# Voltage Drop Calculator

> Estimate the voltage drop along a copper or aluminium cable from its size (mm² or AWG), length and current, for DC, single-phase or three-phase.

Category: Science & engineering · Applies to: Worldwide · Last updated: 2026-10-29
Interactive calculator: https://calculators.geekmidst.com/calculators/voltage-drop-calculator

Current flowing through a cable’s resistance loses some voltage before it reaches the load. The drop grows with current and length and falls as the conductor gets thicker.

Also called: Cable voltage drop calculator, Wire voltage drop calculator.

## Inputs
- System (options: DC (two wires), Single-phase AC, Three-phase AC (balanced); default single)
- Supply voltage (unit V; default 230)
- Load current (unit A; default 16)
- One-way cable length (default 30)
- Length in (options: Metres, Feet; default m)
- Conductor size as (options: mm², AWG; default mm2)
- Conductor cross-section (unit mm²; default 2.5)
- Wire gauge (options: 4/0 AWG, 3/0 AWG, 2/0 AWG, 1/0 AWG, 1 AWG, 2 AWG, 3 AWG, 4 AWG, 6 AWG, 8 AWG, 10 AWG, 12 AWG, 14 AWG, 16 AWG, 18 AWG, 20 AWG, 22 AWG, 24 AWG; default 12)
- Conductor (options: Copper, Aluminium; default copper)
- Conductor temperature (unit °C; default 20; optional)

## Results
- Voltage drop
- Drop as a share of supply
- Voltage at the load
- Conductor cross-section
- Conductor resistance
- Power lost in the cable

## Formula

```
R = ρ × L ÷ A × (1 + α(T − 20 °C));  DC and single-phase: Vd = 2 × I × R;  three-phase: Vd = √3 × I × R
```

Where:
- ρ: resistivity at 20 °C: copper 0.017241 Ω·mm²/m (1/58, 100% IACS), aluminium 0.028265 (61% IACS)
- L: one-way length (m)
- A: cross-section (mm²); AWG uses d = 0.127 mm × 92^((36 − n)/39)
- α: temperature coefficient: copper 0.00393, aluminium 0.00403 per °C
- I: load current (A)

## Worked example
1. Resistance of one conductor: 0.017241 × 30 ÷ 2.5 = 0.2069 Ω.
2. Single-phase current flows out and back: Vd = 2 × 16 × 0.2069 = 6.62 V.
3. 6.62 ÷ 230 = 2.88% of the supply, leaving 223.38 V at the load.

**Result:** 16 A over 30 m of 2.5 mm² copper drops about 6.6 V (2.9%) on a 230 V single-phase circuit at 20 °C.

## What the result means

Wiring rules set maximum drops, often a few per cent depending on the country and the type of circuit. If the result is close to your limit, use a larger conductor or a shorter route, and repeat the calculation at the cable’s operating temperature.

## Assumptions and limits
- Resistive drop only: cable reactance and load power factor are ignored, which is accurate for small and medium cables at mains frequency.
- Three-phase loads are balanced.
- Cable sizing must also meet current-carrying capacity, fault protection and installation rules set by your local wiring regulations.
- Very large conductors (above about 50 mm²) need reactance included; use the manufacturer’s mV/A/m data.

## Questions

### Why multiply by 2 for single-phase?

Current goes out along one conductor and returns along the other, so the cable length is effectively doubled. In a balanced three-phase circuit the factor is √3 instead.

### Which temperature should I use?

For a quick check use 20 °C. For a design check use the cable’s maximum operating temperature (for example 70 °C for PVC insulation), which raises copper resistance by about 20%.

## Related calculators
- [Ohm’s Law Calculator](https://calculators.geekmidst.com/calculators/ohms-law-calculator)
- [Resistor Calculator](https://calculators.geekmidst.com/calculators/resistor-calculator)
- [Length Converter](https://calculators.geekmidst.com/calculators/length-converter)

_Quantities are estimates. Confirm requirements with your engineer or supplier before ordering materials._

Source: GM¹X CalcVerse — https://calculators.geekmidst.com/calculators/voltage-drop-calculator