Wire Size Calculator — Conductor Cross-Section & Voltage Drop
Enter the load current, wire length, supply voltage and maximum allowable voltage drop — get the next standard IEC cross-section (mm²), its AWG equivalent, the actual drop and the heat lost in the wire.
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Conductor material
Next standard IEC 60228 size above the calculated minimum of 1.6 mm²
- 1
Max allowable voltage drop
230 × 3 ÷ 100 = 6.9 - 2
Minimum wire area A_min = 2·L·I·ρ ÷ V_drop
2 × 20 × 16 × 0.01724 ÷ 6.9 = 1.5991A_min = 2·L·I·ρ / V_drop_max — the smallest area that keeps drop within the limit. - 3
IEC standard size (rounded up)
2.50Next IEC 60228 cross-section at or above the calculated minimum.
How does this calculator work?
A_min = 2 × L × I × ρ / V_drop_max gives the minimum conductor area in mm², rounded up to the next IEC standard size and its AWG equivalent. Copper ρ = 0.01724 Ω·mm²/m, aluminium ρ = 0.02825 Ω·mm²/m. IEC/NEC recommend ≤ 3 % drop for branch circuits.
Formula
How this is calculated
Current flowing through a conductor with resistance R produces a voltage drop V = I × R. For a round-trip length of 2L (current goes to the load and returns through the neutral/return wire), the wire resistance is R = 2Lρ/A, where ρ is the conductor resistivity in Ω·mm²/m and A is the cross-sectional area. Rearranging gives the minimum area required to stay within the target drop: A_min = 2LIρ / V_drop_max. Copper at 20 °C has ρ ≈ 0.01724 Ω·mm²/m; aluminium is roughly 64 % more resistive at 0.02825 Ω·mm²/m.
The calculated minimum area is rounded up to the next standard IEC 60228 cross-section (0.75 → 1.0 → 1.5 → 2.5 → 4.0 → 6.0 → … mm²). The actual drop is then computed with that area — it is always at or below the target. The US AWG equivalent is the next gauge whose area meets or exceeds the recommended mm² size. The power dissipated as heat in the wire is P = I²R.
This calculator assumes a single-phase or DC round-trip circuit at ambient temperature with published 20 °C resistivity. In practice resistance rises with temperature, bundled or conduit runs may require current de-rating, and local electrical codes may impose additional safety margins. For safety-critical or high-current installations, always consult a licensed electrician and the applicable national standard (IEC 60364, NEC, BS 7671, or equivalent).
Frequently asked questions
Conductors are manufactured in fixed standard cross-sections (0.75, 1.0, 1.5, 2.5, 4.0 mm² …). The exact calculated minimum rarely coincides with one of those sizes, so the tool always rounds up to guarantee the voltage drop stays within your chosen limit.
IEC 60364 and the US NEC both recommend no more than 3 % drop on a branch circuit and no more than 5 % total (feeder + branch). Use a tighter 1–2 % limit for sensitive electronics or long low-voltage DC runs (12 V, 24 V solar or LED), where a higher percentage represents a large absolute volt loss.
No — the formula uses the factor 2 for a single-phase or DC round trip. For balanced three-phase circuits replace the 2 with √3 ≈ 1.732 in the voltage-drop formula; most three-phase cable-sizing tables already incorporate this factor.
Also known as
TG we-Calculate Editorial Team. (2026). Wire Size Calculator — Conductor Cross-Section & Voltage Drop [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/wire-size-calculator
TG we-Calculate Editorial Team. "Wire Size Calculator — Conductor Cross-Section & Voltage Drop." TG we-Calculate. 2026. https://we-calculate.com/calculator/wire-size-calculator.
TG we-Calculate Editorial Team, "Wire Size Calculator — Conductor Cross-Section & Voltage Drop," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/wire-size-calculator
@misc{wecalculate_wire_size_calculator, title = {Wire Size Calculator — Conductor Cross-Section & Voltage Drop}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/wire-size-calculator}}, year = {2026}, note = {TG we-Calculate} }
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