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Calculate copper or aluminum busbar current rating from dimensions, or work backward from a target current to find the minimum size you need. Enter values in millimeters.
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Touch a busbar under load and it should feel barely warm. If it stings your fingers, or the insulation nearby carries a faint scorched smell, the ampacity has already been exceeded.
Ampacity is not the current flowing through a conductor. It's the maximum current that conductor can carry continuously without overheating, under a specific set of conditions. Two identical-looking copper strips can have very different ampacity ratings depending on ambient temperature, airflow, and how hot you're willing to let them get.
That's the number the calculator above is solving for.
You already have a busbar in hand, or a stock size in mind. Enter its width and thickness in millimeters, pick copper or aluminum, and the calculator returns the cross-sectional area and an estimated ampacity. This is the fastest way to check "is this bar big enough."
You know the current your system needs to carry, but not the bar. Enter the target amperage and the width you have room for, and the calculator solves for the minimum thickness. This is the direction most panel builders actually start from.
Both modes run on the same underlying math.
Worked example: picture a 25 mm wide, 3 mm thick strip of copper on a workbench. Its cross-section is 25 × 3 = 75 mm². At a standard copper current density of roughly 1.2 A/mm², that strip is rated for about 75 × 1.2 = 90 A. Widen it to 5 mm thick and the same strip jumps to 150 A, no extra copper along its length required.
Aluminum follows the identical formula, just with a lower density figure, which is why an aluminum bar has to be noticeably bigger to match a copper one.
Paste the same 25 x 3 mm bar into three different online calculators and you'll likely get three different numbers. None of them are "wrong." They're just built on different assumptions:
A calculator tuned for a ventilated switchgear room and one tuned for a sealed battery box will never agree, and that's expected.
Use this table as a starting reference point, not a final answer for your specific installation.
| Width (mm) | Thickness (mm) | Area (mm²) | Standard Ampacity | Conservative Ampacity |
|---|---|---|---|---|
| 10 | 2 | 20 | ≈24 A | ≈19 A |
| 15 | 3 | 45 | ≈54 A | ≈43 A |
| 20 | 3 | 60 | ≈72 A | ≈58 A |
| 25 | 3 | 75 | ≈90 A | ≈72 A |
| 25 | 5 | 125 | ≈150 A | ≈120 A |
| 40 | 5 | 200 | ≈240 A | ≈192 A |
| 50 | 5 | 250 | ≈300 A | ≈240 A |
| 60 | 10 | 600 | ≈720 A | ≈576 A |
Every row above assumes open-air installation. Enclosed panels need extra margin, covered further down this page.
Same footprint, different metal, different result:
| Size (mm) | Copper Ampacity | Aluminum Ampacity |
|---|---|---|
| 20 x 3 | ≈72 A | ≈48 A |
| 25 x 3 | ≈90 A | ≈60 A |
| 25 x 5 | ≈150 A | ≈100 A |
| 40 x 5 | ≈240 A | ≈160 A |
Aluminum needs roughly a third more cross-section to match copper. It's lighter and cheaper by weight, but bulkier by amperage.
Battery banks are unforgiving. The load is continuous, the space is tight, and there's no fan running.
Real scenario: a DIY builder links four prismatic cells with a thin nickel-plated strip rated "300 A" by the seller. Under sustained 200 A discharge, the connection point gets warm enough to soften the terminal's plastic shroud within an hour. The strip's advertised rating assumed a brief pulse, not hours of continuous draw.
For battery and storage applications, size to the conservative profile in the calculator above, not the standard one. Round up to the next stock thickness. And never let the busiest connection in the pack be the thinnest piece of metal in the circuit.
Continuous ampacity and short-circuit withstand are two separate questions. A fault can push current to ten times the normal rating for a fraction of a second, and the mechanical force from that spike is real. Rigid busbar in a fault event can visibly flex or bend outward, not just heat up.
A bar sized correctly for everyday load can still fail mechanically in a fault if it wasn't checked against the upstream protection device's clearing time. That check sits outside what a simple ampacity calculator covers.
Two identical bars, two different outcomes:
When in doubt, oversize slightly. A busbar is one of the cheapest parts in the system to get right, and one of the most expensive to get wrong.
Not directly. Wire ampacity tables account for insulation and bundling. Busbar tables are based on bare cross-section and open-air cooling, so the two aren't interchangeable.
They're typically continuous operating ratings under stated conditions, not absolute limits. Running above them shortens lifespan and increases fire risk rather than causing instant failure.
No. Parallel bars block airflow to each other, so combined ampacity is usually somewhat less than the sum of each bar calculated on its own.
The base cross-section formula is the same. AC systems add considerations like skin effect and power factor at higher currents, which DC systems don't need to account for.
A common practice is 20–25% above expected continuous load. Enclosed panels or high ambient temperatures usually call for more.
A calculator gives you a starting number. Turning that number into a busbar that actually performs, holds up under fault conditions, and fits your enclosure takes real manufacturing behind it. That's the gap between an online estimate and a part you can bolt in and trust.
This is where SHZHJ comes in. As a China-based manufacturer of copper and aluminum busbars, we work directly with engineers, panel builders, and battery system integrators to turn a target ampacity into a finished bar — cut, punched, plated, and formed to spec, not just sized on paper. Whether you're specifying a single prototype or ordering production volume, our team can review your load requirements and confirm the right size before you commit.
If you have a project that needs more than a ballpark estimate, our engineers are ready to help. Visit lococontact.com to send your specifications, or request a quote below.
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