Wire Size Calculator
Size the cable before the run gets expensive.
Wire Size Calculator
A = (2 × ρ × L × I) / (V × drop%)
Calculated instantly
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What This Calculator Does
The Wire Size Calculator finds the smallest conductor cross-section that keeps voltage drop within a limit you choose. It inverts the voltage-drop equation, so instead of checking a cable you already have, it tells you what to buy.
How It Works
The maximum permitted drop in volts is the supply voltage times the allowed percentage. Setting the voltage-drop equation equal to that limit and solving for area gives area equals two times resistivity times length times current, divided by the allowable drop. Power loss at exactly that size simplifies to current times supply voltage times the allowed fraction.
Example
A 20 A load 30 m from a 230 V supply, with a 3% limit and copper at 0.0175 Ω·mm²/m, needs at least 3.04 mm². The next standard size is 4 mm², which drops 2.28% and wastes 105 W. Choosing 2.5 mm² would exceed the limit.
Formula
Vallow = V × drop% / 100A = 2 × ρ × L × I / VallowPloss = I × VallowAt exactly the allowed drop, the cable dissipates this much regardless of length.
Reading the Results
Minimum conductor cross-section
The calculated area that exactly meets the drop limit. It is a floor, not a standard size — round up, and check ampacity separately.
Power lost at that size
The energy converted to heat in the cable when it runs at the allowed drop. It is independent of length because a longer cable at the same drop has more resistance and less current for a fixed load.
Sources and References
IEC 60364-5-52 — Wiring systems
Conductor sizing and voltage drop
Sets out how voltage drop and current-carrying capacity are both considered when selecting a conductor.
NFPA 70 — National Electrical Code, Article 310
Conductor ampacities and adjustment factors
Ampacity, ambient temperature, and bundling can require a larger conductor than voltage drop alone.
IEEE Std 141 — Recommended Practice for Electric Power Distribution
Voltage-drop limits and conductor selection
Discusses how voltage drop influences conductor choice on long distribution runs.
References identify the method or data behind this calculator. Where a standard or tariff changes, the current edition applies, and the assumptions stated above take precedence over a remembered figure.
Frequently Asked Questions
Is voltage drop enough to choose a cable?
No. A conductor must also carry the current without overheating, which is the ampacity requirement. Installation method, ambient temperature, grouping, and insulation type all affect it, so check the wiring code before buying.
What cross-section should I actually install?
Round the calculated figure up to the next standard size, then verify ampacity. A 3.04 mm² result means 4 mm², not 2.5 mm², because standard sizes jump from 2.5 to 4.
Why is aluminium different?
Aluminium has higher resistivity, about 0.0282 Ω·mm²/m against copper’s 0.0175, so it needs roughly 60% more area for the same drop. It also needs connectors rated for aluminium to avoid corrosion problems.
Does cable length include the return path?
Enter the one-way distance. The calculator doubles it internally because the circuit includes both the outgoing and returning conductors.