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Wire Size For Current Calculator

Work out wire size for current instantly with clear inputs, formula shown and shareable results.

About Wire Size For Current Calculator

Sizing for voltage drop rearranges Ohm's law: required area equals 2 x resistivity x length x current divided by the permitted drop in volts. Copper resistivity is about 0.0172 ohm mm2/m at 20 C. The result must then be checked against the ampacity table for the installation method.

Formula

Cable sizing

A = 2 x rho x L x I / Vdrop_allowed; then verify ampacity for the installation method

Worked example

With the default values below, Wire Size For Current Calculator returns Conductor area required of 4.79. Change any input above and every figure here is recalculated in your browser.

Inputs used

Design current
32 A
Circuit length
30 m
System voltage
230 V
Maximum permitted drop
3 %

Results

Conductor area required
4.79 mm²
Next standard size
6.00 mm²
Drop with the standard size
2.393% %
Drop with the standard size
5.504 V

Step by step

  1. 1Values used
  2. 2Cable sizing
  3. 3Conductor area required= 4.79 mm²
  4. 4Next standard size= 6.00 mm²
  5. 5Drop with the standard size= 2.393 %
  6. 6Drop with the standard size= 5.504 V

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

Is voltage drop or ampacity the limit?

On long runs voltage drop dominates; on short runs ampacity does. Both must be satisfied, and the larger result governs.

Should I use hot resistivity?

For accuracy yes — copper resistivity rises about 0.4% per degree, so a conductor at 70 C is roughly 20% more resistive than at 20 C.

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