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Voltage Drop Calculator

Estimate voltage drop for electrical wire runs by conductor material, length, current, and voltage so you can choose safer circuit designs.

Use calculator Open circuit simulator
Professional workflow

Check whether a wire run stays within an acceptable voltage drop range before you commit to a conductor size.

Vdrop = 2 × K × I × D / CM · %drop = Vdrop / Vsystem × 100
  • Built for branch-circuit and feeder planning where long runs can cause nuisance trips, dim lights, or poor motor starting.
  • Keeps voltage, current, conductor material, and distance visible together so the tradeoff is easy to understand.
  • Best used before final ampacity/code checks, not as a replacement for NEC/IEC requirements.

Calculate voltage drop in electrical cables

💡 Calculates voltage drop considering temperature and material

Use one-way conductor length; the calculator accounts for the return path in the formula.
If the percentage is high, try a larger conductor before changing the load assumptions.
Use realistic operating current rather than breaker size when checking actual voltage drop.

Formula: VD = (2 × ρ × I × L) / A

ρ adjusted for temperature: ρ(T) = ρ(20°C) × [1 + α × (T - 20)]

Acceptable: ≤ 5% voltage drop

Inputs to prepare
  • System voltage
  • Load current in amps
  • One-way wire run length
  • Copper or aluminum conductor
  • Acceptable voltage-drop percentage
Worked example

Example: 20 A load on a 120 V branch circuit

Given
  • • 120 V single-phase circuit
  • • 20 A load current
  • • 75 ft one-way copper run
Steps
  1. 1. Enter 120 V, 20 A, 75 ft, and copper as the conductor material.
  2. 2. Compare the calculated voltage drop percentage against a 3% planning target.
If the result is above 3.6 V on a 120 V circuit, evaluate a larger conductor or a shorter run.
Common use cases
  • Long lighting or receptacle circuits
  • Detached garage or outdoor feeder planning
  • Motor or pump circuits with starting sensitivity
How to read results
  • Around 3% or less is commonly used as a branch-circuit design target.
  • A higher percentage means the load receives less voltage and may run hotter or less efficiently.
  • If the result is high, compare a larger conductor, shorter run, or lower current design.
Code and safety notes
  • Voltage drop recommendations are separate from mandatory ampacity rules.
  • Temperature, conduit fill, bundling, insulation type, and local code can change the final conductor choice.
  • Use manufacturer data and authority-having-jurisdiction guidance for real installations.

Frequently asked questions

What is voltage drop?

Voltage drop is the loss of voltage along a conductor caused by resistance in the wire.

Why does voltage drop matter?

Excessive voltage drop can reduce equipment performance, cause heating, and indicate poor conductor sizing.

Does material matter?

Yes. Copper and aluminum have different resistance values, so the same current and length can produce different drops.

Is this NEC compliant?

Use the output as a planning aid and verify final installations with NEC or your local electrical code.