Size the copper and aluminium busbars that distribute heavy currents inside switchgear and distribution panels. Adjust the current and the bar width and thickness to see the I²R heat and steady-state temperature rise update in real time, and pick a cross-section that stays within its allowed temperature.
Parameters
Current I
A
Steady current carried by the busbar
Conductor material
Sets the operating-temperature resistivity ρ_T
Bar width w
mm
Bar thickness t
mm
Allowed temperature rise ΔT_limit
K
Allowed temperature rise of the bar above ambient
Installation
Sets the cooling condition (effective heat-transfer coefficient)
Results
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Cross-section (mm²)
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Current density (A/mm²)
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Resistance (µΩ/m)
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Heat (W/m)
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Est. temperature rise (K)
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Sizing verdict
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Busbar cross-section — heat generation and dissipation
The central rectangle is the busbar cross-section. Colour shows the temperature rise (blue/green when lightly loaded, orange/red as it nears the limit). Arrows on all four faces are the heat flux leaving the surface; the thermometer on the right reads the temperature rise.
Temperature rise vs current I
Temperature rise vs bar width w
Theory & Key Formulas
$$\dot q = I^{2}\,R',\qquad \Delta T = \frac{\dot q}{h\,A_{surf}}$$
Heat generated per metre $\dot q$ (I: current, R': resistance per metre) and the steady-state temperature rise ΔT (h: effective heat-transfer coefficient, A_surf: surface area per metre). The busbar is sized so that, at the temperature-rise limit, the heat shed from its surface equals the I²R heat generated.
$$R' = \frac{\rho_T}{A},\qquad A = w\,t,\qquad A_{surf} = 2\,(w+t)$$
Resistance per metre R' (ρ_T: operating-temperature resistivity, A: cross-section), cross-section A and surface area per metre A_surf (w: bar width, t: bar thickness). A flat bar has a high surface-to-cross-section ratio and dissipates heat well.
What is the Busbar Sizing Simulator?
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A "busbar" is one of those flat copper plates inside a distribution panel, right? Is it just a substitute for a thick cable?
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Right — it is the "bus" that distributes heavy currents inside distribution panels, switchgear and substations. Carrying hundreds or thousands of amperes with round cable would be bulky and hard to route, so we use rigid rectangular strips of copper or aluminium instead. The interesting part is that busbar size is set not by strength but by heat. When current flows through resistance you get I²R Joule heat, and you choose the cross-section so that heat does not push the temperature up too far.
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It's set by heat! So bigger is always safer? When I raise the "bar width" on the left, the temperature rise keeps dropping.
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Good catch. Widening the bar increases the cross-section, so the resistance falls and the I²R heat drops; at the same time the surface area grows so heat escapes more easily. That is why a wider bar runs much cooler. But if "just make it bigger" were the whole story, sizing would be trivial. Copper and aluminium are expensive metals, so an oversized bar wastes material. The skill is finding the smallest cross-section that still stays within the allowed temperature rise.
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I see. Still, busbars are always thin and flat. Why not a round rod?
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That is the heart of busbar design. Heat generation is fixed by the cross-section — the current density — but heat dissipation is fixed by the surface area. For the same cross-section, a thin, wide flat bar has far more surface than a round or square one. The higher the surface-to-cross-section ratio, the more efficiently it sheds heat, so busbars are deliberately made as thin, wide strips. The "temperature rise vs bar width" chart below shows that falling curve as you widen the bar.
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You can also pick copper or aluminium. How do you choose between them?
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Copper has lower resistivity, so for the same cross-section it produces less heat and carries more current than aluminium. But aluminium is lighter and cheaper, so it is often chosen for long high-current runs and cost-sensitive boards. The catch is that aluminium needs about 1.5-1.6 times the cross-section for the same current capacity, and joint oxidation matters more. Switch the material selector and you will see the resistance and temperature rise change clearly.
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And if the cross-section is too small and the temperature goes over the limit, what happens?
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The scariest spot is the bolted joints. When the bar runs hot the contact faces oxidise and creep, so the contact resistance grows. Then that spot generates even more heat, gets hotter still — a vicious circle, a thermal runaway. It can end in burnout or fire. So with this tool, aim to keep the temperature rise at about 85% of the limit so the verdict reads "comfortable margin". If it reports "undersized", that is your cue to add width or thickness, or switch to copper.
Frequently Asked Questions
When current I flows through a busbar, the conductor resistance R produces I²R heat (Joule heat). That heat is shed from the bar surface by convection and radiation, but if the cross-section is small the heat cannot escape fast enough and the temperature keeps rising. The correct size is the one at which, when the bar reaches its allowed temperature rise (about 40-65 K for an open bar), the heat shed exactly balances the heat generated. Busbar sizing is therefore a thermal problem, not a mechanical one, and this tool compares the estimated temperature rise with the allowed limit to give a sizing verdict.
For the same cross-sectional area, a flat bar has a much larger surface area than a round or square conductor. Heat generation is fixed by the cross-section (current density), while heat dissipation is proportional to surface area, so a flat bar with a high surface-to-cross-section ratio sheds heat far more effectively. In this tool the perimeter 2(width+thickness) sets the surface area, and the temperature-rise versus bar-width chart shows how a wider bar runs cooler. That is why switchgear busbars are thin, wide strips.
This tool uses an operating resistivity of 2.0e-8 ohm-m for copper and 3.2e-8 ohm-m for aluminium. For the same cross-section, copper has lower resistivity, less heat and a higher current rating. Aluminium is lighter and cheaper, so it is chosen for long high-current runs and cost-sensitive boards. When using aluminium you typically need to increase the cross-section by about 1.5-1.6 times for the same current capacity, and you must take care of oxidation at bolted joints.
If the cross-section is too small and the temperature rise exceeds the allowed limit, the bar runs hot and oxidises. The bolted joints are the critical point: as temperature rises the contact faces oxidise and creep, the contact resistance grows, and that spot generates even more heat — a thermal runaway loop that can end in burnout or fire. An oversized bar, on the other hand, wastes expensive copper and aluminium. Keep the temperature rise below about 85% of the allowed limit and select a sensible size with this tool.
Real-World Applications
Low-voltage distribution and panel boards: Inside factory and building low-voltage switchboards, motor control centres (MCCs) and distribution boards, busbars carry power from the main breaker to each branch circuit. The copper bar cross-section is chosen for the panel rating (for example 1000 A, 2000 A or 4000 A), and the temperature rise is checked against the limit as in this tool. Panel makers verify the real rise with temperature-rise type tests (IEC 61439 and similar).
Substations and receiving equipment: In high-voltage and extra-high-voltage receiving and switching equipment, large-section busbars connect transformers, circuit breakers and disconnectors. Outdoor busbars benefit from convective cooling by the wind, so their current rating is higher than indoor bars. The "installation" selector in this tool represents exactly this difference in effective heat-transfer coefficient.
Data centres and electric vehicles: Busbars are central in modern high-current applications too. Data-centre power-distribution units (busways), cell-to-cell connections inside EV battery packs and the internal wiring of fast chargers all use copper and aluminium bars to carry heavy currents compactly. Where weight matters, as in EVs, aluminium bars and a careful copper/aluminium split are increasingly common.
Design verification and troubleshooting: When a panel runs unusually hot or a breaker trips frequently, the cause is often an undersized busbar or a poor joint. A quick estimate like this tool checks the temperature-rise level and helps decide whether the cross-section needs revising or the bolt torque rechecking. Detailed work uses thermal CAE that includes current distribution and proximity effects.
Common Misconceptions and Pitfalls
The most common mistake is fixing the size from a single current-density limit. Rules of thumb like "copper up to 2 A/mm²" are convenient, but the allowable current density depends strongly on installation, bar dimensions, allowed temperature rise and whether bars sit next to each other. For the same cross-section, a thin, wide bar has more surface and tolerates a higher current density, while a stubby section overheats at the same density. Current density is a result, not an input — the right test is whether the temperature rise stays within the limit, exactly as this tool does.
Next, assuming a copper bar can simply be swapped for aluminium. Aluminium's resistivity is about 1.6 times that of copper, so the same cross-section produces about 1.6 times the heat. To keep the same current capacity you must enlarge the cross-section by 1.5-1.6 times, which changes the panel space and joint design. Aluminium also forms an oxide film readily, so if you neglect contact-resistance management at bolted joints (proper surface preparation, Belleville washers, specified torque), those joints become the starting point of local overheating.
Finally, the trap of looking only at steady current. This tool handles the steady-state temperature rise, but a real busbar also faces short-circuit current. During a fault, tens of times the rated current flows for a short time, imposing both a thermal stress (I²R heat rapidly heating the bar) and a huge electromagnetic force between parallel bars (a mechanical stress). A complete busbar design must, on top of the steady temperature rise, check the short-circuit thermal capacity and the support spacing against short-circuit electromagnetic forces. Use this tool for first-pass steady-state thermal sizing.
How to Use
Enter operating current (A) using currentNum or currentRange slider—typical range 100–6000 A for switchgear applications.
Set busbar dimensions: barWidthNum (10–200 mm) and barThickNum (5–50 mm) to define cross-sectional area.
Input allowable temperature rise limit (K) via tempRiseLimitNum; industrial standard is 30–50 K for copper, 40–60 K for aluminium.
Simulator calculates cross-section (mm²), current density (A/mm²), resistance per meter, heat dissipation (W/m), and compares against your temperature rise budget.
Adjust dimensions until the sizing verdict confirms the busbar meets IEC 61439-1 or NFPA 70 compliance.
Worked Example
400 A three-phase distribution in a medium-voltage switchgear. Select copper busbar: width 50 mm, thickness 10 mm (cross-section = 500 mm²). Current density = 400 ÷ 500 = 0.8 A/mm². Copper resistance ≈ 0.0336 µΩ/m. Heat generated = 0.8² × 0.0336 = 0.0215 W/m. With natural convection (≈1.2 W/m·K), temperature rise ≈ 0.0215 ÷ 1.2 = 0.018 K—well below 50 K limit. Verdict: acceptable sizing.
Practical Notes
Aluminium busbars (resistivity 0.0283 µΩ/m) require 1.6× larger cross-section than copper for identical current and temperature rise; use when weight is critical (>500 A systems).
Current density above 2.5 A/mm² in confined panels risks exceeding 50 K rise without active cooling; increase width or thickness proportionally.
Account for skin effect at 50/60 Hz frequencies in round conductors; rectangular busbars minimize this, improving effective current capacity by 3–5%.
Temperature rise compounds with poor ventilation and stacked busbars in compartments; reduce current density by 15–20% in poorly ventilated enclosures.