuous Current Passes, Short Circuit Fails

Imagine a busbar arrangement thermally capable of carrying:

2000 A continuously

But project requirement is:

65 kA RMS for 1 second

If its:

• support spacing
• bar geometry
• joints
• bracing

were only verified for:

50 kA / 1 second

then it should not simply be declared:

65 kA

because the copper cross section “looks sufficient.”

The complete short-circuit design must be verified for the required duty.

Peak Current Also Matters

A short-circuit rating such as:

50 kA RMS / 1 s

does not fully describe the mechanical force at the first peak of the fault.

The initial peak current can be considerably higher than the RMS symmetrical current.

Electromagnetic forces between busbars are strongly influenced by this peak.

That is why busbar supports and bracing are critical.

Do not treat:

Icw

and:

peak withstand current

as the same parameter.

Busbar Support Spacing

Imagine two 100 × 10 mm copper bars.

Design A

Support every 250 mm.

Design B

Support every 600 mm.

Will both withstand the same short-circuit force?

Not automatically.

The span between supports affects mechanical stress and conductor deflection.

This is why a verified busbar system normally defines:

• bar arrangement
• support type
• support spacing
• fault rating

IEC 61439 verification examples explicitly link main-conductor size, current rating, short-time withstand and busbar-support distance — change one and the others must be re-checked.

This is much stronger evidence than:

“We use thick copper.”

Busbar Joints Can Become the Hottest Point

A beautifully sized busbar can still overheat at a poor joint.

Joint resistance can increase because of:

• Incorrect tightening
• Poor contact surface
• Contamination
• Oxidation
• Improper hardware
• Misalignment
• Insufficient contact area
• unsuitable copper/aluminium interface

IEC 61439 does not prescribe one universal busbar-joint overlap dimension; the effectiveness of the joint is ultimately reflected in the assembly's validated temperature-rise performance.

Therefore there is no universal rule such as:

“Every 100 mm bar must overlap exactly X mm.”

Use the verified busbar/joint design and correct assembly procedure.

We have opened older panels for retrofit work where the busbar itself was in perfect condition and every discoloured, annealed spot was at a joint — usually a bolted lap that had been tightened by feel, or a joint where a spring washer had been left out. A joint that adds even a few tens of micro-ohms carries the same current as the rest of the bar but concentrates the loss in a few square centimetres, so it heats far faster than the run of the bar and then loosens further as it cycles. Our recommendation: treat joint hardware, torque and surface preparation as part of the busbar design, record the torque values at FAT, and specify thermographic scanning at the first loaded shutdown. Joints are where busbars actually fail; conductor cross section rarely is.

What About Busbar Torque?

Another common mistake is publishing a universal torque chart and applying it to every busbar.

Required tightening torque depends on:

• Bolt size
• bolt grade
• washer arrangement
• joint design
• material
• plating
• equipment manufacturer

Therefore:

Torque should come from the approved busbar/system/component assembly procedure—not from one generic number copied from another project.

During manufacturing, torque control and traceability can be valuable quality measures.

But temperature-rise performance of the actual design remains the more fundamental electrical validation.

Tinned Copper Busbar — Does It Carry More Current?

Tin plating can provide useful surface and connection benefits, especially depending on:

• environment
• joint interface
• corrosion considerations

But adding tin does not magically convert an undersized copper conductor into a higher-current busbar.

The continuous-current capability still depends on the complete conductor and assembly thermal design.

Use plating because the application requires it, not as a substitute for correct cross section.

Busbar Sleeving and Insulation

Heat-shrink sleeving or insulated/coated busbars may be used for:

• phase identification
• protection against accidental contact
• environmental reasons
• project requirements

But insulation can also influence:

• heat dissipation
• temperature
• clearance/creepage design
• joint accessibility

Therefore, a busbar arrangement verified as bare copper should not automatically be assumed to have exactly the same current capability after adding substantial insulation.

The complete assembly design should account for the actual configuration.

Busbar Orientation

Horizontal and vertical busbars can have different cooling conditions.

Natural convection depends on how air flows around the conductor.

Orientation relative to:

• enclosure
• other busbars
• components

therefore matters.

This again explains why a universal current-density number cannot represent every assembly geometry.

Three Bars in Parallel Do Not Always Share Current Perfectly

Suppose a phase uses:

3 × 100 × 10 mm bars

Ideally each bar would carry one-third of the phase current.

In reality, current distribution can be influenced by:

• geometry
• connection points
• proximity
• AC effects

The connections should therefore be designed so current is shared appropriately across parallel bars.

Simply stacking more bars together without considering connection geometry is not necessarily optimal.

Busbar Connections to ACBs and MCCBs

Another common weak point is:

Large main busbar → comparatively small breaker connection link

The current path must be considered completely:

Incomer terminal → adapter/link → main busbar → vertical busbar → outgoing breaker

If the main horizontal busbar is capable of 3200 A but the connection link overheats at 2500 A, the switchboard is not a 3200 A thermal system.

IEC 61439 evaluates the assembly as a system for this reason.

In our FAT experience, the breaker-to-bus adapter link is the most common weak point in an otherwise sound high-current design. It is short, it is often a single bar where the main bus has three in parallel, it sits directly above an ACB that is itself producing heat, and it is the last item drawn — so it tends to be sized by what fits between the terminal and the bus rather than by current. Our recommendation: dimension the adapter link from the same rated current as the bus it connects to, show it explicitly on the busbar drawing with its own cross section, and include it in the heat-load calculation rather than treating it as hardware.

Do Not Forget Heat From the Circuit Breaker

An ACB carrying 3200 A produces heat.

So do:

• MCCBs
• contactors
• reactors
• VFDs
• control transformers

If the main busbar passes close to these devices, the local ambient inside the enclosure may be significantly higher than the room temperature.

Therefore:

Room ambient ≠ necessarily busbar local ambient.

Assembly-level temperature-rise verification accounts for the way these elements interact.

Example — Same Busbar, Different Panel

Consider:

2 × 100 × 10 mm copper per phase

Panel A

• Large PCC
• natural ventilation
• few components
• 35°C room

Panel B

• compact IP55 enclosure
• VFD section nearby
• 50°C environment
• limited ventilation

It would be poor engineering to assume exactly the same continuous-current rating purely because both panels contain the same copper section.

IP Rating Can Affect Thermal Performance

A higher IP enclosure generally restricts uncontrolled air exchange.

Therefore:

IP55/IP65

can create different thermal conditions than a more ventilated enclosure.

If a customer increases the project specification from:

IP42

to:

IP55

after the panel is designed, the effect should not be treated merely as:

“Change door gasket.”

Thermal performance may also need review.

IEC 61439 verification is tied to the complete assembly configuration.

High Ambient + High IP + High Current Is a Difficult Combination

Consider:

• 4000 A PCC
• IP55
• 50°C ambient
• compact room

This can require substantially more careful thermal engineering than:

• 4000 A
• indoor 35°C
• ventilated switchboard

Possible design responses may include:

• Larger conductors
• Alternative busbar arrangement
• larger enclosure
• controlled ventilation
• equipment derating
• verified proprietary assembly system

There is no one universal answer.

Does Busbar Need Derating at High Altitude?

High altitude changes:

• air density
• cooling
• dielectric performance

Therefore, if the panel will be installed significantly above normal reference altitude, that condition should be specified to the switchboard manufacturer.

Do not design a high-current panel assuming standard service conditions and later install it in a significantly different environment without review.

IEC 61439 includes defined service conditions as part of the assembly framework.

What Is Design Verification?

For an IEC 61439 switchboard, relevant design characteristics are verified through the permitted standard methods.

Key areas include:

• Temperature rise
• Short-circuit withstand
• Dielectric properties
• Protective circuits
• Clearances/creepage
• Component incorporation

IEC 61439-1 sets the general framework, and IEC 61439-2 specifies requirements for PSC assemblies.

This means a busbar design is not “IEC 61439 compliant” merely because:

Copper purity is high

or:

current density is low.

The assembly configuration must satisfy the applicable verification requirements.

Indian Standard for LT Power Switchboards

For Indian projects, BIS currently lists:

IS/IEC 61439 (Part 2):2020

for low-voltage power switchgear and controlgear assemblies, with the BIS listing showing review in 2023.

Therefore, an Indian PCC/MCC specification can explicitly reference the applicable:

IS/IEC 61439 Part 1 / Part 2

rather than simply using older generic language such as:

“Panel shall be type tested.”

The actual required ratings and verification evidence should also be defined.

Is Temperature-Rise Testing Done on Every Customer Panel?

Not necessarily.

Temperature rise is fundamentally a design-verification characteristic of the assembly system.

It is different from routine verification/FAT carried out on every completed project panel.

The project FAT should confirm that the manufactured panel remains consistent with the applicable design, including:

• Busbar sizes
• number of bars
• supports
• spacing
• connections
• equipment arrangement

rather than attempting a full destructive or high-current design-verification test on every supplied panel.

What Should Be Checked During FAT?

Busbar FAT should include careful visual and construction verification.

Check:

• Material
• Size
• Number of bars per phase
• Phase identification
• Main-bus arrangement
• Vertical busbar
• Neutral busbar
• Earth busbar
• Supports
• Support spacing
• Joints
• Hardware
• Torque procedure
• ACB/MCCB connections
• Clearances
• Insulation/sleeving
• Protective bonding

Where the approved quality procedure includes additional:

• resistance
• conductivity
• current-path

checks, these can also provide useful manufacturing-quality evidence.

But they do not replace the assembly's temperature-rise and short-circuit design verification.

Should Busbar Conductivity Be Tested?

Material verification can be useful where required by the project or QA process.

But saying:

“Copper conductivity passed, therefore 3200 A busbar is verified”

would be incorrect.

Conductivity checks can support confidence in the material.

They do not verify:

• heat dissipation
• enclosure thermal behaviour
• joint heating
• short-circuit forces

The complete busbar system must still satisfy the assembly design requirements.

Busbar Sizing Workflow — Practical Engineering Method

A good LT-panel busbar design can follow this sequence:

1. Establish system current

From:

• transformer
• load
• feeder requirement

2. Establish required rated current

Include actual project operating philosophy and future requirement where justified.

3. Select conductor material

Copper or aluminium.

4. Create preliminary cross section

Current-density calculation can be used as an initial estimate.

5. Select practical geometry

Determine:

• width
• thickness
• bars per phase
• spacing

6. Check thermal performance

Against the applicable verified assembly system and ambient/enclosure conditions.

7. Check neutral requirement

Consider imbalance and harmonics.

8. Check short-circuit requirement

Use calculated prospective fault current.

9. Check support system

Including support spacing and peak mechanical forces.

10. Check joints and connections

Including breaker interfaces.

11. Check assembly integration

IP, enclosure, ventilation and nearby heat sources.

12. Confirm applicable IEC/IS verification

Only then freeze the busbar design.

Busbar Sizing Checklist for an RFQ

Before asking a panel manufacturer to quote, provide:

Electrical System

• Voltage
• Frequency
• Rated current
• Transformer rating
• Transformer impedance
• Fault level
• Number of sources
• Bus-coupler philosophy

Busbar

• Copper/aluminium preference
• Rated current
• Neutral requirement
• Earth-bus requirement
• Short-time withstand
• Duration
• Peak withstand where specified

Environment

• Maximum ambient
• Indoor/outdoor
• Altitude
• IP rating
• ventilation restrictions

Loads

• Nonlinear load percentage
• UPS/IT loads
• VFD loads
• single-phase loads
• harmonic information where relevant

Assembly

• Form of separation
• ACB/MCCB arrangement
• cable/busduct connection
• future extension

Quality

• Applicable IEC/IS
• design-verification evidence
• FAT
• TPI
• documentation

This provides a far stronger engineering basis than:

“2500 A busbar, copper, 50 kA.”

Common Busbar Sizing Mistakes

Mistake 1: Using Only A/mm²

Current density is a preliminary tool, not complete thermal verification.

Mistake 2: Selecting the Same Busbar for Every Ambient Temperature

Cooling capability changes with ambient conditions.

Mistake 3: Ignoring IP Rating

A sealed enclosure can have very different thermal behaviour.

Mistake 4: Checking Continuous Current but Ignoring Short Circuit

Both requirements must be satisfied.

Mistake 5: Increasing Copper but Keeping Weak Supports

Short-circuit mechanical strength depends heavily on the support system.

Mistake 6: Reducing Neutral Automatically to 50%

Modern nonlinear/single-phase loads may require a larger neutral.

Mistake 7: Ignoring Breaker Connection Links

The smallest/hottest part of the current path may determine the thermal capability.

Mistake 8: Using One Universal Joint Torque

Torque depends on the approved joint and hardware design.

Mistake 9: Changing Busbar Geometry After Design Verification

Changing bars, spacing or supports can affect thermal and short-circuit performance.

Mistake 10: Assuming a Larger Bar Automatically Means an IEC-Compliant Panel

IEC 61439 compliance applies to the complete assembly.

How Wisdom Techno Solutions Approaches Busbar Engineering

At Wisdom Techno Solutions, busbar selection for PCC, MCC, PMCC and other LT panels should be based on the complete panel requirement rather than one fixed ampere-per-square-millimetre rule.

Wisdom Techno Solutions is a licensed partner for three design-verified switchgear platforms: Rittal Ri4Power (Rittal Solutions Partner since 2021 — the Ri4Power system carries Intertek ASTA design verification to IEC 61439-1 and -2 Edition 3, with copper and aluminium busbar systems up to 6300 A), C&S CX (Licensed Partner since 2023, up to Form 4B Type II) and Siemens SIEPAN Elite/8PU (IEC 61439 Solutions Partner since 2024). We build to each system's verified configuration and routine-verify every assembly in-house at our Vadodara, Gujarat facility, for industrial, EPC and infrastructure projects across India. That places the busbar temperature-rise and short-circuit verification inside a verified design envelope.

Relevant inputs include:

• Rated current
• System voltage
• Fault level
• Short-time withstand
• Copper/aluminium requirement
• Ambient temperature
• IP rating
• Panel construction
• Main breaker arrangement
• Neutral loading
• Harmonics
• Cable/busduct connection
• Applicable assembly design

Depending on project requirements, we engineer busbar systems for customized LT panels while considering:

• Main busbar
• vertical distribution bus
• neutral bus
• earth bus
• busbar supports
• ACB/MCCB connections
• short-circuit duty
• temperature-rise requirements
• customer specifications

The objective should not be:

“Use the minimum copper that passes an A/mm² calculation.”

Nor should it be:

“Add excessive copper everywhere to be safe.”

The objective is:

Use a busbar system whose material, geometry, joints, supports and assembly configuration are technically suitable for the required continuous current and fault duty.

WTS In-House FAT and Busbar Checks

WTS performs applicable in-house panel FAT using calibrated test instruments and equipment.

Depending on the project and approved procedure, busbar-related inspection can include:

• Busbar dimensional verification
• Material verification
• Joint inspection
• Connection inspection
• Torque control
• Phase identification
• Neutral/earth verification
• Protective continuity
• conductivity/current-path related checks where applicable
• IR testing
• HV/dielectric testing
• comparison with approved drawings

Customer and consultant representatives can witness FAT where required.

We also facilitate Third Party Inspection according to the agreed project inspection plan.

Design-verification characteristics such as temperature rise and short-circuit withstand should be supported by the applicable assembly design/evidence rather than being confused with routine FAT.

A Better Way to Compare Two Panel Quotations

Suppose:

Manufacturer A

2500 A PCC

Copper:

2000 mm²/phase

Manufacturer B

2500 A PCC

Copper:

3000 mm²/phase

Which is better?

Do not immediately select Manufacturer B because it contains more copper.

Ask:

Parameter	Manufacturer A	Manufacturer B
Rated assembly current	?	?
Ambient condition	?	?
Busbar configuration	?	?
Temperature-rise verification	?	?
Fault rating	?	?
Icw duration	?	?
Busbar support spacing	?	?
Neutral rating	?	?
Joint design	?	?
Applicable IEC 61439 evidence	?	?

Manufacturer B may indeed have the stronger design.

But kg of copper alone is not the technical comparison.

The verified performance of the assembly is.

Conclusion

Busbar sizing begins with current.

But it does not end with current density.

The simple preliminary formula:

Area = Current / Current Density

can help estimate conductor size.

After that, the real engineering begins.

Check:

• Material
• geometry
• bars per phase
• ambient temperature
• enclosure
• IP rating
• ventilation
• neutral current
• harmonics
• joints
• supports
• fault level
• short-circuit duration
• breaker connections
• applicable assembly verification

A 2500 A busbar is not 2500 A simply because:

2500 ÷ 1.25 = 2000 mm²

It is 2500 A when the complete busbar and switchboard design is shown to be suitable for that current under the required installation and verification conditions.

And a busbar that carries 2500 A thermally may still be unsuitable for:

65 kA / 1 second

unless its conductors, supports, joints and complete assembly can withstand that fault duty.

So the correct busbar-design question is not:

“How many A/mm² should I use?”

It is:

“What busbar arrangement will safely carry the required continuous current, remain within acceptable temperature limits and withstand the calculated short-circuit duty inside this actual switchboard?”

That is the difference between a simple copper calculation and proper LT panel engineering.

Planning a PCC, MCC, PMCC or high-current LT panel?

Share the SLD, transformer data, rated current, short-circuit level, ambient conditions, IP requirement and preferred busbar material with Wisdom Techno Solutions for project-specific panel and busbar engineering.

Related Guides

• Icu vs Ics vs Icw and panel fault rating
• Load list and transformer/incomer sizing
• Panel heat dissipation and cooling
• LT panel technical specification and RFQ guide
• Product page: IEC 61439 type-tested panels

Frequently Asked Questions

How is LT panel busbar size calculated?

Start with a preliminary estimate from Area = Current / Assumed Current Density to get a working cross section. Then select a practical geometry — bar width, thickness, number of bars per phase and spacing — and check it against the assembly's temperature-rise performance at the actual ambient and IP rating. Separately, check the short-circuit duty: conductor section for the thermal stress, and support type and spacing for the peak electromagnetic force. The busbar is only sized when both the continuous-current and the fault checks pass for that specific enclosure and support arrangement.

What current density should be used for copper busbar?

There is no single IEC 61439 current-density value suitable for every copper busbar. The allowable current depends on geometry, enclosure, ambient, ventilation and the verified assembly design.

Is 1.2 A/mm² compulsory for copper busbar?

No. IEC 61439 contains no such figure — 1.2 A/mm² is an internal design assumption used by some organisations for first-pass estimating, nothing more. Two busbars at the same 1.2 A/mm² can run at quite different temperatures depending on bar shape, number of parallel bars, spacing, enclosure ventilation and ambient. Use it to get a starting cross section, then justify the final selection on temperature-rise performance for the actual assembly.

Is 0.8 A/mm² compulsory for aluminium busbar?

No. As with copper, this is a preliminary estimating figure rather than a standard requirement. Aluminium needs a larger cross section than copper for the same duty because of its lower conductivity, but the final selection still depends on the complete thermal, mechanical and assembly design — including joint design and surface preparation, which matter more for aluminium than for copper because of oxide formation and thermal expansion at the connection.

What is the current of a 1000 kVA transformer at 415 V?

Approximately 1391 A, from I = kVA × 1000 / (√3 × V) = 1,000,000 / (1.732 × 415). An LT incomer for such a transformer is therefore commonly engineered around the next standard rating above this figure, typically 1600 A. Note that this is the transformer's rated full-load current, not a busbar rating — the 1600 A busbar still has to be verified for temperature rise in its actual enclosure and for the short-circuit duty at that location.

What is the current of a 2000 kVA transformer at 415 V?

Approximately 2782 A at rated apparent power, using the same relationship. A switchboard for this transformer would commonly be evaluated around a 3200 A standard rating. Bear in mind that the fault level rises with transformer size too, so a 2000 kVA transformer usually brings a higher prospective short-circuit current and therefore a more demanding support and bracing requirement than a 1000 kVA unit at the same voltage.

Is 2 × 100 × 10 mm copper suitable for 2500 A?

Its total cross-sectional area is 2000 mm², giving an arithmetic current density of 1.25 A/mm² at 2500 A. That calculation alone is not sufficient to declare it suitable; thermal and assembly verification are also required.

Why does ambient temperature affect busbar current?

Higher ambient temperature reduces the temperature margin available before the assembly reaches its permitted thermal limits. Manufacturer IEC-rated busbar data therefore applies ambient-dependent current ratings.

Does IP55 affect busbar sizing?

It can. More enclosed construction can change internal heat dissipation, so the complete switchboard thermal design should be reviewed rather than assuming the same busbar rating in every enclosure.

Should neutral busbar be 50% of phase busbar?

Not automatically. Load imbalance and harmonic current, particularly in systems with substantial nonlinear/single-phase loads, should be evaluated. IEC 60364-5-52 includes conductor-sizing considerations where harmonic currents are present.

Can neutral busbar be 100% of phase?

Yes, and in some installations it should be. A full-rated or specifically engineered neutral is commonly appropriate where single-phase loading is heavy or where triplen harmonics are significant — data-centre and IT loads, large UPS installations, extensive LED lighting and switch-mode supplies. Third-harmonic currents add arithmetically in the neutral instead of cancelling, so a neutral can carry substantial current even when the phase currents look balanced. The requirement should come from the load composition and harmonic data, and IEC 60364-5-52 includes conductor-sizing considerations where harmonic currents are present.

Does busbar size determine short-circuit rating?

Not by itself. Short-circuit withstand also depends on bar geometry, support type/spacing, joints and the complete assembly construction. IEC 60865-1 covers calculation of thermal and mechanical effects of short-circuit currents.

Which standard is used to calculate short-circuit current?

IEC 60909-0:2016 (Edition 2.0) is the current IEC standard for calculation of short-circuit currents in three-phase AC systems.

Which standard applies to LT switchboard busbars?

For applicable power switchgear and controlgear assemblies, IEC 61439-1:2020 provides the general requirements and IEC 61439-2:2020 the specific requirements, together forming the verification framework. Neither prescribes a busbar current density; instead they require the assembly's temperature rise and short-circuit withstand to be verified as characteristics of the complete design. For the short-circuit calculation itself, IEC 60909-0 is used to establish the prospective fault current, and IEC 60865-1 for the resulting thermal and mechanical effects on conductors.

What is the Indian standard for LT power switchgear assemblies?

BIS lists IS/IEC 61439 (Part 2):2020 for low-voltage power switchgear and controlgear assemblies, with the listing showing review in 2023. It is the Indian adoption of the corresponding IEC part, so the verification framework is the same. Specifying IS/IEC 61439 Part 1 and Part 2 explicitly, together with the required current rating, Icw and duration, ambient and IP, gives bidders a far clearer basis than the older phrasing "panel shall be type tested".

Does IEC 61439 specify a fixed busbar joint overlap?

No universal overlap dimension is specified. Joint effectiveness is instead reflected in the validated temperature-rise performance of the complete assembly, so a joint is judged by how the assembly behaves thermally rather than by a dimension copied from another project. Use the busbar system's own joint design, hardware and assembly procedure, and keep the joint arrangement consistent with whatever configuration the verification was based on.

Should busbar joints be torque tightened?

Connections should be assembled according to the approved manufacturer/system joint procedure, including specified hardware and tightening requirements. One universal torque value should not be applied to every joint.

Is more copper always better?

Not necessarily. More conductor area can reduce current density, but proper switchboard design also requires suitable geometry, thermal performance, short-circuit withstand, supports and connections.

What data should I provide for busbar sizing?

Provide system voltage, rated current, transformer rating and impedance, calculated prospective fault level, required short-time withstand and its duration, copper or aluminium preference, neutral requirement, maximum ambient temperature, IP rating, altitude, enclosure and ventilation conditions, harmonic and nonlinear-load information, and the applicable project standards. Maximum ambient and IP rating are the two most frequently omitted items, and both directly change the busbar that can be offered. Send the calculated fault current at the panel location rather than the transformer kVA alone, so the manufacturer is not left inferring the short-circuit duty.