Voltage Drop Calculator
Calculate AC or DC voltage drop, percentage drop, power loss and the minimum conductor size that keeps the drop within limits.
Inputs
Formula
- ρ_T = ρ₂₀ × (1 + α(T − 20))
- DC / 1-ph: ΔV = 2 × ρ_T × L × I / A
- 3-ph: ΔV = √3 × ρ_T × L × I / A
- Loss = k × I² × R, ΔV% = ΔV / V × 100
Voltage drop is the ohmic loss along the conductor and returns as heat. Because copper resistivity rises about 0.4% per kelvin, a conductor running at 70 °C has roughly 20% more resistance than at 20 °C — ignoring this under-predicts the drop. The factor 2 for single-phase accounts for the return conductor; three-phase uses √3 because line-to-line voltage is compared with per-phase drop.
Step-by-step calculation
Resistivity at temperature
ρ₂₀ × (1 + α(T − 20))
0.02058 Ω·mm²/m
Effective area
A × parallel runs
25 mm²
Voltage drop
√3 × ρ × L × I / A
5.703 V
Percentage drop
ΔV / V × 100
1.374%
Power loss
3 I²R
493.92 W
Worked example
- 50 A over 80 m in 25 mm² copper at 70 °C, three phase 415 V.
- ρ₇₀ = 0.0172 × (1 + 0.00393 × 50) = 0.02058 Ω·mm²/m.
- ΔV = 1.732 × 0.02058 × 80 × 50 / 25 = 5.70 V = 1.37% — within the 3% limit.
Assumptions
- Resistive drop only; conductor reactance is neglected, valid for sizes up to about 120 mm².
- Balanced three-phase loading and uniform conductor temperature.
Tips
- Doubling the cross-section halves the drop; doubling the length doubles it.
- Running two smaller cables in parallel is often cheaper than one very large cable.
Warnings
- For cables above 120 mm² and long runs, include reactance — the drop can be 10–20% higher than the resistive value.
Standards & references
- IEC 60364-5-52 Annex G
- IS 732:2019
- NEC 210.19(A)
Frequently asked questions
What is an acceptable voltage drop?
3% for lighting circuits and 5% for power circuits, measured from the origin of the installation, is the widely accepted limit in IEC-based codes.
Why use 70 °C instead of 20 °C resistivity?
Because a fully loaded PVC cable runs at its 70 °C rated conductor temperature, where resistance is about 20% higher than the 20 °C table value.
Does voltage drop waste energy?
Yes. The dropped voltage times the current is dissipated as heat in the cable — this calculator reports that loss in watts and kWh per year.
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