Voltage drop calculator
Work out the voltage drop in an electrical circuit from the cable length, wire size, current or power, in single-phase or three-phase circuits, with AWG or mm² wire sizes, compare it with the usual 3% and 5% limits and see the smallest wire size that meets them.
How to useVoltage drop in the cable
Simplified formula, without reactance and with resistivity at 20 °C (68 °F). It does not replace a design by a qualified professional.
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Voltage drop calculator: what it is for
Work out the voltage drop in a cable with the simplified formula ΔV = k × ρ × L × I × cos φ ÷ S, with k equal to 2 in single-phase circuits and √3 in three-phase circuits, for copper or aluminum wire. Enter the current or power, the length in feet or meters and the wire size in AWG or mm²: the tool shows the drop in volts and in percent, compares it with the chosen limit (3%, 4% or 5%), shows the power lost in the cable and points out the smallest wire size that meets the limit, with a table of sizes.
The calculations run in your browser. The tool is an estimate and does not replace a design by a qualified professional.
How to use
- Choose the circuit type, the voltage and the conductor.
- Enter the current or power and the length.
- Choose the wire size and see whether the drop is within the limit.
When to use it
- Checking the wire size of a long run, such as for a gate opener or a pump.
- Installations in barns, workshops and outdoor areas.
- Studying for electrical exams.
Frequently asked questions
How is voltage drop calculated?
With the simplified formula, the drop in volts is ΔV = k × ρ × L × I × cos φ ÷ S, where k is 2 for single-phase circuits and √3 (1.73) for three-phase circuits, ρ is the resistivity of the conductor (0.0172 Ω·mm²/m for copper), L is the cable length in meters (one way), I is the current, cos φ is the power factor and S is the cross-section in mm². The drop in percent is ΔV divided by the circuit voltage.
What is the recommended limit?
A common recommendation, such as the informational notes in the US National Electrical Code (NEC), is a maximum of 3% on a branch circuit and 5% in total, from the service to the farthest outlet. Standards in other countries set similar limits, such as 4% for final circuits. Larger drops make motors run hot, lights dim and equipment misbehave.
Does the calculator replace an electrical design?
No. It uses the simplified formula, which ignores cable reactance and the rise in resistance with temperature. Sizing a cable also depends on its current-carrying capacity (ampacity), the installation method and the protection. Installations should be designed by a qualified professional.
Is it really free?
Yes, 100% free. No sign-up, no subscription and no usage limits. All Tools123 tools are free and run right in your browser.