Quick voltage drop calculator Construction tools
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Quick Voltage Drop Calculator + Cable Size Guide

Calculate voltage drop (V and %) from voltage, current, distance, and conductor conditions.

The allowable threshold is a reference Pass/Warn comparison only. Select a standard set to see cable-size candidates ranked by voltage drop.

Distance rule always visible (single: 2L / three-phase: L)
Choose preset or manual resistance
Copy the result or share it by URL

How to use

  1. Choose single-phase or three-phase, then enter voltage, current, and one-way distance.
  2. Set the conductor material (Cu/Al), temperature, and power factor.
  3. Choose a preset resistance by standard and size, or enter a manual value.
  4. Enter the reference voltage-drop % and review ΔV, ΔV%, and Pass/Warn in the result card.
  5. Copy the result or share the URL when needed.

Sample

Case 1: Single-phase 200 V / 20 A / 30 m / Cu 25 mm²

Input

Circuit=single, V=200 V, I=20 A, L=30 m, material=Cu, temp=60 °C, pf=1.0, standard=IEC, size=25 mm², allowable=3.0%

Output

ΔV=0.96V, ΔV%=0.48%, Pass

Case 2: Three-phase 400 V / 80 A / 80 m / Al 16 mm²

Input

Circuit=three-phase, V=400 V, I=80 A, L=80 m, material=Al, temp=60 °C, pf=0.9, standard=IEC, size=16 mm², allowable=3.0%

Output

ΔV=20.40V, ΔV%=5.10%, Warn

Quick formulas

  • R(T) = R(20) × (1 + α × (T - 20))
  • Single-phase (2-wire): ΔV = 2 × I × R(T) × L[km] × pf
  • Three-phase (3-wire): ΔV = √3 × I × R(T) × L[km] × pf
  • ΔV% = (ΔV / V) × 100

FAQ

Should I enter round-trip distance?

No. Enter one-way distance. Internally, single-phase uses round-trip (2L), while three-phase uses one-way distance with the √3 formula.

Does Pass mean code compliance?

No. Pass/Warn is only a reference comparison. Always prioritize regulations, internal standards, design documents, and manufacturer data.

When should I use manual resistance instead of preset?

Use manual input when you have resistance values from the manufacturer or a design sheet. Presets are approximations based on conductor cross-sectional area.

What changes when power factor is not 1.0?

In this quick model, the resistive drop is multiplied by cosφ. Reactance is not included.

Notes

  • This tool provides a reference calculation only. Prioritize applicable regulations, standards, design documents, and manufacturer data.
  • The model is simplified and resistive. It does not account for reactance, installation conditions, ampacity, short-circuit performance, or protection coordination.
  • Enter one-way distance. Single-phase uses round-trip (×2); three-phase uses one-way distance with the √3 formula.