Voltage Drop Calculator

What this calculator does

The Voltage Drop Calculator estimates how much voltage is lost along a conductor between a source and a load. Resistance in the wire converts part of the supply voltage to heat before it reaches the equipment. Long runs, small conductors, high current, and poor conductivity increase the loss.

The calculator supports DC, AC single-phase, and AC three-phase circuits, parallel conductors, custom resistance values, and common AWG wire sizes for copper and aluminum. Results include the voltage drop in volts, the drop as a percentage of the supply voltage, and the voltage that remains at the load.

How to use this calculator

1. Choose the circuit type: DC, AC single-phase, or AC three-phase.
2. Enter the supply voltage and the load current.
3. Enter the one-way distance and choose its unit.
4. Select either a custom resistance or an AWG wire size.
5. For a custom resistance, enter the value and choose the matching unit.
6. For an AWG wire, choose copper or aluminum and the wire gauge.
7. If several conductors run in parallel per phase, choose the conductor count.
8. For AC circuits, enter the power factor and click Calculate.

The default example uses 120 V, 1 A, a 500 ft one-way run, and 1.2 Ω/km. With a power factor of 0.85, the estimated drop is about 0.31 V, or 0.26% of the supply.

Voltage drop formulas

DC:             Vdrop = 2 × I × R × L
AC single-phase: Vdrop = 2 × I × R × L × PF
AC three-phase:  Vdrop = √3 × I × R × L × PF

In these equations, I is current in amperes, R is the conductor resistance per unit length, L is the one-way length, and PF is the power factor. The factor of two accounts for the outgoing and return path in a single-phase or DC circuit. Three-phase circuits use √3 instead. When conductors are installed in parallel, the resistance is divided by the number of conductors.

What the results mean

The voltage drop is the amount of potential lost in the conductors. The percentage drop compares that loss with the supply voltage; many designers aim to keep it below 3% for branch circuits and below 5% overall. The voltage at the load is the supply voltage minus the drop. A value that is too low can cause dim lights, weak heating, unreliable electronics, or overheated motors.

The resistance per kilometer lets you compare wire sizes and materials. The one-way resistance is the run resistance after length, unit, and parallel-conductor adjustments.

Assumptions and limitations

• The model uses resistance only. It does not include conductor reactance, capacitance, skin effect, or temperature correction.
• The power factor scales the resistive drop in AC modes. A PF of 1 assumes a purely resistive load.
• The AWG table is an approximation of uncoated copper and aluminum resistance at 75 °C and may differ from the exact values in the National Electrical Code.
• The calculator does not check ampacity, insulation temperature ratings, conduit fill, or code compliance.
• Results are estimates for planning. A licensed electrician should verify conductor sizing for an actual installation.

Frequently asked questions

What is a normal voltage drop?

A common design target is no more than 3% on a branch circuit and no more than 5% from the service panel to the farthest outlet. Local codes and project specifications can set different limits.

Why does three-phase use √3 instead of 2?

In a balanced three-phase circuit, the line-to-line voltage and the phase currents combine so that the effective loop factor is the square root of three rather than two. The calculator assumes a balanced load.

Does a larger wire always reduce voltage drop?

Yes, for the same material and length, a larger cross-sectional area lowers resistance and therefore lowers voltage drop. The trade-off is cost, weight, and installation difficulty.

Reference tables

Wire material resistance

AWG reference

Values are approximate at 20 °C. Use the wire gauge and material specified by local electrical code.

Source

Reference: Calculator.net — Voltage Drop Calculator.

Disclaimer

This calculator provides estimates for educational and planning purposes. It is not a substitute for engineering review, local electrical codes, or the advice of a licensed electrician.