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Voltage Drop Calculator

Estimate voltage drop, voltage drop percentage, and voltage at load from current, wire length, conductor material, phase, and wire size.

Nominal source voltage in volts.

Load current in amperes.

One-way conductor length in feet.

Status: initial

Results

Awaiting calculation

Electrical planning estimate

Introduction

The Voltage Drop Calculator estimates voltage loss in a conductor run from voltage, current, length, conductor material, phase type, and wire size.

Voltage drop is the reduction in voltage between the source and the load caused by conductor resistance.

Formula and method guide

Single-phase or DC

VD = 2 x K x I x L / CM

The factor 2 accounts for the outgoing and return conductor path.

Three-phase

VD = 1.732 x K x I x L / CM

Three-phase voltage drop uses the square-root-of-three factor.

Voltage drop percent

Drop % = voltage drop / system voltage x 100

Percent drop helps compare results across different voltage systems.

Inputs explained

System voltage

The nominal source voltage for the circuit.

Load current

The current in amperes carried by the conductor.

One-way length

The one-way distance from source to load in feet.

Material

Copper and aluminum have different resistance constants.

Wire size

Wire area in circular mils used by the voltage drop equation.

FAQs

What does this Voltage Drop Calculator do?
It estimates conductor voltage drop, percent drop, and voltage at the load.
What units does it use?
It uses volts, amperes, feet, and common AWG/kcmil wire sizes.
Should I enter one-way or round-trip length?
Enter one-way length. The formula applies the required phase factor.
Does it support three-phase circuits?
Yes. Select three-phase AC to use the 1.732 factor.
Does it support DC circuits?
Yes. DC uses the same round-trip factor as single-phase in this estimate.
Does copper drop less voltage than aluminum?
For the same size, current, and length, copper usually has less voltage drop.
What is circular mil area?
Circular mil area is a wire-area measure used in conductor calculations.
What is the 3% guidance?
A 3% branch-circuit drop is a common planning target, but requirements depend on the project and code context.
Can this calculator size wire for code compliance?
No. It estimates voltage drop only and does not replace code-compliant conductor sizing.
Does it check ampacity?
No. It does not check ampacity, derating, insulation, or terminal temperature limits.
Does voltage drop matter for motors?
Yes. Motors can be sensitive to voltage drop, especially during starting.
Why does longer wire increase voltage drop?
Longer conductors have more resistance, which increases voltage loss.
Why does larger wire reduce voltage drop?
Larger wire has more circular-mil area, which reduces resistance in the formula.
Is the K value exact?
No. K values are common approximations and real conductor resistance changes with conditions such as temperature.
Who should verify final electrical design?
A qualified electrician, engineer, or authority having jurisdiction should verify final design requirements.

References

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