Wire Size Calculator
Select conductor size from current, run length and voltage drop limit.
About Wire Size Calculator
Single phase circuits count the run twice because current returns along the neutral, while three phase uses the square root of three. Long runs are usually governed by voltage drop rather than by ampacity. A cable sized only for its current rating can still deliver unacceptable voltage at the far end, which is why drop is the binding constraint on anything beyond a short run.
Formula
Wire Size
Area = k × ρ × L × I ÷ allowable drop, where k is 2 single phase and 1.732 three phase
- ρ
- Resistivity of copper, about 0.0175 Ω·mm²/m
- factor of 2
- Single phase current flows out and back, doubling the effective length
Worked example
With the default values below, Wire Size Calculator returns Conductor Area Required of 4.870. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Load currentminimum 0.1 A
- 32 A
- One-way run lengthminimum 0.1 m
- 30 m
- Supply voltageminimum 12 V
- 230 V
- Maximum voltage dropaccepts 0.1–10 %
- 3 %
- Circuit typeone of Single phase, Three phase
- Single phase
Results
- Conductor Area Required
- 4.870 mm2
- Next Standard Size
- 6.0 mm2
- Voltage Drop at That Size
- 5.600 V
- Voltage Drop at That Size
- 2.435%
- Allowable Voltage Drop
- 6.900 V
Step by step
- 1Values used
- 2Wire Size
- 3Conductor Area Required= 4.870 mm2
- 4Next Standard Size= 6.0 mm2
- 5Voltage Drop at That Size= 5.600 V
- 6Voltage Drop at That Size= 2.435
- 7Allowable Voltage Drop= 6.900 V
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 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.
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 Size calculated?
Area = k × ρ × L × I ÷ allowable drop, where k is 2 single phase and 1.732 three phase Single phase circuits count the run twice because current returns along the neutral, while three phase uses the square root of three. Long runs are usually governed by voltage drop rather than by ampacity.
Why does Wire Size matter?
A cable sized only for its current rating can still deliver unacceptable voltage at the far end, which is why drop is the binding constraint on anything beyond a short run.
What values do I need to enter?
This calculator takes 5 inputs: Load current, One-way run length, Supply voltage, Maximum voltage drop, Circuit type. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.