Voltage Drop Calculator

Calculate voltage drop for electrical cables quickly and accurately. Supports single-phase and three-phase circuits with copper or aluminium conductors.

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* Disclaimer: This calculator is provided for educational and estimation purposes only. Always consult a licensed electrical professional and adhere to your local electrical codes and standards before making any decisions based on these calculations.

How to use this calculator

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.

Formulas

1. R = ρ × L / A

R = ρ × L / A

ρ (rho): 0.0175 Ω·mm²/m (Cu) / 0.0282 Ω·mm²/m (Al)
L: Cable Length
A: Cable Size (mm²)

2. Single Phase

Vd = 2 × I × R

3. Three Phase

Vd = √3 × I × R

4. Percentage Drop

Drop % = (Vd / V) × 100

Worked Example

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%

3.80%, above the common 3% guideline

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%

2.37%, within the 3% guideline

A 16 mm² cable is adequate for this run.

Copper vs Aluminium

Property	Copper	Aluminium
Resistivity at 20°C	0.0175 Ω·mm²/m	0.0282 Ω·mm²/m
Voltage drop at same size	Lower	Higher
Size needed for same drop	Smaller	Substantially larger
Weight and cost	Heavier, usually costlier	Lighter, often cheaper
Connections	Standard	Requires anti-oxidant paste to prevent loosening

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.

Frequently Asked Questions

What is an acceptable voltage drop?

A common guideline is 3% for lighting circuits and 5% for other loads, but the limit depends on your local electrical code.

How does cable length affect voltage drop?

Voltage drop increases proportionally with cable length. As the cable gets longer, its total resistance increases, causing a greater loss of voltage along the run.

Should I use copper or aluminium?

Copper is a better conductor, meaning you can use a smaller cable for the same voltage drop. Aluminium is cheaper and lighter but has higher resistance, requiring a larger cross-sectional area to achieve the same performance.

What happens if the voltage drop is too high?

Equipment at the end of the cable receives less voltage than it needs. Lights may dim, motors can run hot or struggle to start, and electronics may behave unreliably. The lost voltage is also wasted as heat in the cable.

Why does single phase multiply by 2?

Current flows to the load and back again, so it passes through the cable twice. That is why single phase uses Vd = 2 × I × R, where R is the resistance of one conductor, and why you enter only the one-way length.

Which voltage should I enter for a three-phase system?

Enter the line-to-line voltage, such as 400 V or 415 V, depending on your supply. The percentage drop is calculated against the value you enter.

Is the cable length one-way or round-trip?

Enter the one-way distance from the supply to the load. The formulas already include the return path.

Does this calculator include temperature, power factor or reactance?

No. It uses conductor resistance at 20°C and ignores power factor and reactance. A loaded cable runs warmer than 20°C and its resistance rises, so treat the result as an estimate and verify final designs with local electrical standards.

Voltage Drop Calculator

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Voltage Drop

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Current Result

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Power Triangle Results

How to use this calculator

Choose the phase

Choose the conductor

Enter the supply voltage

Enter the load current

Enter the cable length

Select the cable size

Formulas

1. R = ρ × L / A

2. Single Phase

3. Three Phase

4. Percentage Drop

Worked Example

Single phase copper

Three phase copper

Copper vs Aluminium

Increase the cable size

Shorten the distance

Reduce the load

Frequently Asked Questions

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