AZCalculate logo

Engineering

Voltage Drop Calculator

Use this voltage drop calculator to estimate voltage lost over a simple conductor run from one-way length, current, resistance per meter, and system voltage.

voltage dropcablecurrentelectricalvoltage drop calculatorcable voltage drop

Calculator

Calculate instantly

Calculator Inputs

Choose what you want to calculate, enter realistic values, adjust units where shown, and use the scenario section for repeated-use planning.

Input and Formula Breakdown

The calculator uses a simplified resistance-based circuit estimate. It doubles the entered one-way length to represent the outgoing and return conductor path, then compares the drop with system voltage.

One-way cable length
30 ft(9.14 m)
Current
20 A
Resistance per meter
0.01 ohm/m
System voltage
240 V
Voltage drop
1.83 V
Voltage drop percent
0.76%

Scenario and Comparison

Current

1.83 V

Projected

21.95

Adjusted

2.01

Visual Comparison

Current result1.83
Projected usage21.95
Comparison scenario2.01

Sign in to save this calculation and access it later.

Sign inCreate account

Trust note

Important estimate

Electrical results are estimates and do not replace applicable codes, qualified design, installation, inspection, or safety review.

High risk contextDifficulty: intermediateConfidence estimate: 90/100Reviewed 2026-06-25Reviewed by: AZCalculate electrical calculator reviewCalculator v2.1 / Formula v2.13 sources listed

Formula and Explanation

Voltage drop = 2 x one-way length x current x conductor resistance per meter

The calculator uses a simplified resistance-based circuit estimate. It doubles the entered one-way length to represent the outgoing and return conductor path, then compares the drop with system voltage.

Variable descriptions

L(m)
One-way cable length: Length from source to load before the return-path multiplier is applied.
I(A)
Current: Load current through the conductor.
R(ohm/m)
Resistance per meter: Conductor resistance for the selected material, size, and temperature assumption.
V(V)
System voltage: Nominal system voltage used to calculate drop percent.

Formula Notes

  • The entered length is one-way; the formula multiplies by 2 for a simple outgoing and return path.
  • The estimate is resistance-based and does not automatically model AC reactance, power factor, conduit fill, temperature correction, or code-specific requirements.
  • Use conductor resistance data that matches the material, size, and temperature assumption being modeled.

Common uses

  • Size cables
  • Check electrical runs
  • Compare conductor options
  • Estimate simple conductor voltage drop
  • Compare conductor resistance assumptions

Assumptions

What this calculation assumes

  • The entered length is one-way and the formula doubles it for a simple return path.
  • Resistance per meter is entered by the user and should match the conductor material, size, and temperature assumption.
  • The model is a planning estimate and does not certify code compliance or installation safety.

Avoid mistakes

Quick checks before you rely on the result

  • Entering total loop length when the calculator expects one-way length.
  • Using ohms per kilometer or ohms per 1000 feet without converting to ohms per meter.
  • Treating a simplified estimate as a substitute for electrical design review.

Step-by-Step Explanation

Follow the reasoning, not only the final number.

  1. 1

    Set up the calculation

    Enter one-way cable length in meters.

    Starting with clearly defined values and units prevents the most common calculation errors.

  2. 2

    Work through step 2

    Enter the load current in amps.

    This step transforms the known values into the form required by the formula.

  3. 3

    Work through step 3

    Enter conductor resistance per meter.

    This step transforms the known values into the form required by the formula.

  4. 4

    Work through step 4

    Multiply by 2 to include a simple return path.

    This step transforms the known values into the form required by the formula.

  5. 5

    Interpret the result

    Divide the voltage drop by system voltage to calculate the percent drop.

    Compare the result with your real-world goal, such as size cables.

Worked example

Cable run example

Known values

  • One-way length: 30 m
  • Current: 20 A
  • Resistance: 0.005 ohm/m
  • System voltage: 240 V

Calculation

  1. 1. Circuit length factor is 2 x 30 m.
  2. 2. Voltage drop is 2 x 30 x 20 x 0.005 = 6 V.
  3. 3. Voltage drop percent is 6 / 240 x 100.

Result and meaning

Estimated voltage drop is 6 V, or about 2.5% of a 240 V system.

Calculator guide

About this Voltage Drop Calculator

Estimate voltage drop over cable length from current and conductor resistance. This page includes an interactive calculator, concise formula notes, worked examples, FAQs, related calculators, and practical guidance you can revisit whenever needed.

voltage dropcablecurrentelectricalvoltage drop calculatorcable voltage dropwire voltage dropelectrical cable calculatorvoltage drop percent

References

Sources used for this calculator

Official

SI units and constants

National Institute of Standards and Technology

Last checked: 2026-06-25 | Next review: 2026-12-25

Found something that does not look right?

We work hard to keep every calculator accurate and useful. If you notice a calculation error, missing option, or unclear explanation, please let us know so we can review and correct it promptly.

Report an Issue

Calculator usage

Usage information loads after the calculator is ready.

FAQ

Voltage Drop Calculator FAQs

How does the voltage drop calculator work?+

It uses Voltage drop = 2 x length x current x resistance per meter and calculates the result from the values you enter.

Can I copy or print the result?+

Yes. AZCalculate calculator pages include copy, share, and print actions.

What does the voltage drop calculator estimate?+

It estimates the voltage lost along a conductor run from one-way length, current, conductor resistance per meter, and system voltage.

Does the length input mean one-way or total circuit length?+

The input is one-way cable length. The formula doubles it to estimate the outgoing and return path in a simple circuit.

What resistance value should I enter?+

Enter the resistance per meter for the conductor size, material, and temperature assumption you want to model.

Does this calculator handle AC reactance or power factor?+

No. It uses a simplified resistance-based estimate. AC impedance, reactance, power factor, conduit conditions, and temperature effects may require a fuller design method.

Can I use this result as electrical-code compliance?+

No. It is an estimate only and does not replace applicable electrical codes, qualified design, installation, inspection, or safety review.

Why does voltage drop percent matter?+

The percent value compares the voltage lost with the system voltage, making it easier to compare different circuit voltages and conductor options.

How can I reduce voltage drop?+

Common options include shorter runs, larger conductors, lower current, higher system voltage, or a different conductor material, subject to professional review.

Why can real voltage drop differ from the estimate?+

Temperature, conductor material, installation conditions, AC effects, connections, and actual load current can change real voltage drop.

Related tools

Related Calculators

Continue with closely related tools selected from the same subject and matching calculation needs.

Engineering

Ohm's Law Calculator

Calculate voltage, current, resistance, and power from Ohm's law relationships.

PopularTrending
Engineering

Cable Size Calculator

Estimate required cable area from current, allowable density, and safety factor.

Physics

Current Calculator

Calculate current from voltage and resistance.

Physics

Voltage Calculator

Calculate voltage from current and resistance.

Engineering

Solar Panel Calculator

Estimate solar panel count from energy use, sun hours, panel wattage, and system losses.

Engineering

Power-to-Weight Ratio Calculator

Calculate vehicle or system power-to-weight ratio, required power for a target ratio, and maximum weight for the available output.

Explore all Engineering calculators