Voltage drop calculator
Calculate the voltage drop along a copper cable run for single-phase or three-phase circuits.
How it is calculated
Single-phase: ΔV = 2 × ρ × I × L ÷ A. Three-phase: ΔV = √3 × ρ × I × L ÷ A. Copper resistivity ρ = 0.01724 Ω·mm²/m at 20 °C. Percent drop = ΔV ÷ V × 100.
Worked example
Three-phase, 50 A, 100 m, 16 mm², 400 V: ΔV = 1.732 × 0.01724 × 50 × 100 ÷ 16 = 9.33 V, or 2.33 %.
Steps
- Select single-phase or three-phase.
- Enter the load current and the one-way cable length.
- Enter the conductor size and the supply voltage, then compare the percent drop with your limit.
Frequently asked questions
What is a typical limit?
Many codes recommend 3 % for branch circuits and 5 % total from supply to the farthest outlet. Check your local code.
Is this for aluminum cable?
No. Aluminum resistivity is about 1.6 times copper, so the drop is higher for the same size.
Does temperature matter?
Yes. Resistance rises with temperature. This calculator uses the 20 °C value, so hot cables drop slightly more.
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References
Copper resistivity 0.01724 Ω·mm²/m at 20 °C (IEC 60228 conductor resistance). Voltage drop limits from NEC NFPA 70 informational notes and local code.
Estimates for information only. Verify critical values with a qualified engineer or your supplier standards.