Calculation reviewed: 4 October 2026 · Engineering Desk Tools
How to use the Voltage drop calculator
Estimate cable voltage drop, percentage drop and load-end voltage for copper or aluminium conductors. Start with measurements or documented values for the equipment and reporting period you actually want to assess. The calculator does not fill in a typical installation for you. Its blank fields help keep a demonstration value from being mistaken for your own operating data.
Choose the required options, enter the values in the labelled units and select Calculate. If a field is rejected, check its range and unit before changing it. The result is accompanied by the equation and assumptions so you can decide whether it answers your engineering question.
Inputs and measurement basis
- Supply. Three-phase uses the √3 factor and line-to-line voltage; single-phase uses a factor of 2 for the out-and-back conductors.
- Conductor material. Pick the conductor metal from the cable data sheet. Aluminium has about 64 percent higher resistivity than copper.
- Load current (A). Use the design or measured running current of the circuit, not the breaker rating.
- One-way cable length (m). Measure from the source to the load along the cable route, one way only.
- Conductor cross-section (mm²). Conductor cross-sectional area in mm², from the cable data sheet. Do not enter AWG numbers. The value must be greater than zero.
- Supply voltage (V). Nominal supply voltage at the source; line-to-line for three-phase. The value must be greater than zero.
- Conductor operating temperature (°C). Conductor temperature in service, for example 70 °C for a fully loaded PVC cable, or ambient for a lightly loaded one. The allowed maximum is 160.
Record the measurement source and operating condition. Retain unrounded values for verification.
Method and interpretation
Conductor resistance rises with temperature, so a cable at its operating temperature drops more voltage than a cold one. The calculator corrects the 20 °C resistivity to the temperature you enter. For three-phase the √3 factor converts the per-conductor drop to the line-to-line drop; for single-phase the current flows out and back, so the factor is 2. In larger cables conductor reactance adds to the resistive drop, and a lagging power factor makes it matter more, so use manufacturer impedance data for big feeders.
ρ(T) = ρ20 × (1 + α × (T − 20)); R = ρ(T) × length / area. Single-phase drop = 2 × I × R. Three-phase drop = √3 × I × R. Drop (%) = drop / supply voltage × 100. Copper: ρ20 = 0.017241 Ω·mm²/m, α = 0.00393 per °C. Aluminium: ρ20 = 0.028264 Ω·mm²/m, α = 0.00403 per °C.
Enter the one-way length from source to load. The result is resistive drop only: conductor reactance, power factor, parallel cables, harmonics and voltage drop during motor starting are not included, and reactance becomes more important in larger cables. Three-phase voltage is line-to-line. The permitted drop depends on the applicable code, equipment tolerance and project specification. This is not a cable size or protection recommendation.
Worked example
This example does not populate the form. Enter your own installation data.
Calculated result: 2.382 % voltage drop. Resistive drop only. Reactance, power factor and starting current are not included. Compare the percentage with the limit in your applicable code or project specification.
Check the arithmetic independently using the displayed equation. Retain intermediate precision and compare the final value with your equipment documentation.
Quick-reference table
These are illustrative values, not certified ratings or manufacturer data. Use the form for your own conditions.
Common mistakes
Entering the total (out and back) length instead of the one-way length; mixing mm² and AWG sizes; forgetting that a three-phase limit is expressed against line-to-line voltage; judging a cable by the drop at running current when starting current may cause a deeper dip. Before relying on the output, check the decimal format, unit scale and source of each value. Recheck unusual results against the formula.
Frequently asked questions
Are the example values used automatically?
No. Inputs remain blank when the tool opens or when you clear them. The worked example and table are reading material below the calculator. Enter the actual values you want to check each time, and label saved reports so their context is not lost.
What does the result mean?
The result is the voltage lost along the cable at the entered current, as volts and as a percentage of the supply. Compare the percentage with the limit in your code or specification, and check the load-end voltage against the equipment tolerance.
Can I use a zero or a negative value?
The allowed range follows the meaning of each input. A divisor needs a positive value; other fields may allow zero. Check the form labels and error message rather than forcing an invalid value. A rejected input must be corrected before a result can be shown.
Is this a final design or equipment approval?
No. Enter the one-way length from source to load. The result is resistive drop only: conductor reactance, power factor, parallel cables, harmonics and voltage drop during motor starting are not included, and reactance becomes more important in larger cables. Three-phase voltage is line-to-line. The permitted drop depends on the applicable code, equipment tolerance and project specification. This is not a cable size or protection recommendation. Use the calculation as support for a checked decision, and review equipment documentation and applicable requirements before changing an installation or operating setting.
How can I save or share a calculation?
Calculate first, then use Copy result or Print report. Some maintenance tools also provide a CSV summary. Reports contain your own entered data, so review them before sharing. You can share the public page URL without including plant records or confidential measurements.
How do I report an unexpected result?
Use the Contact page and include the tool URL, input values, units, expected output and the reference you used. Explain the measurement basis. Exclude confidential information.
Related tools and records
Use related calculations with consistent units, periods and equipment scope.
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