Flexible Busbar Sizing & Ampacity Guide
A practical guide to sizing flexible copper busbars: how to calculate the cross-section from your current rating, what current density to assume, and which derating factors matter. Written by the engineering team at SVS Maverick.
Quick Sizing Rule
Cross-section (mm²) = Continuous current (A) ÷ Current density (A/mm²)
For bare copper in enclosed switchgear, start with 1.5-2 A/mm². A 1,000 A connection therefore needs roughly 500-660 mm² of copper. Then verify temperature rise and apply deratings for ambient, insulation, and ventilation. For a guaranteed rating, ask us to size the busbar against your actual operating conditions.
How to Size a Flexible Busbar in 6 Steps
1. Define the Continuous Current
Establish the maximum continuous (not peak) current the busbar must carry, including any duty-cycle or overload requirements. Add a 15-20% safety margin for load growth and aging.
2. Estimate the Cross-Section
Apply S = I / k with a current density appropriate to the installation: 1.5-2 A/mm² is a conservative start for enclosed assemblies. Example: 1,000 A ÷ 1.7 A/mm² ≈ 590 mm².
3. Check Temperature Rise
Verify the chosen section against the permissible temperature rise for your standard and plating (bolted joints with bare copper are typically limited to lower rises than plated ones). Consider ambient temperature, enclosure ventilation, and nearby heat sources.
4. Apply Derating Factors
Derate for elevated ambient (≈8-10% per 10°C above reference), altitude above 1,000 m, insulation over the busbar body, bundled or stacked conductors, and restricted airflow.
5. Define the Mechanical Envelope
Choose width, stack thickness, and lamination count to fit the space and provide the required flexibility. Thinner foils (0.1 mm) flex more; thicker foils (0.3-0.5 mm) suit mostly static links.
6. Specify Terminals and Finish
Define hole patterns or studs per your mating hardware, plating (tin for general use, nickel for welding to aluminium, silver for lowest contact resistance), and insulation class.
Indicative Ampacity Reference Table
| Copper Cross-Section | Typical Stack Dimensions | Indicative Continuous Current* |
|---|---|---|
| 50 mm² | e.g. 20 × 2.5 mm stack | ≈ 100 - 150 A |
| 120 mm² | e.g. 32 × 4 mm stack | ≈ 250 - 320 A |
| 250 mm² | e.g. 50 × 5 mm stack | ≈ 450 - 600 A |
| 500 mm² | e.g. 63 × 8 mm stack | ≈ 800 - 1,100 A |
| 800 mm² | e.g. 80 × 10 mm stack | ≈ 1,200 - 1,600 A |
| 1,200 mm² | e.g. 100 × 12 mm stack | ≈ 1,700 - 2,300 A |
| 2,000 mm² | e.g. 125 × 16 mm stack | ≈ 2,600 - 3,500 A |
| 4,000 mm² | multiple parallel stacks | ≈ 5,000 - 10,000 A |
*Indicative ranges for bare copper at 35-40°C ambient with moderate ventilation, spanning conservative enclosed-panel to ventilated installations. Actual ratings depend on temperature rise limits, enclosure, insulation, and duty cycle - always verify for your installation or request a sized recommendation from our engineers. For a quick interactive estimate, try our busbar ampacity calculator.
Derating Factors to Consider
Ambient Temperature
Ratings assume 35-40°C ambient. Derate roughly 8-10% per additional 10°C. Inside sealed enclosures, use the internal air temperature, not the room temperature.
Insulation
Insulated busbars dissipate heat less effectively than bare copper. Apply a 10-15% derating or verify by temperature-rise test, and check the insulation temperature class.
Enclosure & Ventilation
Free-air ratings drop significantly inside unventilated enclosures. Forced ventilation or generous spacing between conductors recovers capacity.
Duty Cycle
Intermittent loads allow smaller sections than continuous duty. Short-time ratings (e.g. fault currents for 1 s) are governed by adiabatic heating, not steady-state ampacity.
Altitude
Above approximately 1,000 m, thinner air reduces convective cooling. Apply standard altitude correction factors for your applicable standard.
Joint Quality
Bolted joints add contact resistance and local heating. Proper contact pressure, plating, and joint overlap (typically 5-10× bar thickness) keep joints cooler than the bar itself.
Sizing for Flexibility, Not Just Current
Two busbars with identical cross-sections can have very different mechanical behavior. Flexibility is set by the lamination thickness and count: a 500 mm² busbar built from fifty 0.1 mm foils bends easily, while the same section built from ten 0.5 mm strips is much stiffer. Specify the expected movement - thermal expansion range, vibration amplitude, and installation offset - alongside the current rating.
The flexible length between the solid terminal pads also matters: a longer free length distributes bending over more material, reducing stress per cycle and extending fatigue life. Our engineers balance these parameters when designing to your application.
Sizing FAQ
How do I calculate the cross-section of a copper busbar?
A practical first estimate divides the continuous current by an allowable current density: S = I / k, where S is the cross-section in mm², I is the current in amperes, and k is the current density in A/mm². For bare copper in enclosed assemblies, k = 1.5-2 A/mm² is a conservative starting point; well-ventilated or short busbars can run higher. The final size must be verified against permissible temperature rise for the specific installation.
What current density is safe for copper busbars?
Typical design values range from 1.2-2 A/mm² for enclosed switchgear with limited cooling, up to 2-3 A/mm² for short, well-ventilated busbars. Higher densities mean higher temperature rise. Standards-based design works from permissible temperature rise (commonly 30-65 K depending on the standard and plating) rather than a fixed density.
How does ambient temperature affect busbar ampacity?
Published ratings usually assume 35-40°C ambient. As a rule of thumb, every 10°C of additional ambient above the reference reduces ampacity by roughly 8-10%, because less temperature-rise margin remains before the conductor reaches its limit.
Does insulation reduce a busbar’s current rating?
Yes, slightly. Insulation impedes heat dissipation from the copper surface, so an insulated busbar of the same cross-section runs warmer than a bare one. The insulation’s temperature class can also become the limiting factor - PVC at 105°C limits the design more than Kapton at 200°C. Account for this in sizing or specify a higher temperature-class insulation.
Why specify a safety margin when sizing a busbar?
A 15-20% margin above the calculated continuous current covers load growth, harmonics, voltage fluctuations, contact resistance aging at bolted joints, and hotter-than-expected ambient conditions. If your system requires 2,000 A continuous, sizing for roughly 2,400 A is common practice.
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Send us your continuous current, ambient conditions, space envelope, and movement requirements. Our engineering team will recommend a verified cross-section, lamination build, and terminal design - free of charge with your quotation.
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