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How to Size a Laminated Busbar – Cross-Section and Ampacity

10/14

2026

By A&JLINK Engineering Team · Last updated October 2026 · 7 min read
The short answer
Sizing a busbar is a short sequence — from current to cross-section, then to heat, drop, and clearance.
  • ✓Start from current. Continuous current sizes the conductor; peak current sets the short-time withstand.
  • ✓Temperature rise is the real limit. Size for the permitted rise, then confirm the cross-section.
  • ✓Then verify. Resistance, voltage drop, and insulation clearance finish the job.
A laminated busbar is a custom part, and its most consequential dimension is the conductor cross-section. Choose it too small and the busbar runs hot, ages its insulation early, or drops too much voltage; choose it too large and you pay in material, mass, and space. Getting from "my circuit carries X amps" to "my conductor is Y mm²" is not guesswork — it is a short, repeatable calculation.
Scope: this article covers how to size a laminated busbar — from current to cross-section, temperature rise, resistance, and insulation clearance.

1. Why Sizing Comes First

Cross-section drives almost everything downstream. It sets the temperature rise (and therefore the insulation you can use and how long the part lasts), the voltage drop (and therefore the efficiency), and the mass and cost of the conductor. Because a laminated busbar is built to order, the section is a design decision, not a catalog pick — which is why sizing is the first thing to settle, before the layout is frozen.
Get it right and the rest of the design follows; get it wrong and every later decision inherits the problem. That is why knowing how to size a laminated busbar properly — not just quickly — pays off across the whole project.

2. Start from Current — Continuous and Peak

Sizing begins with two numbers, and they are not the same.
Continuous current is what heats the conductor. It is the steady-state current the busbar will carry for long periods, and it is the number that drives the cross-section.
Peak current is what stresses it briefly — inrush, fault, or a short overload. It does not set the steady-state section, but it sets the short-time withstand the design must survive, and sometimes a minimum section to avoid overheating during the pulse. For a deeper look at how much current a busbar can carry, see our guide on busbar current capacity.
A common sizing error is to size for the average current and ignore the duty cycle — the conductor sees the RMS of the actual waveform, not its arithmetic mean.

3. From Ampacity to Cross-Section (and Temperature Rise)

The link between current and area is the current density, J, measured in amps per square millimetre:
A = I / J
The whole difficulty is that J is not a fixed number. It depends on how the busbar is cooled and how much temperature rise the design allows. A naturally cooled busbar has to reject heat by convection and radiation from its surface, so it is designed at a low current density; forced-air or liquid cooling removes heat faster, so the same section can carry more.
That is why temperature rise is the real limit, not current alone. The conductor's steady-state temperature is ambient plus the rise from I²R heating, and it must stay below what the insulation class can survive over the service life. Two busbars of the same section can therefore have different ratings depending on orientation, enclosure, and airflow. How the rise behaves across a real installation is covered in our guide on busbar temperature performance.
The practical rule: size for the permitted temperature rise, then confirm the cross-section that achieves it — not the other way around. In a laminated busbar, the layers of insulation and the bonded structure also change how heat leaves the part, so the same copper section can behave differently depending on how it is built.

4. Material Changes the Section

The same current does not produce the same section in copper and aluminum. Copper conducts better, so an aluminum busbar needs roughly 1.6 times the cross-section for the same resistance. That single factor flows straight into sizing: an aluminum design will be bulkier but lighter, which is exactly the trade-off that makes aluminum attractive in weight-sensitive products.
So the sizing step is material-dependent. Choose the material first, then size — or size both and compare, since the section determines mass and cost as much as it determines temperature.

5. Check Resistance and Voltage Drop

Once the section is set, confirm the electrical result. Resistance follows the familiar relationship:
R = ρ · L / A
where ρ is the material's resistivity, L the conductor length, and A the cross-section. Multiplying by current gives the voltage drop, which has to stay within what the circuit can tolerate. For a high-current, short busbar the drop is usually small; over a long run or at low voltage (a 48 V or 12 V system, for example) it can become the constraint that forces a larger section.
Resistance also sets the I²R loss, which is both a power cost and an additional heat input to the thermal calculation. Our guide on busbar resistance covers how loss and efficiency interact with the design.
A useful check: if the voltage drop is well within spec, the section is likely comfortable on resistance; if it is marginal, the conductor may need to grow — and that growth will also reduce the temperature rise, so the two checks reinforce each other.

6. Voltage, Insulation, and Clearance

Cross-section is only one dimension of "size." The others come from voltage.
Higher voltage needs thicker insulation and larger clearance and creepage distances between conductors and from conductor to ground — and those distances set the overall envelope just as the conductor area does. They are governed by insulation coordination standards (IEC 60664-1), and they cannot be traded away for a smaller conductor. A design that sizes for current alone and meets the voltage rules afterward is a design that will be re-drawn.
In practice, current sets the conductor area and voltage sets the insulation and spacing, and the two together decide the final envelope.

7. A Sizing Workflow You Can Follow

If you want a repeatable method for how to size a laminated busbar, the sequence below is short enough to follow in order:
  1. Collect the inputs: continuous current, peak current and duration, system voltage, allowed temperature rise, ambient, cooling method, and available space.
  2. Choose the material (copper or aluminum) for the application.
  3. Pick a working current density for the cooling method and allowed rise.
  4. Compute the cross-section: A = I / J.
  5. Confirm the temperature rise at that section — including the I²R loss as heat.
  6. Check resistance and voltage drop at the design current.
  7. Set insulation thickness and clearance for the system voltage.
  8. Compare against the envelope — and iterate if the section or the spacing does not fit.
Because steps 3 to 5 are interdependent, sizing is usually one or two passes rather than a single calculation. The goal is a section that meets the thermal limit with margin, fits the space, and holds the drop within spec.

8. Frequently Asked Questions

How do you calculate the cross-section of a laminated busbar?
Divide the continuous current by an allowable current density (A = I / J), where J is set by the cooling method and the permitted temperature rise — then confirm the result against the thermal limit.
What is the main limit when sizing a busbar?
Temperature rise. The conductor must stay within the insulation class's limit over the service life, so sizing is really a thermal problem that happens to be solved with a cross-section.
Does the material change the size?
Yes. An aluminum busbar needs roughly 1.6 times the section of copper for the same resistance, so the material choice must come before the final sizing.
Is sizing only about current?
No. Voltage sets the insulation thickness and the clearance, which are part of the overall envelope and cannot be reduced by choosing a smaller conductor.

9. Summary

Knowing how to size a laminated busbar turns the choice of cross-section into a short, repeatable sequence: start from the continuous and peak current, convert to a cross-section using a current density that matches the cooling and the allowed temperature rise, adjust for the material, and then verify resistance, voltage drop, and insulation clearance. Current decides the conductor area; voltage decides the insulation and spacing; together they define the envelope. Done in that order, the sizing stops being guesswork and becomes a design step you can document and defend. Talk to our engineers if you would like us to size a busbar for your circuit.
Want Us to Size a Busbar for Your Circuit?
Send your current, voltage, cooling, and space — our engineers will come back with a recommended cross-section.
Talk to an Engineer

References & Standards

  1. IEC 60664-1 — Insulation coordination for equipment within low-voltage supply systems — Part 1: Principles, requirements and tests. International Electrotechnical Commission. webstore.iec.ch

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