Wire Voltage Drop Calculator
Enter the wire size, run length, current and supply voltage. Choose round trip when the current goes out and back, or one way when you only want a single conductor. For learning and design checks, not safety certification.
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Formula
- Conductor length:
L = run length × 2for a round trip, or× 1for one way (1 ft = 0.3048 m) - Resistance:
R = ρ × L ÷ A, with ρ at 20 C: copper 1.7241e-8 ohm m, aluminium 2.65e-8 ohm m - AWG area:
d = 0.127 mm × 92^((36 − n) ÷ 39),A = π ÷ 4 × d² - Drop:
V_drop = I × R, percent= V_drop ÷ V_supply × 100
Worked example
Copper 2.5 mm², 20 m run, round trip, 10 A on 230 V: L = 40 m, R = 1.7241e-8 × 40 ÷ 2.5e-6 = 0.2759 ohm, drop = 10 × 0.2759 = 2.759 V, which is 1.20% of 230 V. The load sees 227.24 V and the wire wastes 27.6 W.
Frequently asked questions
How do I calculate voltage drop in a wire?
Find the resistance R = resistivity x conductor length / area, then multiply by the current. Copper, 2.5 mm2, 20 m round trip (40 m) at 10 A: R = 0.2759 ohm, so the drop is 2.76 V.
What is the difference between one way and round trip?
Current flows out on one conductor and back on another, so a 20 m run has 40 m of conductor. Round trip doubles the length and the drop; one way counts the run once (1.38 V in the same example).
What resistivity values are used?
At 20 C: copper 1.7241e-8 ohm m and aluminium 2.65e-8 ohm m. Hot wires have higher resistance, so real drops are a little larger than shown.
What voltage drop is acceptable?
Many designers aim for a few percent at most, but limits depend on the equipment and local rules; 2.76 V on 230 V is 1.2%. This tool is for learning and design checks, not safety certification.