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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.

  • voltage drop
  • wire voltage drop
  • electrical voltage drop
  • conductor voltage
  • three phase voltage drop

Nominal source voltage in volts.

Load current in amperes.

One-way conductor length in feet.

Status: initial

Results

Awaiting calculation

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Voltage drop targets

Explore the voltage drop calculator result visually while keeping this page's detailed guidance and examples.

Voltage drop targets
2.4%
ExcellentUnder 3%3–5%HighCritical

Use this result well

  1. 1Verify the calculator inputs.
  2. 2Compare the key result relationships.
  3. 3Review the page guidance before acting.

Use this as planning support, not a replacement for electrical-code design.

Calculator guide

Introduction

The Voltage Drop Calculator estimates voltage loss in a conductor run from voltage, current, length, conductor material, phase type, and wire size. It also suggests listed wire sizes for 3% and 5% planning targets.

What is voltage drop?

Voltage drop is the reduction in voltage between the source and the load caused by conductor resistance. Too much drop can reduce equipment performance, dim lighting, and create motor-starting problems.

Voltage drop method

This calculator uses common circular-mil voltage drop formulas with copper and aluminum K constants. It reports voltage drop, percent drop, voltage at the load, target comparisons, maximum length, and nearby wire-size effects.

Variable explanations

Understand what each input and result means before calculating.

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.

Phase type

DC and single-phase use a round-trip factor; three-phase uses the square-root-of-three factor.

Material

Copper and aluminum have different resistance constants.

Wire size

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

Planning target

Common design references use 3% for branch circuits and 5% combined feeder plus branch as planning guidance.

Reviewed by the Calculator.org.in Editorial Team

Formula behavior, validation cases, explanatory examples, and cited sources are checked before publication. This review supports educational accuracy and is not a substitute for qualified professional advice.

Last reviewed: 2026-08-03

Review process

Formula guide

See the calculation logic, variable definitions, and practical meaning.

Single-phase or DC

VD = 2 x K x I x L / CM

  • K is conductor resistance constant.
  • I is current.
  • L is one-way length.
  • CM is circular mil area.

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

Three-phase

VD = 1.732 x K x I x L / CM

  • 1.732 is the square root of 3.

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

Voltage drop percent

Drop % = voltage drop / system voltage x 100

  • System voltage is the nominal source voltage.

Percent drop helps compare results across different voltage systems.

Required circular mils

CM = phase factor x K x I x L / allowable voltage drop

  • Allowable voltage drop is system voltage x target percent.

This is used to suggest the smallest listed conductor that meets a target.

Maximum one-way length

L = allowable voltage drop x CM / (phase factor x K x I)

  • Uses the selected wire size and target voltage drop.

This estimates how far the selected conductor can run while staying under the target.

Voltage at load

Load voltage = system voltage - voltage drop

  • System voltage is nominal source voltage.

This estimates voltage available at the equipment.

Material comparison

Copper and aluminum use different K values

  • Copper K = 12.9, aluminum K = 21.2 in this planning formula.

For the same size, current, and length, aluminum usually has more voltage drop.

Worked examples

Follow realistic inputs through the calculation step by step.

1

Worked example

120 V branch circuit

  1. 1System voltage is 120 V, load current is 20 A, and one-way run is 100 ft.
  2. 2Using 10 AWG copper, voltage drop is about 4.97 V.
  3. 3Voltage drop percent is about 4.14%, so the selected wire is above a 3% planning target.
2

Worked example

Three-phase feeder

  1. 1Three-phase uses the 1.732 factor rather than the round-trip factor of 2.
  2. 2Larger conductors reduce drop by increasing circular-mil area.
  3. 3Compare the target table before selecting a final conductor.
3

Worked example

Aluminum conductor comparison

  1. 1Aluminum uses a higher K value than copper.
  2. 2For the same size, current, and length, aluminum usually has more voltage drop.
  3. 3The material comparison table shows the selected size with both materials.
4

Worked example

Maximum length check

  1. 1The max-length output estimates how far the selected conductor can run at a 3% target.
  2. 2If the actual run is longer, compare a larger conductor or revise the design.

Common mistakes

Avoid these common input and interpretation errors.

Using round-trip length

Enter one-way length. The formula applies the phase factor internally.

Confusing ampacity and voltage drop

This calculator estimates voltage drop only. It does not size conductors for ampacity.

Ignoring installation conditions

Temperature, terminals, insulation, raceway fill, harmonics, and motor starting can affect real installations.

Treating a planning estimate as design approval

Electrical design should be reviewed by qualified professionals and applicable codes.

Assuming the 3% target is always mandatory

Voltage drop notes are commonly treated as design recommendations, while project specifications and local rules can vary.

Selecting wire only by voltage drop

Final conductor size must also satisfy ampacity, temperature, terminal, grounding, and installation rules.

Ignoring equipment startup current

Motors and compressors can draw higher current during starting, which may make voltage drop more important.

Frequently asked questions

Quick answers to the questions users ask most often.

What does this Voltage Drop Calculator do?
It estimates conductor voltage drop, percent drop, voltage at the load, suggested wire size for planning targets, and maximum one-way run length.
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.
What is the 5% guidance?
A 5% total drop from feeder plus branch circuit is a common planning reference.
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.
How does the recommended wire size work?
It calculates the circular mil area needed to stay under the selected target and chooses the smallest listed AWG or kcmil size that meets it.
What is maximum one-way length?
It is the estimated longest one-way run for the selected conductor while staying within a 3% voltage drop target.
Is the K value exact?
No. K values are common approximations and real conductor resistance changes with conditions such as temperature.
Why might a manufacturer calculator give a different result?
Some tools use NEC resistance/reactance tables, temperature corrections, power factor, or manufacturer-specific conductor data.
Should I use this for solar or battery DC circuits?
It can estimate DC voltage drop, but final design should also account for equipment manuals, protection devices, temperature, and applicable code.
Can I use this for service or feeder design?
Use it for planning only. Feeders and services need full code review, ampacity checks, grounding, and local authority requirements.
Who should verify final electrical design?
A qualified electrician, engineer, or authority having jurisdiction should verify final design requirements.

References

Sources used to support the calculator guidance.

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Version history

A transparent record of calculator content updates.

Updated 2026-08-03
  • 1.1.0 · 2026-08-03

    Added recommended wire sizes for 3% and 5% targets, max run length, target comparison, wire-size comparison, material comparison, and expanded design guidance.

  • 1.0.0 · 2026-07-06

    Initial voltage drop calculator release.