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Cable Ampacity Calculator

Work out cable ampacity instantly with clear inputs, formula shown and shareable results.

About Cable Ampacity Calculator

Tabulated ampacity assumes a single circuit at 30 degrees C ambient. Real installations need two corrections: an ambient factor, which follows the square root of the ratio of remaining temperature rise, and a grouping factor for cables bunched together that cannot shed heat independently.

Formula

Ampacity correction

I = Itabulated x Ca x Cg, with Ca = sqrt((Tmax - Tambient) / (Tmax - 30))

Worked example

With the default values below, Cable Ampacity Calculator returns Corrected ampacity of 26.46. Change any input above and every figure here is recalculated in your browser.

Inputs used

Base ampacity from the table
47 A
Ambient temperature
40 °C
Circuits grouped together
4 circuits
Insulation ratingone of 70 C PVC, 90 C XLPE / thermoset
70 C PVC

Results

Corrected ampacity
26.46 A
Ambient correction factor
0.866
Grouping correction factor
0.650
Reduction from base rating
43.7% %

Step by step

  1. 1Values used
  2. 2Ampacity correction
  3. 3Corrected ampacity= 26.46 A
  4. 4Ambient correction factor= 0.866
  5. 5Grouping correction factor= 0.650
  6. 6Reduction from base rating= 43.7 %

Assumptions and limits

  • The result is derived only from the 4 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 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
  • Electrical results are planning estimates only. Verify against the applicable wiring rules (IEC 60364, BS 7671, NEC or local code) and have all installation work designed and checked by a qualified electrician.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Electrical results are planning estimates only. Verify against the applicable wiring rules (IEC 60364, BS 7671, NEC or local code) and have all installation work designed and checked by a qualified electrician.

Frequently Asked Questions

Why does grouping cut capacity so much?

Bunched cables heat each other, so each must run cooler to stay within its insulation limit. Six circuits together can lose 40% of rating.

Which standard should I follow?

Use the tables in the applicable code — BS 7671 Appendix 4, IEC 60364-5-52 or NEC 310 — since the correction factors and base values differ.

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Cable Ampacity Calculator – Calculate Corrected ampacity