# Voltage Drop Calculator

> Calculate voltage drop step by step: Single-phase / DC: Vd = 2 × I × R.

Page: https://freecalculator.tech/tools/voltage-drop-calculator
Last reviewed: 2026-11-03

## Formula

Single-phase / DC: Vd = 2 × I × R. Three-phase (line-to-line): Vd = √3 × I × R, where R = ρ × L ÷ A for one conductor.

- The factor 2 counts the outgoing and return conductors; √3 applies to balanced three-phase line voltage.
- Resistive method only — cable reactance and power factor are ignored, which suits small conductors.
- Compare the percentage with the limit in the wiring code you follow.
- Single-phase / DC: Vd = 2 × I × R. Three-phase (line-to-line): Vd = √3 × I × R, where R = ρ × L ÷ A for one conductor.
- Inputs used: System, Supply voltage, Load current, One-way cable length, Conductor size, Conductor resistivity.

## Inputs

- System
- Supply voltage
- Load current
- One-way cable length
- Conductor size
- Conductor resistivity

## Worked example: 16 A over 30 m of 2.5 mm² copper, 230 V

Single-phase branch circuit.
1. R = 1.68 × 10⁻⁸ × 30 ÷ 2.5 × 10⁻⁶ = 0.2016 Ω.
2. Vd = 2 × 16 × 0.2016 = 6.45 V.
3. 6.45 ÷ 230 = 2.80%.

## Common questions

### What voltage drop is acceptable?

It depends on the code you work under; many guides recommend a few percent for branch circuits. Check the rule that applies to your installation.

### Why enter one-way length?

The calculator multiplies by 2 (single-phase) or √3 (three-phase) itself, so entering round-trip length would double-count.

### Does cable temperature matter?

Yes. Hot conductors have higher resistance and more drop. Enter a resistivity for the operating temperature for a closer answer.

### How do I use the Voltage Drop Calculator?

Enter the required values for System, Supply voltage, Load current, One-way cable length, Conductor size, and the other fields shown. The calculator applies the formula on this page and shows the main result with any supporting breakdown so you can check the arithmetic.

### What formula does the Voltage Drop Calculator use?

Single-phase / DC: Vd = 2 × I × R. Three-phase (line-to-line): Vd = √3 × I × R, where R = ρ × L ÷ A for one conductor. The visible formula section above lists the calculation path and the edge cases the page handles, so the result can be checked without relying on the form alone.

### Can the Voltage Drop Calculator be used for exact decisions?

Use it as a calculation aid, not as a substitute for checking the underlying rule, contract, policy, or professional advice that applies to your situation. When a result depends on local rules, personal details, prices, or dates, enter those values yourself and confirm them before acting.

## Limitations and assumptions

- Ignores reactance, so large cables at low power factor are underestimated.
- Balanced three-phase load assumed.
- Does not size the cable for current-carrying capacity.
- Results are estimates for planning and learning. Real designs, installations and bids must be checked by a qualified, licensed professional against the codes that apply where you work.
- The result depends on the values you enter for this voltage drop calculator; it does not supply missing rates, rules, prices, dates, or assumptions for you.

## Sources

- OpenStax (Rice University) — https://openstax.org/details/books/university-physics-volume-2

## Cite this page

- APA: Free Calculator. (2026, November 3). Voltage Drop Calculator. https://freecalculator.tech/tools/voltage-drop-calculator
- MLA: "Voltage Drop Calculator." Free Calculator, 3 Nov. 2026, https://freecalculator.tech/tools/voltage-drop-calculator.
- Link: Voltage Drop Calculator — Free Calculator, https://freecalculator.tech/tools/voltage-drop-calculator