Wire Ampacity Calculator
Get a quick, indicative current-carrying capacity for a conductor from its cross-section and an assumed current density. It is a back-of-envelope sanity check, not a substitute for the AS/NZS 3008 ampacity tables.
Enter Values
Minimum 0
Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.
How to use this calculator
- Enter the conductor cross-section in square millimetres.
- Leave current density at the default 4 A/mm2, or enter a value that suits your conductor and cooling conditions.
- Read the indicative ampacity, then confirm the real rating against the wiring rules for your install method.
How it works
The tool multiplies conductor cross-section by an assumed current density: ampacity = csa x density. A density of around 4 A/mm2 is a common rule of thumb for insulated copper in air, but the true safe current depends heavily on insulation, how the cable is installed, grouping with other cables and ambient temperature.
Worked example
Worked example. A 10 mm2 conductor at 4 A/mm2 gives 10 x 4 = 40 A. A 25 mm2 conductor at the same density gives 25 x 4 = 100 A. These are starting estimates only — the tabulated rating may be higher or lower.
Common mistakes
- Treating the result as a certified rating — it is only a rough guide.
- Using the default density in conditions (buried, bunched, hot) where the real capacity is much lower.
- Confusing cross-section (mm2) with diameter (mm); this tool needs the area.
Frequently asked questions
Is 4 A/mm2 always correct?
No. It is a rough figure for insulated copper in open air. Bundling, thermal insulation or high ambient temperature can push the safe density well below that, so always verify against AS/NZS 3008.
Can I use this for aluminium?
You can, but aluminium is typically run at a lower current density than copper for the same temperature rise, so lower the density value accordingly and treat the result as indicative.
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Cite this tool
Zerdly. (2026). Wire Ampacity Calculator. Retrieved from https://www.zerdly.com/tools/wire-ampacity-calculator/
Tip: Enter any known values to calculate the remaining results.
Designed to run locally in your browser. Tool inputs are not intentionally stored by Zerdly unless stated otherwise.