Voltage Drop Calculator
Estimate voltage drop for DC, single-phase and balanced three-phase circuits. Choose copper or aluminum, enter a conductor size in mm² or AWG, and see the drop in volts and percent, load-end voltage, conductor power loss, maximum one-way length and required cross-sectional area for a chosen drop target.
Circuit inputs
Use the one-way cable length. The circuit factor below accounts for the return path in DC/single-phase or the √3 relationship in balanced three-phase.
Advanced settings: target drop, energy loss & custom resistivity
How to use, formulas, worked example, FAQs & related calculators
How to Use the Voltage Drop Calculator
Choose DC, single-phase AC or balanced three-phase AC. Enter the source voltage, load current, one-way cable length, conductor material and conductor size. The tool reports voltage drop, percentage drop, load-end voltage, conductor loss, maximum one-way length at the selected drop target and the theoretical conductor area needed to meet that target.
Voltage Drop Formula
ToolVault uses a resistance-based conductor model. First, resistance per metre is calculated from resistivity and cross-sectional area:
R = ρ × L ÷ AFor DC and single-phase circuits, the outgoing and return paths produce the factor of 2:
ΔV = 2 × I × RFor a balanced three-phase circuit using line-to-line voltage:
ΔV = √3 × I × RThe percentage and load-end voltage are:
Drop % = ΔV ÷ Vsource × 100 • Vload = Vsource − ΔVWorked Example: 48 V DC Circuit
Assume a 48 V DC circuit carries 20 A over a 30 m one-way run using 6 mm² copper. With copper resistivity of 1.724 × 10⁻⁸ Ω·m:
Resistance per metre ≈ 0.002873 Ω/m.
Using the DC factor of 2, the estimated drop is approximately 3.448 V. The percentage drop is about 7.18%, so the approximate load-end voltage is 44.55 V.
This example demonstrates the resistance-only model used by the calculator. For final cable selection, manufacturer resistance/impedance data and the applicable electrical standard should be checked.
What Is Voltage Drop?
Voltage drop is the reduction in voltage between the source and load caused by the impedance of the conductors and connections. As cable length increases, current increases or conductor cross-sectional area decreases, voltage drop generally increases in a resistance-dominated model.
One-Way Cable Length
Enter the physical distance from the supply to the load. The calculator applies the appropriate circuit factor internally. For DC and single-phase, this represents the outbound plus return conductor path; for balanced three-phase, the √3 relationship is used with the entered line-to-line voltage.
AWG and mm²
AWG and metric cross-sectional area are different labeling systems. The AWG selection is converted to an equivalent cross-sectional area for the resistance calculation. Metric mode uses the entered mm² value directly. A larger conductor area generally produces lower resistance and lower voltage drop for the same material, current and length.
Maximum Cable Length and Required Area
The inverse calculations answer two different questions. Maximum length finds how far the selected conductor can run while staying below the target voltage-drop percentage. Required area finds the theoretical cross-sectional area that satisfies the same target under the same resistance model.
Lmax = Vmax ÷ (Factor × I × R per metre)Amin = Factor × I × L × ρ ÷ VmaxThese are voltage-drop constraints only. They do not prove that a conductor is suitable for ampacity, short-circuit withstand, temperature, installation method or code compliance.
Important Accuracy and Scope Notes
The default resistivity values are reference values at approximately 20°C. Actual conductor resistance changes with temperature. AC circuits can also have inductive reactance and power-factor effects, while terminals and connections add resistance. For a final design, use the cable manufacturer's resistance/impedance data and applicable electrical requirements.
Frequently Asked Questions
What is an acceptable voltage drop?
The permitted or recommended value depends on the electrical standard, system type, installation and design objective. Use the target percentage in this calculator as a design input, then verify against the applicable requirements.
Does longer cable increase voltage drop?
Yes, in the resistance-based model, voltage drop is directly proportional to one-way length when current, conductor area and material remain constant.
Does thicker cable reduce voltage drop?
Yes. Increasing conductor cross-sectional area lowers resistance, which reduces voltage drop for the same current and length.
Is this a complete wire-size calculator?
No. It calculates voltage-drop-related conductor area and length. Final wire or cable sizing must also consider ampacity, protection, installation conditions, temperature correction, fault current and the applicable electrical code.