Education / Electrical
Voltage Drop Calculator: Copper, Aluminium, AWG and mm²
Estimate resistive voltage drop and percentage drop at 20 °C or 75 °C, with metre and foot lengths. Compare 3% or 5% guidance and see circuit assumptions.
Voltage Drop Calculator: Copper, Aluminium, AWG and mm²: Uses twice the one-way resistance for DC and single-phase circuits, or the square-root-of-three factor for a balanced three-phase circuit. Drop percentage is voltage drop divided by supply voltage times 100. AC results assume unity power factor and negligible reactance. Check sizes with the [wire gauge converter](/wire-gauge-converter). Runs 100% locally in your browser with zero server file uploads.
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Formula: DC/single-phase ΔV = 2 × I × L × ρ/A; balanced three-phase ΔV = √3 × I × L × ρ/A. L is one-way metres; A is mm². Drop % = 100 × ΔV ÷ supply voltage. ρ(T) = ρ20 × [1 + α × (T − 20)].
Used: 30 m / 98.43 ft, 16 A, 2.5 mm², 230 V, 20 °C. Resistivity 0.01724 Ω·mm²/m; one conductor resistance 0.006896 Ω/m. Difference from 3% guidance: -0.122 percentage points.
Assumes steady current, equal conductors, unity power factor and negligible AC reactance. Three-phase voltage is line-to-line. This is not an ampacity, protection, earth-fault or installation safety check. A drop at or above the supply voltage means the model is outside a usable operating range.
Reference resistivities (Ω·mm²/m): copper 0.01724 at 20 °C and 0.020966426 at 75 °C, α = 0.00393/K; aluminium 0.02828 at 20 °C and 0.03434606 at 75 °C, α = 0.0039/K. The 75 °C values are linear estimates; alloys and stranding vary. NIST copper tables · University of Victoria: conductor properties
NEC 210.19(A) informational note recommends about 3% on a branch circuit and 5% total feeder plus branch for reasonable operation; an informational note is guidance. IEC 60364-5-52 Annex G describes voltage-drop guidance by supply and load type, commonly 3% lighting / 5% other uses on public low-voltage supplies. Check local adopted rules and the entire circuit path. NFPA 70 catalogue · IEC 60364-5-52 catalogue
Conductor resistance
NIST Handbook 100 copper tables and University of Victoria ELEC220 Lecture 5 give annealed-conductor properties. The model uses copper ρ20 = 0.01724 Ω·mm²/m, α = 0.00393/K; aluminium ρ20 = 0.02828 Ω·mm²/m, α = 0.0039/K. At 75 °C, linear estimates are 0.020966426 and 0.03434606 respectively. Actual conductor alloys and stranding vary. Sources: https://nvlpubs.nist.gov/nistpubs/Legacy/hb/nbshandbook100.pdf ; https://www.ece.uvic.ca/~ece220/revised_notes/revised%205.pdf
Voltage-drop guidance
NEC 210.19 informational note describes about 3% for a branch circuit and 5% total feeder plus branch for reasonable operation. NFPA's own public development report reproduces the note: https://www.nfpa.org/api/files?path=%2Ffiles%2FAboutTheCodes%2F70%2F70_A2022_NEC_P02_SD_SRStatements.pdf
IEC 60364-5-52 Annex G gives guidance by supply/load type, commonly 3% lighting and 5% other uses on public low-voltage supplies. Consult the current standard and national adoption; this calculator is a resistive estimate. Standards-body catalogue: https://webstore.iec.ch/en/publication/1878
How to use it
- Select circuit type, conductor material and temperature.
- Enter one-way length, current, supply voltage and AWG or metric cross-section.
- Read the voltage drop, receiving voltage and difference from the selected guidance.
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Frequently asked questions
Is length the full cable loop?
Enter one-way source-to-load length. The DC and single-phase formulas already include the return conductor. Three-phase uses line-to-line supply voltage and balanced current.
Does this certify a cable size?
No. It does not check ampacity, protection, fault currents, earthing or installation method. Reactance and power factor can materially change AC results. Follow local adopted wiring rules with a qualified electrician.
Are 3% and 5% mandatory everywhere?
No. NEC informational notes give efficiency guidance. IEC guidance varies by supply and load type, and local rules may differ. Consider the total feeder and final-circuit path.
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