What formula is used?

VD = (2 × ρ × L × I) / A — where ρ is the resistivity of the conductor material (Ω·mm²/m), L is the one-way run length in meters, I is the current in amperes, and A is the conductor cross-sectional area in mm². The factor 2 accounts for the full round-trip path (hot + neutral/return).

What resistivity values are used?

Copper: 0.01724 Ω·mm²/m (standard annealed copper). Aluminum: 0.02825 Ω·mm²/m (1350-H19 electrical grade aluminum). These are at 20 °C; actual values rise slightly at higher operating temperatures.

What is an acceptable voltage drop?

The NEC recommends no more than 3% drop on branch circuits and 5% total drop from service panel to outlet. Many designers target 2–3% on branch circuits for energy efficiency and equipment longevity.

Does this handle three-phase circuits?

This tool calculates single-phase or DC circuits (the 2× round-trip factor). For balanced three-phase circuits, replace the factor 2 with √3 (≈1.732). A dedicated three-phase tool gives more accurate results for those systems.

What AWG sizes are supported?

AWG 18 through 4/0 (0000) plus 250, 300, 350, 400, 500 kcmil are listed, covering the most common residential, commercial, and light industrial wire sizes.

Why does aluminum wire need a larger gauge?

Aluminum has about 64% of the conductivity of copper, so it has higher resistance per unit length. To carry the same current with the same voltage drop, aluminum wire must be approximately two AWG sizes larger than copper.

Voltage Drop Calculator

Enter current, wire length, conductor material, and gauge to instantly calculate voltage drop and receiving-end voltage.

Circuit Parameters

Conductor Material

Gauge System

Wire Gauge

One-Way Length (feet)

Load Current (A)

Source Voltage (V)

Unit:

Result

Enter values and press Calculate

Formula Reference

Voltage Drop

VD = 2 × ρ × L × I / A

Resistivity (ρ)

Copper: 0.01724 Ω·mm²/m

Aluminum: 0.02825 Ω·mm²/m

NEC Guideline

Branch circuits: ≤3% drop

Total (feeder + branch): ≤5% drop

Result copied!

Summary

This calculator computes the voltage drop along a conductor run using the standard formula VD = 2 × ρ × L × I / A, where ρ is resistivity, L is one-way length, I is current, and A is cross-sectional area. It supports copper and aluminum conductors in both AWG and metric (mm²) sizes, and works for single-phase and DC circuits. The result includes total voltage drop, percent drop, and the actual voltage at the load end.

How it works

Select the conductor material (copper or aluminum).
Choose the wire gauge system: AWG or metric (mm²).
Pick the wire gauge from the list.
Enter the one-way circuit length in feet or meters.
Enter the load current in amperes.
Enter the source voltage to see percent drop and receiving-end voltage.

Use cases

Size wire for residential branch circuits per NEC 210.19 recommendations.
Verify that a long feeder run stays within the 3% or 5% voltage drop limit.
Design low-voltage DC systems such as solar panels or battery banks.
Calculate wire gauge needed for motor branch circuits.
Check voltage at the end of a long extension cord or temporary power run.
Plan voltage drop for EV charging station sub-feeder runs.
Validate wire sizing for outbuildings, barns, or detached garages.
Troubleshoot dimming lights or underperforming appliances caused by excessive drop.

Frequently Asked Questions

Last updated: 2026-05-23 · Reviewed by Nham Vu