IEC 61439 Busbar Sizing for LV Switchboards: How to Calculate Thermal Limits, Short-Circuit Withstand, and Spacing

Why IEC 61439 matters for busbar sizing

IEC 61439 is the core standard for low-voltage switchgear and controlgear assemblies up to 1000 V AC and 1500 V DC, with rated currents up to 6300 A. For busbar design, its value is simple: it turns “looks big enough” into a verified engineering process.

The older IEC 60439 approach relied heavily on legacy type testing. IEC 61439 instead requires design verification by one of three routes:

1. Testing
2. Comparison with a verified reference design
3. Calculation

For busbars, the most important verifications are in Clause 10:

• 10.10 - temperature rise
• 10.9 - short-circuit withstand
• 10.7 - mechanical strength
• 10.11 - dielectric properties and spacing

If you are building an LV switchboard for a plant room, MCC, main distribution board, or SCADA-controlled power center, these are not academic details. They determine whether your assembly survives normal service, fault events, and hot ambient conditions without nuisance trips, insulation damage, or busbar deformation.

Step 1: Start with the actual design basis

Before sizing copper or aluminum, define the assembly conditions:

• Rated operational current, $I_n$
• Prospective short-circuit current at the installation point
• Required short-time withstand current, $I_{cw}$
• Peak withstand current, $I_{pk}$
• Ambient temperature, usually 35°C per IEC 61439-1 Table 6
• Enclosure type and IP rating
• Ventilation, grouping, and installation altitude
• Joint technology, plating, and support spacing

A frequent mistake is to size busbars only for current and forget the fault duty. Another common mistake is to ignore ambient derating. A busbar that passes in a climate-controlled workshop may fail in a 40°C plant room.

Thermal limits and temperature rise verification

Clause 10.10 and IEC 61439-1 Table 6

IEC 61439-1 Table 6 sets permissible temperature rises for parts of the assembly. For busbars and conductors, the typical limits are:

Component	Bare Copper/Aluminium (K)	Bare Silver-Plated Copper (K)	External Electrical Insulation (K)	Terminals for External Conductors (K)
Busbars and conductors	105 / 90	115	70	70
Connections and joints	70	75	-	85
Manual contacts	70	75	-	-
Built-in components	Per manufacturer	-	-	-

The key rule is that the lowest applicable limit governs the connection. For example, if a copper busbar can rise by 105 K but the circuit breaker terminal is limited to 85 K, then the connection must stay at or below 85 K.

At the standard reference ambient of 35°C, a 105 K rise means a maximum conductor temperature of 140°C. That may be acceptable for bare copper, but not necessarily for nearby insulation, cable lugs, or device terminals.

How to estimate current capacity

A practical first-pass sizing relation is:

$$ I_n = A \times J \times K $$

Where:

• $A$ = cross-sectional area in mm²
• $J$ = current density in A/mm²
• $K$ = derating factor for ambient, grouping, enclosure, and installation conditions

For copper busbars in LV assemblies, a typical starting point is 1.0 to 1.6 A/mm², depending on enclosure ventilation and temperature-rise target.

A simplified thermal rise estimate can also be expressed as:

$$ \Delta \Theta \propto \frac{I^2}{A^2} $$

This is not a substitute for a verified IEC calculation or test, but it is useful for early layout decisions.

Worked example: 2000 A copper busbar

Assume:

• Required current: 2000 A
• Ambient: 35°C
• Enclosed board with moderate ventilation
• Copper busbars, single layer, painted or tinned
• Target temperature rise: about 70 K

A common starting arrangement is 2 x 80 x 10 mm copper bars.

That gives:

$$ A_{eff} = 2 \times 80 \times 10 = 1600 \text{ mm}^2 $$

If we use a rough current density of 1.25 A/mm²:

$$ I \approx 1600 \times 1.25 = 2000 \text{ A} $$

This is a credible starting point, but you still need to check:

• Joint temperature rise
• Enclosure derating
• Grouping of phases
• Skin and proximity effects
• Hot spots near bends and terminations

A practical derating chain might look like this:

• IP54 enclosure factor: 0.95
• Grouping factor: 0.90

So the effective rating becomes:

$$ 2000 \times 0.95 \times 0.90 = 1710 \text{ A} $$

That means the design is undersized for a true 2000 A continuous duty, so you would typically step up to a larger section or add another bar.

Typical vendor sizing guidance

Vendors such as ABB, Schneider Electric, and Eaton publish IEC 61439-compliant sizing tables and validated assembly data. A representative guide looks like this:

Size (mm, Single Cu Bar, Painted)	I_n (A) @ 70 K Rise	I_n (A) @ 50 K Rise
20×5	200	170
40×5	400	350
60×10	800	700
80×10	1000	900

Use these as screening values, not final proof. Final compliance still depends on the exact assembly geometry and verification method.

Joint temperature is often the real limit

In field work, joints fail before straight busbar runs. A bolted joint can run 10 to 20 K hotter than the adjacent conductor if contact pressure, plating, or surface prep is poor.

That is why many manufacturers, including Schneider Electric in Blokset and ABB in several LV platforms, verify joints using prototype testing and FEA-based thermal models. For design work, assume the joint is the critical point and specify:

• Clean mating surfaces
• Correct torque
• Spring washers or Belleville washers where approved
• Tin or silver plating where appropriate
• Adequate joint overlap

Short-circuit withstand: thermal and mechanical checks

Clause 10.9 and IEC 60865-1

IEC 61439-1 Clause 10.9 requires verification that the assembly can withstand short-circuit stresses. This includes:

• Thermal withstand for the duration of the fault
• Dynamic withstand from electromagnetic forces

IEC 60865-1:2011 is the main calculation standard for the effects of short-circuit currents on busbar systems.

Thermal short-circuit check

A simplified adiabatic form is:

$$ \Delta \Theta_{sc} = \frac{I_{sc}^2 \times t}{k \times A^2} $$

Where:

• $I_{sc}$ = short-circuit current
• $t$ = fault duration in seconds
• $k$ = material constant
• $A$ = conductor area

For copper, a commonly used constant is around 143 for bare conductors.

Example: 50 kA for 1 second

Assume:

• Busbar: 80 x 10 mm copper
• Area: 800 mm²
• Fault current: 50 kA
• Duration: 1 s

Then:

$$ \Delta \Theta_{sc} = \frac{50{,}000^2 \times 1}{143 \times 800^2} $$

$$ \Delta \Theta_{sc} \approx 55 \text{ K} $$

If the busbar was already operating with a 70 K rise at steady state, the post-fault temperature can be estimated as:

$$ 35 + 70 + 55 = 160^\circ \text{C} $$

That is below a typical copper damage threshold of around 200°C for bare copper, so the thermal check passes in this simplified example.

Dynamic force check

Short-circuit current creates very large electrodynamic forces between parallel conductors. A simplified force relation is:

$$ F_m \propto \frac{I_{pk}^2 \times L}{d} $$

Where:

• $I_{pk}$ = peak short-circuit current
• $L$ = unsupported length
• $d$ = spacing between bars

The practical implications are straightforward:

• Larger fault current means much higher force
• Longer unsupported spans mean more bending
• Smaller spacing means stronger attraction or repulsion depending on phase arrangement

For busbar support systems, the mechanical design must keep deflection within acceptable limits and prevent insulator failure.

Example support strategy

For a board rated at 50 kA / 1 s, a reasonable arrangement might be:

• 80 x 10 mm copper bars
• Supports every 600 mm
• Epoxy or cast-resin insulators
• Verified support load rating above the calculated force

Vendor systems such as ABB Prisma, Schneider Okken, and Rittal Ri4Power typically publish support spacing guidance and short-circuit ratings. Use those data where available, but ensure the exact bar geometry matches the tested configuration.

Busbar spacing and mechanical design

Clause 10.7 and Clause 10.11

IEC 61439-1 Clause 10.7 covers mechanical strength, while Clause 10.11 covers dielectric properties. In practice, this means you need to check:

• Bar-to-bar spacing
• Bar-to-earth spacing
• Creepage and clearance
• Deflection under fault forces
• Clearance after thermal expansion

A common rule of thumb from IEC 60865-1-based design practice is:

Configuration	Min Spacing (mm) @ 1000 V
Phase-phase	20-30
Phase-earth	10-20
Trefoil 3-phase center spacing	about 40

These are starting values, not universal approvals. Pollution degree, humidity, altitude, and enclosure design can all push the required spacing higher.

Why spacing matters more than many teams think

Spacing affects all three major design risks:

1. Thermal performance - tighter spacing reduces air circulation
2. Dielectric performance - lower clearance raises flashover risk
3. Mechanical performance - smaller gaps can increase fault-force interaction

For dusty or contaminated environments, increase spacing and consider the impact of IEC 61439 environmental conditions plus the installation’s pollution level. If the assembly is in a harsh industrial area, a conservative 20% spacing margin is often justified.

Support materials

Typical support choices include:

• Porcelain or glass supports for lower-current applications
• Cast-resin or epoxy supports for higher fault duty
• Manufacturer-approved spacers and phase barriers

As a practical threshold, many designers move to robust epoxy supports above 800 A or whenever the short-circuit duty is substantial. The support spacing is often between 0.6 m and 1.2 m, depending on bar size and fault level.

A practical engineering workflow

Here is a reliable workflow for IEC 61439 busbar design:

1. Define the duty
   • $I_n$
   • $I_{cw}$
   • $I_{pk}$
   • ambient temperature
   • enclosure type
2. Select a preliminary busbar size
   • Use current density as a first pass
   • Include ambient and grouping derating
3. Verify thermal rise
   • Use manufacturer tables, calculation tools, or tested reference designs
4. Check short-circuit withstand
   • Use IEC 60909 for fault current estimation
   • Verify thermal and mechanical effects per IEC 61439 and IEC 60865-1
5. Check spacing and support
   • Confirm clearances, creepage, and support intervals
6. Prototype test if needed
   • Especially for high current, custom geometry, or nonstandard enclosures
7. Document the design verification file
   • This is essential for CE marking and technical compliance

Example comparison of common LV platforms

Different vendors solve the same IEC 61439 problem in slightly different ways.

Vendor / Family	Typical Strength	Notes
ABB UniGear ZS1	High short-circuit performance	Often used for demanding distribution and MCC applications
Schneider Electric Blokset	Modular LV switchboards	Strong documentation and validated busbar systems
Eaton xEnergy	Flexible Cu/Al options	Useful for configurable distribution assemblies
Siemens Sivacon S8 / S8x	Large-scale LV distribution	Strong integration with Siemens ecosystem
Rittal Ri4Power	Enclosure-based modularity	Good for standardized panel builds

For automation-heavy facilities, the busbar design often sits alongside PLC and SCADA architecture. A plant may use Siemens S7, Rockwell ControlLogix, Beckhoff TwinCAT, Ignition, AVEVA System Platform, or COPA-DATA zenon for control and monitoring, but the switchboard still has to satisfy the same IEC 61439 electrical and thermal rules. The control platform does not reduce the fault duty on the copper.

Common mistakes to avoid

• Sizing only for continuous current and ignoring short-circuit duty
• Assuming a vendor table applies to a different enclosure geometry
• Ignoring joint temperature rise
• Using too little support spacing for the fault level
• Forgetting ambient derating above 35°C
• Mixing tested and untested components without re-verification
• Treating aluminum like copper with the same cross-section

For North American projects, cross-check IEC work against NFPA 70, NFPA 70E, NEC 408.36, and where applicable UL 891. IEC remains the primary reference for global LV assembly design, especially in CE-marked projects under EN/IEC frameworks.

Final design takeaways

If you want a busbar system that survives real service, not just CAD review, focus on three things:

• Thermal rise under normal load
• Short-circuit withstand under fault conditions
• Spacing and support under both thermal expansion and electrodynamic forces

In practice, the safest approach is to start with vendor-verified busbar systems, confirm the exact geometry against IEC 61439-1 Clause 10, and only then optimize for cost or footprint. That is especially true in high-availability industrial plants, where a busbar failure can stop production, damage downstream drives such as ABB ACS or Siemens Sinamics, and create a long outage across the SCADA-connected power network.

If you need help checking an IEC 61439 busbar design, validating a switchboard layout, or reviewing a panel against thermal and short-circuit limits, Powerfabric can help you move from assumptions to a compliant build - start the conversation here: /contact