Wire Ampacity Calculator
Determine the derated current-carrying capacity of a cable in its installed conditions.
About Wire Ampacity Calculator
Derating factors multiply rather than add. A cable derated 12 per cent for temperature and 20 per cent for grouping retains only 70 per cent of its tabulated rating, not 68 per cent by subtraction. Grouping and thermal insulation are the most commonly overlooked deratings, and both routinely cost more capacity than ambient temperature does.
Formula
Wire Ampacity
Derated ampacity = tabulated ampacity × ambient factor × grouping factor × insulation factor
- derating factors
- Multiplied, not added, because each independently limits heat dissipation
Worked example
With the default values below, Wire Ampacity Calculator returns Derated Ampacity of 32.38. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Tabulated ampacityminimum 0 A
- 46 A
- Derating for ambient temperatureaccepts 0–100 %
- 12 %
- Derating for groupingaccepts 0–100 %
- 20 %
- Derating for thermal insulationaccepts 0–100 %
- 0 %
- Design currentminimum 0 A
- 32 A
Results
- Derated Ampacity
- 32.38 A
- Combined Derating Factor
- 0.7040
- Utilisation of Derated Capacity
- 98.8%
- Spare Capacity
- 0.38 A
- Verdict
- Adequate but with little margin
Step by step
- 1Values used
- 2Wire Ampacity
- 3Derated Ampacity= 32.38 A
- 4Combined Derating Factor= 0.7040
- 5Utilisation of Derated Capacity= 98.8
- 6Spare Capacity= 0.38 A
- 7Verdict= Adequate but with little margin
Assumptions and limits
- The result is derived only from the 5 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
- Figures are rounded for display to 4 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
Frequently Asked Questions
How is Wire Ampacity calculated?
Derated ampacity = tabulated ampacity × ambient factor × grouping factor × insulation factor Derating factors multiply rather than add. A cable derated 12 per cent for temperature and 20 per cent for grouping retains only 70 per cent of its tabulated rating, not 68 per cent by subtraction.
Why does Wire Ampacity matter?
Grouping and thermal insulation are the most commonly overlooked deratings, and both routinely cost more capacity than ambient temperature does.
What values do I need to enter?
This calculator takes 5 inputs: Tabulated ampacity, Derating for ambient temperature, Derating for grouping, Derating for thermal insulation, Design current. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.