Voltage Drop Calculator

Estimate conductor voltage drop for common copper and aluminum AWG wire sizes

This is an engineering estimate, not a wire-sizing or electrical-code approval. Verify conductor ampacity and local code requirements separately.

Your result will appear here.

About the Voltage Drop Calculator

Voltage drop is the reduction in voltage that occurs as current flows through conductor resistance. Long cable runs, high current and small conductors increase the drop and can affect equipment performance.

This calculator estimates the drop from conductor properties, length and current. Final conductor selection should still follow applicable electrical codes and equipment requirements.

How to Use the Voltage Drop Calculator

  1. Enter the circuit current.
  2. Enter one-way conductor length and select the circuit configuration expected by the calculator.
  3. Choose conductor size/material or enter resistance data.
  4. Calculate voltage drop and percentage drop, then compare it with the design limit appropriate to the installation.

How the Calculation Works

Basic voltage drop is Vdrop = I×Rtotal. Conductor resistance depends on resistivity, length and cross-sectional area.

For a two-wire circuit, the current path includes the outgoing and return conductor, so total circuit length is commonly twice the one-way run. Percentage drop = Vdrop ÷ supply voltage × 100.

Worked example

Voltage Drop Calculator Worked Example

120 V circuit with 3 V drop

Percentage drop = 3÷120×100 = 2.5%. The load would receive approximately 117 V under that operating condition.

Result guide

Understanding Your Voltage Drop Calculator Result

A percentage result helps compare voltage drop across systems with different supply voltages. Acceptable limits depend on the equipment and governing design rules.

Before you rely on the answer

Assumptions and Limitations

  • Current represents the relevant operating load.
  • Conductor material, size and circuit length are entered correctly.
  • Temperature, connections and AC reactance are excluded unless modeled.

Practical tip

Using This Result in Practice

Use the actual one-way run and confirm whether the calculator already doubles it for the return path. Accidentally doubling twice is a common source of overly pessimistic results.

Related calculations

Useful Calculations to Continue With

If you know load power but not current, calculate amps first with the Watts to Amps Calculator.

If you need to verify how voltage, current and resistance relate before evaluating conductor drop, use the Ohm's Law Calculator.

Common questions

Voltage Drop Calculator FAQ

Why does a longer wire cause more voltage drop?

Resistance increases with conductor length, so the same current produces a larger I×R drop.

Why does thicker wire reduce voltage drop?

A larger conductor cross-sectional area has lower resistance for the same material and length.

Can this calculator choose a safe wire size by itself?

No. Ampacity, temperature rating, installation method, protection and code requirements also control conductor sizing.