Busbar Sizing Calculator
This busbar sizing calculator finds the copper or aluminum busbar size for a load current. Enter the current, the bar material, and the thickness, and the busbar sizing calculator returns the cross-section you need, the next standard bar size, the real current density, and a short-circuit check, so you can pick a bar for a switchboard or panel in one step.
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The Busbar Sizing Calculator is part of our Cable & Protection collection. Browse more cable & protection to size, convert, and check your work alongside this tool.
What the busbar sizing calculator does
A busbar is a solid flat bar that carries current inside switchboards, panels, and bus ducts. Its size has to be large enough that the bar stays within its temperature limit at full load and survives the heat of a short circuit. The busbar sizing calculator divides the load current by a design current density to get the required cross-section, rounds it up to a standard width at the thickness you choose, and checks the bar against the short-circuit current for the fault duration.
Busbar size formula
The first step of any busbar calculation is the cross-section from the current density:
I is the load current and J the allowed current density. With several bars per phase the heat dissipates less well, so the calculator divides by a parallel bar factor of 0.9 for two bars, 0.82 for three, and 0.73 for four. The short-circuit check uses the adiabatic formula:
Here k is 122 for copper and 81 for aluminum, the values for a bar that starts at 90 °C and may reach 200 °C during the fault. The larger of the two areas sets the bar size.
Busbar sizing calculator example
A panel feeder carries 800 A on copper bars 10 mm thick, with a 25 kA fault for 1 second:
The load current governs. The next standard bar at 10 mm is 80 x 10 mm, which gives 800 mm² and a real current density of 1.0 A/mm². The bar weighs 7.17 kg per meter. In aluminum the same current needs 1,000 mm², so a 100 x 10 mm bar.
How to use the busbar size calculator
Enter the full load current of the circuit in amps and choose copper or aluminum. Leave the current density at 0 to use the typical design values of 1.2 A/mm² for copper and 0.8 A/mm² for aluminum, or enter your own value from a specification. Pick the bar thickness and how many bars run in parallel per phase. For the short-circuit check, enter the prospective fault current in kA and its duration, usually 1 second. Press Calculate.
Reading the busbar current rating result
The recommended busbar is the smallest standard bar that meets both the load and the short-circuit check. A real current density close to the design value means the bar is well used; a much lower value means the short circuit or the standard width step pushed the size up. Current density sizing is a design rule of thumb: the final busbar current rating depends on the enclosure, ventilation, ambient temperature, bar spacing, and paint, so confirm a switchboard design with the manufacturer tables or a temperature rise test to IEC 61439.
When to use a busbar calculation
- Sizing main and distribution busbars in a low voltage panel
- Choosing copper or aluminum bars for a bus duct or rising main
- Checking whether existing bars can take a higher load
- Checking busbar short circuit withstand after a transformer upgrade
- Comparing one thick bar with two thinner bars in parallel
Common busbar calculation mistakes
- Using free air ratings inside an enclosure. Closed panels run hotter, so use a design current density.
- Forgetting the short circuit. A bar sized only for load can fail under fault.
- Treating two bars as double. Parallel bars share heat and carry less than twice the current.
- Ignoring joints. Bolted joints are hot spots and need enough overlap and torque.
Tips for busbar sizing
- Keep the current density near 1.2 A/mm² for copper in closed panels.
- Use several thinner bars instead of one very wide bar for high currents.
- Check the bar supports for short-circuit forces as well as heating.
- Treat a cable feeder with its own ampacity rules, not bar current density.
Copper busbar size versus aluminum busbar size
Aluminum conducts about 61% as well as copper, so an aluminum bar needs roughly 1.5 times the cross-section for the same current. It still weighs about half as much as the copper bar, which is why long bus ducts and rising mains often use aluminum. Copper wins where space is tight, at bolted joints, and where the bars carry high fault currents. To compare cost and handling, the copper weight calculator gives the weight of any copper bar length.
Busbar ampacity in real panels
The current density method gives a sound first size, but the real busbar ampacity depends on how the heat leaves the bar. Vertical bars cool better than flat horizontal ones, painted bars radiate more heat than bare ones, and bars in a closed panel at 40 °C ambient can carry far less than the same bar in free air. Alternating current also crowds toward the edges of wide bars, so above about 2,000 A several thinner bars in parallel with a gap equal to their thickness work better than one very wide bar. The short-circuit level comes from the supply transformer, which the fault current calculator estimates from its kVA and impedance.
Busbar current density sizing chart (10 mm thick bars)
| Load current | Copper at 1.2 A/mm² | Aluminum at 0.8 A/mm² | Copper, 2 bars per phase |
|---|---|---|---|
| 250 A | 25 x 10 mm | 40 x 10 mm | 2 x 12 x 10 mm |
| 400 A | 40 x 10 mm | 50 x 10 mm | 2 x 20 x 10 mm |
| 630 A | 60 x 10 mm | 80 x 10 mm | 2 x 30 x 10 mm |
| 800 A | 80 x 10 mm | 100 x 10 mm | 2 x 40 x 10 mm |
| 1,250 A | 120 x 10 mm | 160 x 10 mm | 2 x 60 x 10 mm |
| 1,600 A | 160 x 10 mm | 200 x 10 mm | 2 x 80 x 10 mm |
| 2,000 A | 200 x 10 mm | 2 x 160 x 10 mm | 2 x 100 x 10 mm |
Busbar Sizing Calculator FAQ
How do you calculate busbar size?
Divide the load current by the current density, for example 800 A ÷ 1.2 A/mm² = 667 mm², then pick the next standard bar, here 80 x 10 mm in copper.
What current density is used for copper busbars?
About 1.2 A/mm² is a common design value for copper inside enclosures and 0.8 A/mm² for aluminum. Open, well-ventilated bars can run higher.
How many amps can a 50 x 10 copper busbar carry?
At 1.2 A/mm² a 500 mm² bar is sized for 600 A. Its thermal rating in free air is higher, but enclosures and ambient heat reduce it.
Why check the busbar for short circuit?
A fault heats the bar in a fraction of a second. If the cross-section is too small, the bar can exceed its temperature limit and soften or damage the insulation and supports.