What is the maximum current density for copper busbars?

The maximum allowable current density for copper busbars depends on the installation conditions. For busbars inside enclosed switchgear panels with limited ventilation, the typical maximum is 1.2 A/mm². For busbars in ventilated enclosures, the limit increases to approximately 1.6 A/mm². For busbars in free air with unrestricted convection, up to 2.0 A/mm² is acceptable. These values ensure the busbar temperature rise remains within the limits set by BS EN 61439-1 (70K above ambient for bare busbars). Higher current densities cause more resistive heating and can exceed the temperature limit, degrading insulation and increasing connection resistance at joints.

Should I use copper or aluminium busbars?

Copper is the standard choice for busbars within distribution boards and switchgear assemblies. It has higher conductivity (allowing smaller cross-sections), higher mechanical strength, and excellent corrosion resistance. Aluminium is preferred for long busbar trunking runs (rising mains, horizontal distribution in large buildings) where the weight saving is significant — aluminium is only one-third the weight of copper for a given volume. Aluminium is also less expensive per unit length, which matters for long runs. The main disadvantage of aluminium is that it forms an oxide layer on exposed surfaces that increases contact resistance — this is managed by using plated contact surfaces (tin or silver) at all connection points. In practice, most panel busbars are copper and most busbar trunking systems offer both copper and aluminium options.

How do I calculate busbar short-circuit withstand?

The thermal withstand of a busbar under short-circuit conditions is checked using the adiabatic equation from BS 7671: minimum cross-sectional area = root(I²t) / k, where I is the prospective short-circuit current in amperes, t is the disconnection time of the protective device in seconds, and k is the material factor from BS 7671 Table 54.6 for bare conductors (copper: 159 under normal conditions; aluminium: 105). The calculated minimum area must be less than the actual busbar cross-section. For example, with a fault current of 25kA and a disconnection time of 0.1 seconds using a bare copper busbar under normal conditions: minimum area = root(25000² x 0.1) / 159 = 7906 / 159 = 49.7 mm². Any copper busbar with a cross-section greater than 49.7 mm² will withstand this fault. For type-tested assemblies, the manufacturer verified withstand rating to BS EN 61439 takes precedence. The mechanical forces also need checking for high fault levels.

What is busbar trunking and when is it used?

Busbar trunking (also called busway) is a pre-fabricated system of enclosed busbars used for power distribution in commercial and industrial buildings. It consists of aluminium or copper busbars enclosed in a sheet metal housing, with standardised tap-off points at regular intervals where distribution boards or individual circuits can be connected. Busbar trunking is used instead of large cables for high-current distribution (typically 250A to 5000A). It is commonly used for rising mains in multi-storey buildings, horizontal power distribution in large open-plan buildings, and factory floor distribution where flexible tap-off points are needed. Busbar trunking is faster to install than equivalent cable systems, takes up less space, and allows easy modification by adding or relocating tap-off points.

How does busbar orientation affect current rating?

Busbar orientation significantly affects heat dissipation and therefore current rating. A vertically mounted flat busbar dissipates heat more effectively than a horizontally mounted one because vertical orientation promotes natural convection — hot air rises past the busbar surfaces, creating an air flow that carries heat away. A horizontal busbar oriented on its edge (narrow face up) is better than flat (wide face up) because the edge orientation allows convection on both faces. For multiple busbars per phase, spacing them apart (typically one busbar thickness) allows air to circulate between them, further improving heat dissipation. The Elec-Mate calculator accounts for busbar orientation when calculating temperature rise.

What voltage drop limit applies to busbars under BS 7671?

BS 7671 requires that voltage drop from the origin of the installation to any socket-outlet or fixed equipment terminal shall not exceed 3% for lighting and 5% for other circuits (Reg 525.202, referencing Appendix 4, Section 6.4). For a busbar trunking run feeding distribution boards, the voltage drop in the trunking itself plus the drop in the final circuits must remain within these limits. Busbars have very low resistance per unit length compared to cables, so voltage drop is rarely the critical constraint — thermal rating and short-circuit withstand are usually the governing factors. However, for very long busbar runs (over 50 metres) supplying sensitive equipment, voltage drop should be calculated and verified. The Elec-Mate busbar sizing calculator includes voltage drop calculation as part of its output.

Busbar Calculator

Busbar Sizing Calculator — Current Rating and Cross-Section Tool

Enter the required current rating, busbar material, and installation conditions. The calculator determines the correct busbar dimensions, verifies temperature rise, calculates voltage drop, and checks short-circuit withstand capacity. Size busbars with confidence.

Current RatingCross-SectionTemperature RiseShort-Circuit Check

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Key Takeaways

1Busbar current rating is determined by the cross-sectional area and the maximum allowable current density: typically 1.2 A/mm² for copper busbars in enclosed panels and up to 2.0 A/mm² for busbars in free air.
2Copper busbars have approximately 60% higher current carrying capacity than equivalent aluminium busbars, but aluminium is lighter and less expensive per unit length.
3Temperature rise is the governing design factor — BS EN 61439-1 limits the temperature rise of busbars in switchgear assemblies to 70K above ambient (105°C total at 35°C ambient).
4Short-circuit withstand must be verified using the adiabatic equation: minimum CSA = root(I²t) / k, with k from BS 7671 Table 54.6 for bare conductors — 159 for copper and 105 for aluminium under normal conditions.
5Elec-Mate calculates busbar sizing for copper and aluminium, with temperature rise, voltage drop, and short-circuit verification all built in.

What Is Busbar Sizing?

Busbar sizing is the process of selecting the correct cross-sectional dimensions for a conductor bar (busbar) that carries electrical current within switchgear assemblies, distribution boards, busbar trunking systems, and power distribution infrastructure. Busbars are used instead of cables where high currents need to be distributed efficiently within a compact space.

The sizing process must satisfy four independent requirements:

Current carrying capacity — the busbar must carry the full load current without exceeding the maximum allowable temperature.
Temperature rise — the busbar temperature must not exceed the limits set by the enclosure standard (BS EN 61439) under sustained load conditions.
Voltage drop — the resistive voltage drop along the busbar length must be within acceptable limits, particularly for long busbar trunking runs.
Short-circuit withstand — the busbar must withstand the thermal and mechanical effects of the prospective short-circuit current for the time taken by the protective device to clear the fault.

In practice, either the temperature rise or the short-circuit withstand usually governs the busbar size. The Elec-Mate busbar sizing calculator checks all four criteria simultaneously and highlights which one is the governing factor.

Copper vs Aluminium Busbars

Busbars are manufactured from either copper (Cu) or aluminium (Al). Each material has distinct properties that affect the busbar sizing and the overall design of the switchgear assembly.

Copper Busbars

Resistivity: 17.2 n-ohm-m at 20°C
Density: 8,900 kg/m³
Current density: 1.2-2.0 A/mm²
k factor (adiabatic): 159 bare, normal conditions (Table 54.6)
Higher current capacity per mm²
More compact installation

Aluminium Busbars

Resistivity: 28.3 n-ohm-m at 20°C
Density: 2,700 kg/m³
Current density: 0.8-1.2 A/mm²
k factor (adiabatic): 105 bare, normal conditions (Table 54.6)
67% lighter than copper
Lower material cost per metre

Copper is the standard choice for most switchgear and distribution board busbars due to its higher conductivity and mechanical strength. Aluminium is preferred for long busbar trunking runs where weight is a concern, and for high-current applications where the lower material cost offsets the need for a larger cross-section. Many modern busbar trunking systems use aluminium conductors with copper-plated or tin-plated contact surfaces to prevent oxide build-up at connections.

Current Density Method

The current density method is the most common approach to busbar sizing. The required cross-sectional area is calculated by dividing the design current by the allowable current density for the busbar material and installation conditions.

A = I / J

A = required cross-sectional area in mm²

I = design current in amperes

J = allowable current density in A/mm²

Typical Current Density Values

Copper, enclosed panel: 1.2 A/mm² — the most conservative value, used for busbars inside enclosed switchgear with limited ventilation.
Copper, ventilated enclosure: 1.6 A/mm² — for busbars inside ventilated panels or trunking with adequate airflow.
Copper, free air: 2.0 A/mm² — for exposed busbars with unrestricted airflow, such as open busbar systems in substations.
Aluminium, enclosed panel: 0.8 A/mm² — lower than copper due to the higher resistivity of aluminium.
Aluminium, free air: 1.2 A/mm² — equivalent to copper in an enclosed panel.

For example, a copper busbar in an enclosed panel carrying 800A would require a minimum cross-section of 800 / 1.2 = 667 mm². A standard busbar size of 50mm x 15mm (750 mm²) would be selected.

Temperature Rise

Temperature rise is typically the governing factor in busbar sizing. As current flows through the busbar, resistive heating (I²R losses) causes the busbar temperature to rise above ambient. The temperature must not exceed the limits set by the enclosure standard.

BS EN 61439-1 Temperature Limits

Bare copper/aluminium busbars: maximum temperature rise of 70K above ambient (maximum 105°C at 35°C ambient).
Insulated busbars: maximum temperature depends on the insulation class — typically 90°C or 105°C for common insulation types.
Connection points: bolted connections must not exceed 105°C for bare connections or 90°C for insulated connections (due to the higher resistance at joints).

The temperature rise depends on the I²R losses, the surface area of the busbar (which determines heat dissipation), the orientation of the busbar (vertical busbars dissipate heat better than horizontal due to convection), the proximity of other heat sources, and the enclosure ventilation. Multiple busbars per phase (stacked or spaced) increase the surface area and improve current capacity per unit of conductor material.

The Elec-Mate busbar sizing calculator computes the temperature rise based on the busbar dimensions, material properties, and installation configuration, alerting you if the temperature limit is exceeded.

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Voltage Drop in Busbars

Voltage drop in busbars is calculated using the same fundamental principle as cable voltage drop — the current flowing through the resistance of the conductor produces a voltage drop proportional to the current and the conductor length.

Vd = I x R x L

Vd = voltage drop in volts

I = current in amperes

R = resistance per metre (ohm/m), calculated from resistivity and cross-sectional area

L = busbar length in metres

For short busbars within a distribution board (typically less than 1 metre), the voltage drop is negligible and rarely a concern. For long busbar trunking runs (which can extend tens of metres through a building), the voltage drop must be calculated and verified against the BS 7671 voltage drop limits of 3% for lighting and 5% for other circuits (Reg 525.202 and Appendix 4, Section 6.4).

Regulation 525.202 requires that the voltage drop between the origin of the installation and any socket-outlet or the terminals of fixed current-using equipment shall not exceed the values stated in BS 7671 Appendix 4, Section 6.4 — 3% for lighting circuits and 5% for other circuits.

Note that busbar resistance increases with temperature — the resistance at operating temperature is higher than at 20°C. The calculator uses the resistance at the expected operating temperature for an accurate voltage drop calculation.

Short-Circuit Withstand

Busbars must be able to withstand the thermal and mechanical effects of a short circuit for the time it takes the protective device to clear the fault. The thermal withstand is checked using the adiabatic equation, and the mechanical withstand is checked by calculating the electromagnetic forces between the busbars during the fault.

Thermal Withstand Check

A_min = root(I²t) / k

A_min = minimum cross-sectional area in mm²
I²t = let-through energy of the protective device in A²s
k = material factor from BS 7671 Table 54.6 for bare conductors — 159 for copper, 105 for aluminium under normal conditions (see note on k values below)

Important — 5 s limit (Reg 543.1.3): The adiabatic equation is only valid for disconnection times not exceeding 5 s. Where a fuse or MCCB has an operating time above 5 s, the equation does not apply — consult BS 7454 or use a type-test verified device with a confirmed withstand rating.

k Values — Temperature and Insulation Class

k is a factor that accounts for the resistivity, temperature coefficient, and heat capacity of the conductor material at the appropriate initial and final temperatures (Reg 543.1.3). The value of k therefore varies depending on the conductor material and the initial and final temperatures assumed.

Bare copper (Table 54.6): k = 228 where the busbar is visible and in restricted areas, 159 under normal conditions, 138 where there is a fire risk. The condition determines the permissible final temperature, so check which column applies to your assembly.
Bare aluminium (Table 54.6): k = 125 visible and in restricted areas, 105 under normal conditions, 91 where there is a fire risk.
Type-tested assemblies: for busbars inside a switchgear or busbar trunking assembly, the manufacturer's verified short-circuit withstand rating to BS EN 61439 takes precedence over a generic adiabatic check — use the manufacturer's data where it exists.

The mechanical forces during a short circuit can be enormous. The force between two parallel busbars carrying fault current is proportional to the square of the current and inversely proportional to the distance between them. For high fault levels (above 25kA), the busbar support structures must be designed to withstand these forces — this may require closer spacing of busbar supports, stronger fixings, or additional bracing.

The prospective fault current calculator can determine the fault level at the point where the busbar is installed, which is the input needed for the short-circuit withstand check.

Busbar Trunking Enclosure as CPC (Reg 543.2.2)

Where the metal enclosure of a busbar trunking system is used as the circuit protective conductor (CPC), Reg 543.2.2 requires that its cross-sectional area is at least equal to that determined by Reg 543.1 (the adiabatic calculation above). The enclosure continuity must also be assured — whether by construction or by suitable connections — and must be protected against mechanical, chemical, and electrochemical deterioration. This is a frequently overlooked design requirement for rising-mains trunking: using the trunking enclosure as the CPC is only permissible if the metalwork satisfies the adiabatic area requirement and its continuity is maintained throughout the run.

Busbar sizing with fault current verification

Elec-Mate's busbar sizing calculator checks current density, temperature rise, voltage drop, and short-circuit withstand in one calculation. Enter the design current and fault level, select copper or aluminium, and get the correct busbar dimensions. Works offline on site.

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Worked Examples

Example 1: Main Distribution Board Busbar

A main distribution board requires a 630A copper busbar in an enclosed panel. The prospective short-circuit current is 25kA with a disconnection time of 0.1 seconds.

Current density method: A = 630 / 1.2 = 525 mm²

Short-circuit check (k = 159, bare copper, normal conditions): A = root(25000² x 0.1) / 159 = 7906 / 159 = 49.7 mm²

The current density requirement governs (525 mm² vs 44.9 mm²). Select a standard busbar: 40mm x 15mm (600 mm²) or 50mm x 12mm (600 mm²).

Example 2: Busbar Trunking Run

A 1600A aluminium busbar trunking run is 30 metres long, feeding a three-phase distribution board. Calculate the voltage drop.

Required CSA: 1600 / 0.8 = 2000 mm² per phase

Resistance at 70°C: (28.3 x 10⁻⁶ x 1.20) / (2000 x 10⁻⁶) = 0.0170 ohm/m

Vd = 1600 x 0.0170 x 30 / 1000 = 0.82V per phase

As a percentage of 230V: 0.82 / 230 x 100 = 0.36% — well within the 5% limit.

How to Size a Busbar

Follow these six steps to select the correct busbar dimensions for any distribution board, switchgear assembly, or busbar trunking installation.

Determine the design current

Calculate the maximum continuous current the busbar must carry. This is the diversified maximum demand of all circuits fed from the busbar, taking into account load diversity and future growth.

Select the busbar material

Choose copper or aluminium based on the application. Copper is standard for switchgear busbars. Aluminium is used for long busbar trunking runs where weight and cost are factors.

Calculate the cross-sectional area

Divide the design current by the allowable current density for the material and installation type. Select the next standard busbar size above the calculated area.

Verify temperature rise

Calculate the I²R heating and verify the busbar temperature does not exceed the BS EN 61439-1 limit of 105°C (70K rise above 35°C ambient) under sustained full load.

Check voltage drop

For busbar trunking runs, calculate the voltage drop along the busbar length. Verify it is within the BS 7671 limits of 3% for lighting and 5% for other circuits.

Verify short-circuit withstand

Check that the busbar cross-section can withstand the thermal effects of the prospective short-circuit current using the adiabatic equation. Verify the busbar supports can withstand the electromagnetic forces.

Busbar Sizing Calculator Features

Everything you need to size busbars correctly for distribution boards, switchgear, and busbar trunking systems.

Instant Sizing Calculation

Enter the design current and installation conditions. The calculator determines the minimum busbar cross-section and selects the nearest standard busbar…

Copper and Aluminium

Full support for both copper and aluminium busbars with accurate material properties. Compare the two materials side by side for any given current rating.

Temperature Rise Verification

Calculates the busbar temperature under sustained load and verifies compliance with BS EN 61439-1 temperature limits. Flags busbars that will overheat.

Voltage Drop Calculation

Calculates the resistive voltage drop along the busbar length, accounting for temperature-dependent resistance. Essential for long busbar trunking runs.

Short-Circuit Withstand Check

Verifies the busbar can withstand the thermal effects of the prospective fault current using the adiabatic equation with correct k factors for copper and…

Standard Sizes Database

Built-in database of standard busbar dimensions. The calculator selects the nearest standard size above the calculated requirement, eliminating guesswork.

Frequently Asked Questions About Busbar Sizing

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Busbar Sizing Calculator — Current Rating and Cross-Section Tool

Key Takeaways

What Is Busbar Sizing?

Copper vs Aluminium Busbars

Copper Busbars

Aluminium Busbars

Current Density Method

Typical Current Density Values

Temperature Rise

BS EN 61439-1 Temperature Limits

The calculators live where your certs and quotes are

Voltage Drop in Busbars

Short-Circuit Withstand

Thermal Withstand Check

k Values — Temperature and Insulation Class

Busbar Trunking Enclosure as CPC (Reg 543.2.2)

Busbar sizing with fault current verification

Worked Examples

Example 1: Main Distribution Board Busbar

Example 2: Busbar Trunking Run

How to Size a Busbar

Determine the design current

Select the busbar material

Calculate the cross-sectional area

Verify temperature rise

Check voltage drop

Verify short-circuit withstand

Busbar Sizing Calculator Features

Instant Sizing Calculation

Copper and Aluminium

Temperature Rise Verification

Voltage Drop Calculation

Short-Circuit Withstand Check

Standard Sizes Database

Frequently Asked Questions About Busbar Sizing

What is the maximum current density for copper busbars?

Should I use copper or aluminium busbars?

How do I calculate busbar short-circuit withstand?

What is busbar trunking and when is it used?

How does busbar orientation affect current rating?

What voltage drop limit applies to busbars under BS 7671?

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Three-Phase Installation Guide

What electricians say

One App for Everything!

Fantastic app for electricians

Absolutely amazing

Trusted by electricians across the UK

Size Busbars in Seconds on Your Phone