How to Use the Voltage Drop Calculator
Choosing the right cable size is crucial for safety and efficiency. This free voltage drop calculator gives instant estimates. Follow these six steps.
- 1
Choose the phase
Select single phase (typically 120 V or 230 V for homes) or three phase (commonly 400 V or 415 V for commercial and industrial supplies).
- 2
Choose the conductor
Pick copper or aluminium. Copper has lower resistance, so it gives less voltage drop for the same size.
- 3
Enter the supply voltage
Type the voltage at your source in volts (V).
- 4
Enter the load current
Input the maximum expected current in amps (A). If you only know the wattage, use the Watts to Amps calculator first.
- 5
Enter the cable length
Enter the one-way distance from the power source to the load, in metres or feet. The formulas already account for the return path.
- 6
Select the cable size
Choose the cross-sectional area in mm². The results update instantly.
How Voltage Drop Is Calculated
Voltage drop occurs because every conductor has natural electrical resistance. The longer the wire or the higher the current, the greater the drop.
1. Resistance of the conductor
- ρ (rho)
- Resistivity at 20°C: 0.0175 Ω·mm²/m for copper and 0.0282 Ω·mm²/m for aluminium.
- L
- One-way cable length in metres.
- A
- Cross-sectional area of the cable in mm².
2. Single phase
Multiplying by 2 accounts for the outgoing and return conductors.
3. Three phase
√3 is approximately 1.732.
4. Percentage drop
Divide the voltage drop by the supply voltage and multiply by 100.
Worked Examples
Two typical cases, calculated step by step with the formulas above.
Single phase copper
- System230 V, 20 A, single phase
- CableCopper, 50 m, 4 mm²
- Resistance0.0175 × 50 / 4 = 0.21875 Ω
- Voltage drop2 × 20 × 0.21875 = 8.75 V
- Percentage(8.75 / 230) × 100 = 3.80%
Step up to 6 mm² to bring the drop to about 2.54%.
Three phase copper
- System400 V, 50 A, three phase
- CableCopper, 100 m, 16 mm²
- Resistance0.0175 × 100 / 16 = 0.109375 Ω
- Voltage drop1.732 × 50 × 0.109375 = 9.47 V
- Percentage(9.47 / 400) × 100 = 2.37%
A 16 mm² cable is adequate for this run.
Copper vs Aluminium
Choosing between the two is a trade-off. Copper has lower resistivity, while aluminium is lighter and often more cost-effective for long, high-current runs, but needs a larger cross-section for the same voltage drop.
| Property | Copper | Aluminium |
|---|---|---|
| Resistivity at 20°C | 0.0175 Ω·mm²/m | 0.0282 Ω·mm²/m |
| Voltage drop at the same size | Lower | Higher |
| Size needed for the same drop | Smaller | Substantially larger |
| Weight and cost | Heavier, usually costlier | Lighter, often cheaper |
| Connections | Standard | Need care (anti-oxidant paste) to prevent loosening |
Common Mistakes and How to Reduce Drop
If your voltage drop is too high, you have three main ways to fix it:
Increase the cable size
A thicker cable has lower resistance, so the voltage drop falls.
Shorten the distance
Reroute the cable so the physical run is shorter.
Reduce the load
Splitting the load into several circuits lowers the current on each wire.