Copper Busbar Size Chart, Current Carrying Capacity & Weight Calculation
Anyone who has stood near an overloaded LT panel knows the signs: a faint warm smell, discoloured joints, a busbar that is hotter than it should be. Most of the time the cause is not a faulty breaker. It is a busbar that was sized in a hurry.
Choosing the right copper busbar size comes down to three questions. How much current will it carry? How much will it heat up? And how much will it weigh, and therefore cost? This guide gives you a practical size chart, explains how current capacity is worked out, and shows the weight formula with an example you can reuse.
What Is a Copper Busbar?
A busbar, often called a busbar copper patti in the Indian market, is a flat strip or bar of conductor that collects and distributes electrical power inside panels, switchgear, distribution boards and substations. It replaces bundles of cables where large currents are involved, because it is compact, rigid and carries heat away well.
Copper is the preferred material for the job. It has excellent conductivity, handles heat well and resists corrosion better than most alternatives. Most busbars are made from ETP copper (Electrolytic Tough Pitch), which is about 99.9 percent pure and widely used for electrical conductors. In India, copper bars for electrical use are covered under IS 613, which sets requirements for the material itself.
Understanding the Basics Before the Chart
Three terms will help you read any size chart correctly.
Cross-sectional area. This is width multiplied by thickness, in square millimetres. A 50 x 10 mm bar has a cross-section of 500 mm². Current capacity depends mainly on this number.
Current density (A/mm²). This is how many amperes you allow per square millimetre of copper. In Indian panel practice, a common design value for copper is around 1.2 A/mm² for conservative sizing, and some manufacturers go higher for small sections or well-ventilated layouts. Aluminium is typically designed at a lower value, which is why Copper busbars are smaller for the same load.
Temperature rise. Every conductor heats up when it carries current. The aim is to keep the rise within the limits set for the enclosure and the connected equipment, as covered in standards such as IEC 61439 for low-voltage assemblies. A busbar that runs hot ages its insulation, loosens joints and wastes energy.
Copper Busbar Size Chart (Indicative)
The table below shows common copper busbar sizes used in LT panels, with cross-section, weight per metre and an indicative current rating. The current values use a conservative 1.2 A/mm² and apply to a single bare bar in a typical indoor panel.
| Size (width x thickness, mm) | Cross-section (mm²) | Weight (kg/m) | Indicative current (A) |
|---|---|---|---|
| 15 x 3 | 45 | 0.40 | 54 |
| 20 x 3 | 60 | 0.53 | 72 |
| 20 x 5 | 100 | 0.89 | 120 |
| 25 x 5 | 125 | 1.11 | 150 |
| 30 x 5 | 150 | 1.34 | 180 |
| 40 x 5 | 200 | 1.78 | 240 |
| 50 x 5 | 250 | 2.23 | 300 |
| 50 x 6 | 300 | 2.67 | 360 |
| 60 x 6 | 360 | 3.20 | 432 |
| 75 x 10 | 750 | 6.68 | 900 |
| 100 x 10 | 1000 | 8.90 | 1200 |
Please treat these values as a planning guide, not a certified rating. Real-world capacity depends on the factors discussed below, and your panel designer should confirm the final selection against the applicable standard.
Copper Busbar Current Carrying Capacity: What Changes the Number?
The busbar ampacity you can safely use is rarely the same as the number in a simple table. Here is what moves it up or down.
1. Ambient temperature. A panel sitting in a hot plant room or a rooftop enclosure in an Indian summer cannot carry as much as one in an air-conditioned space. Higher ambient temperature means less headroom, so you derate the bar.
2. Orientation and spacing. A bar mounted on its edge (vertical orientation of the wide face) sheds heat better than one lying flat. Spacing between phases also matters, since closely packed bars heat each other.
3. Number of bars per phase. Two bars side by side do not carry exactly double the current of one. Because of the skin and proximity effects at 50 Hz, the second and third bars are used less effectively. Designers apply a derating factor, and the gap between the bars should be kept consistent, often equal to the bar thickness.
4. Enclosure and ventilation. A sealed box traps heat. Ventilated panels, louvres or forced air can raise the usable rating.
5. Surface finish. A tinned copper busbar resists oxidation and keeps joint resistance stable over the years, which helps in humid, coastal or chemically polluted environments. Bare copper is fine in clean, dry conditions, but its surface darkens over time and joint care becomes more important.
6. Joints and terminations. A busbar is only as good as its weakest connection. Clean contact surfaces, correct bolt size and proper torque keep resistance low. Many overheating cases trace back to a loose or poorly prepared joint, not to the bar.
Copper Busbar Weight Calculation
Weight matters for two reasons: it decides how much support your panel structure needs, and it is how busbars are usually priced, by the kilogram.
The busbar weight formula is simple. The density of copper is about 8.89 to 8.9 g/cm³, so:
Weight per metre (kg) = Width (mm) x Thickness (mm) x 8.9 / 1000
Worked example. Suppose you need a 50 x 10 mm bar, 10 metres long.
- Cross-section: 50 x 10 = 500 mm²
- Weight per metre: 500 x 8.9 / 1000 = 4.45 kg
- Total weight for 10 m: 4.45 x 10 = 44.5 kg
Multiply that by the current rate per kilogram and you have your material cost. Keep a small allowance, usually a few percent, for cutting and wastage, especially if the bars will be bent or drilled.
A quick sanity check: a bar of 1000 mm² cross-section weighs close to 8.9 kg per metre. If a supplier's bar feels noticeably lighter than the calculation suggests, the dimensions may be undersized or the material may not be what was promised.
How to Select the Right Busbar Size
Here is a simple, practical way to approach it.
- Start with the load current. Take the rated current of the incomer or the feeder, and add a margin for future expansion, commonly 20 to 25 percent.
- Divide by your chosen current density. For example, a 600 A requirement at 1.2 A/mm² needs about 500 mm², which points to a 50 x 10 mm bar.
- Check temperature rise. Consider ambient conditions, orientation and enclosure type, and increase the section if the bar will run hot.
- Check short-circuit strength. The bar and its supports must withstand the mechanical force and heating during a fault. This often decides support spacing and sometimes the bar size.
- Choose the finish. Select tinned or bare copper depending on the environment.
- Confirm weight and cost. Use the weight formula to compare options before you order.
Common Mistakes to Avoid
- Sizing only on the table value. Ignoring ambient temperature and enclosure type is the most common error.
- Using multiple bars without derating. Two bars are not twice as good as one.
- Skipping the short-circuit check. A bar that carries load comfortably can still fail under fault forces if supports are too far apart.
- Buying on the lowest price per kilogram. Poor material quality or incorrect dimensions cost more later in heat, losses and failures.
- Ignoring joint preparation. Dirty or loose joints cause hot spots even on a perfectly sized bar.
- Mixing metals carelessly. Where copper meets aluminium, use suitable bimetallic connectors to avoid corrosion.
Why Material Quality Matters
Two busbars with the same dimensions can behave very differently if the copper is not consistent. Impurities raise resistance, which means more heat for the same current. Poor surface finish and uneven thickness cause hot spots and difficult joints.
This is why panel builders and electrical contractors look for suppliers who offer ETP copper with controlled purity, accurate dimensions and clean edges. Koprex supplies copper busbars made for LT panels and power distribution, so engineers can work from the dimensions and weights they calculated, without surprises on site.
Frequently Asked Questions
What is the standard current density for a copper busbar?
- A conservative design value in Indian practice is around 1.2 A/mm², though actual values depend on size, ventilation and temperature rise limits.
How do I calculate copper busbar weight?
- Multiply width in mm by thickness in mm, then by 8.9, and divide by 1000. The result is the weight in kilograms per metre.
Which is better, tinned or bare copper busbar?
- Tinned bars resist oxidation and are better in humid or corrosive environments. Bare copper works well in clean, dry indoor panels.
What size busbar do I need for 400 A?
- At 1.2 A/mm², you need roughly 333 mm², so a 50 x 8 mm or 60 x 6 mm bar is a sensible starting point, subject to temperature rise and enclosure checks.
What does IS 613 cover?
- It specifies requirements for copper rods and bars used for electrical purposes. It sets material standards rather than giving ready current ratings for every installation.
Final Thoughts
A good busbar is invisible. It carries current quietly for decades, stays cool and never makes the news. Getting there takes a few simple steps: work out the load, choose a sensible current density, account for the real conditions inside your panel and verify the weight and cost before ordering.
If you are planning a new LT panel, an upgrade or a bulk requirement, the team at Koprex can help you with standard and custom copper busbar sizes. Share your current rating, bar dimensions and finish preference, and get a clear quote.
Need ETP copper busbars you can rely on? Contact Koprex today for sizes, samples and pricing.