In DC or single-phase, ΔV = 2 × L × I × ρ ÷ S, with L in meters, I in amps, ρ in Ω·mm²/m and S in mm². For copper at 20 °C, 20 m, 25 A and 6 mm²: 2 × 20 × 25 × 0.0172 ÷ 6 = 2.87 V. At 220 V that is 1.30 %.
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Calculate voltage drop in volts and percent in a copper or aluminum cable, see the voltage that reaches the load and compare sizes against a limit you choose.
Every conductor has resistance, and the current flowing through it makes the voltage fall along the cable. The resistance of a wire is R = ρ × L ÷ S, where ρ is the metal's resistivity in Ω·mm²/m, L is the length in meters and S is the cross-section in mm². In a DC or two-wire single-phase circuit the current travels out and back, so ΔV = 2 × L × I × ρ ÷ S. In a balanced three-phase circuit, using the line-to-line voltage, ΔV = √3 × L × I × ρ ÷ S. The percentage is ΔV ÷ V × 100 and the voltage that reaches the load is V − ΔV. The tool uses the resistivity of annealed copper, 0.0172 Ω·mm²/m at 20 °C, and of aluminum, 0.0282 Ω·mm²/m (Wikipedia table, 1.72e-8 and 2.82e-8 Ω·m). When the cable heats up, resistivity rises: ρ(T) = ρ20 × [1 + α × (T − 20)], with α about 0.0039 per kelvin for both metals. Copper at 70 °C has a resistivity of 0.02058 Ω·mm²/m, almost 20 % above its 20 °C value, and the drop follows. Example: a 25 A, 220 V shower circuit with 20 m of 6 mm² cable loses 2 × 20 × 25 × 0.0172 ÷ 6 = 2.87 V, which is 1.30 %. With 2.5 mm² and 30 m at 20 A, the drop is 8.26 V, or 3.75 %. At low voltage the effect weighs more: 10 A on 12 V over 5 m of AWG 14 wire (2.08 mm²) loses 0.83 V, almost 7 % of the voltage. The AWG gauge comes from d = 0.127 mm × 92^((36 − n) ÷ 39), and the cross-section is the area of that circle: AWG 14 is 2.08 mm² and AWG 10 is 5.26 mm². This calculator considers resistance only: it ignores the reactance of long, large-section AC cables, the skin effect and grouping. The drop limit is your own field, because each design and each code sets its own, and the tool does not replace cable sizing, which also depends on the maximum current the conductor can carry.
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In DC or single-phase, ΔV = 2 × L × I × ρ ÷ S, with L in meters, I in amps, ρ in Ω·mm²/m and S in mm². For copper at 20 °C, 20 m, 25 A and 6 mm²: 2 × 20 × 25 × 0.0172 ÷ 6 = 2.87 V. At 220 V that is 1.30 %.
Length is the one-way distance from the source to the load. The return path is already in the math.
Resistivity used: 0.0172 Ω·mm²/m.
Resistivity at 20 °C: annealed copper 1.72e-8 Ω·m (0.0172 Ω·mm²/m) and aluminum 2.82e-8 Ω·m (0.0282 Ω·mm²/m), temperature coefficients 0.00393 and 0.0039 per kelvin (Wikipedia, Electrical resistivity and conductivity).
| mm² | ΔV (V) | ΔV (%) | Voltage at the load | Cable loss | Meets |
|---|---|---|---|---|---|
| 34.4 | 15.64 | 185.6 | 860 | ✗ | |
| 22.93 | 10.42 | 197.1 | 573.3 | ✗ | |
| 17.2 | 7.818 | 202.8 | 430 | ✗ | |
| 11.47 | 5.212 | 208.5 | 286.7 | ✗ | |
| 6.88 | 3.127 | 213.1 | 172 | ✓ | |
| 4.3 | 1.955 | 215.7 | 107.5 | ✓ | |
| 2.867 | 1.303 | 217.1 | 71.67 | ✓ | |
| 1.72 | 0.7818 | 218.3 | 43 | ✓ | |
| 1.075 | 0.4886 | 218.9 | 26.87 | ✓ | |
| 0.688 | 0.3127 | 219.3 | 17.2 | ✓ | |
| 0.4914 | 0.2234 | 219.5 | 12.29 | ✓ | |
| 0.344 | 0.1564 | 219.7 | 8.6 | ✓ | |
| 0.2457 | 0.1117 | 219.8 | 6.143 | ✓ | |
| 0.1811 | 0.0823 | 219.8 | 4.526 | ✓ | |
| 0.1433 | 0.06515 | 219.9 | 3.583 | ✓ | |
| 0.1147 | 0.05212 | 219.9 | 2.867 | ✓ | |
| 0.09297 | 0.04226 | 219.9 | 2.324 | ✓ | |
| 0.07167 | 0.03258 | 219.9 | 1.792 | ✓ | |
| 0.05733 | 0.02606 | 219.9 | 1.433 | ✓ |
Limit of this tool. It calculates only the voltage drop from conductor resistance. It does not check ampacity, heating, overcurrent protection or any code minimum size, and it does not size the cable. That depends on the design and belongs to a licensed professional following your local electrical code.