Cable Size Calculator
Select a cable size that satisfies both current capacity and voltage drop.
About Cable Size Calculator
Cable selection must satisfy two independent criteria, and the larger requirement wins. Short heavy-current runs are governed by ampacity; long runs are almost always governed by voltage drop. Knowing which criterion governs tells you what to change: a shorter route helps a voltage-drop limited circuit, while better ventilation helps an ampacity-limited one.
Formula
Cable Size
Area is the larger of that needed for ampacity and that needed to hold voltage drop within limits
- governing factor
- Which of the two criteria sets the size
Worked example
With the default values below, Cable Size Calculator returns Minimum Conductor Area of 8.889. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Design currentminimum 0 A
- 40 A
- Route lengthminimum 0 m
- 60 m
- Nominal voltageminimum 1 V
- 400 V
- Permitted voltage dropaccepts 0–100 %
- 5 %
- Phasesone of Single phase, Three phase
- Three phase
Results
- Minimum Conductor Area
- 8.889 mm2
- Next Standard Size
- 10.0 mm2
- Area Needed for Current Capacity
- 8.889 mm2
- Area Needed for Voltage Drop
- 4.300 mm2
- Governing Factor
- Current capacity governs — the run is short relative to the load
Step by step
- 1Values used
- 2Cable Size
- 3Minimum Conductor Area= 8.889 mm2
- 4Next Standard Size= 10.0 mm2
- 5Area Needed for Current Capacity= 8.889 mm2
- 6Area Needed for Voltage Drop= 4.300 mm2
- 7Governing Factor= Current capacity governs — the run is short relative to the load
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 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 Cable Size calculated?
Area is the larger of that needed for ampacity and that needed to hold voltage drop within limits Cable selection must satisfy two independent criteria, and the larger requirement wins. Short heavy-current runs are governed by ampacity; long runs are almost always governed by voltage drop.
Why does Cable Size matter?
Knowing which criterion governs tells you what to change: a shorter route helps a voltage-drop limited circuit, while better ventilation helps an ampacity-limited one.
What values do I need to enter?
This calculator takes 5 inputs: Design current, Route length, Nominal voltage, Permitted voltage drop, Phases. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.