How do I calculate voltage drop?

Voltage drop is calculated from the circuit current and the resistance of the conductor over the cable run.

Why does cable length affect voltage drop?

Because longer conductors have more resistance, which increases the voltage lost along the run.

Does conductor size affect voltage drop?

Yes. Larger conductors have lower resistance, so they usually produce less voltage drop over the same distance.

Can this calculator show voltage drop as a percentage?

Yes. Voltage drop percentage is shown in the results and is calculated by dividing the voltage drop by the supply voltage.

Does material type matter?

Yes. Different conductor materials have different resistivity values, so copper and aluminium do not behave the same way.

Does this calculator replace electrical design checks?

No. It is an estimating tool only. Final design, cable selection, and compliance checks should be confirmed separately.

Voltage Drop Calculator

Use this voltage drop calculator to estimate the voltage lost along a cable run. It is useful for planning cable sizes, checking long runs, and understanding how cable length, conductor size, load current, and system type affect the final voltage at the load. This calculator follows the voltage-drop limits set out in BS 7671 (IET Wiring Regulations, 18th Edition).

Voltage (V)

Load Current (A)

One-way Length

Power Factor

Material

Ambient Temp (°C)

Size Standard

Conductor Size

Auto-size conductor for target drop

Your Results

Calculation Summary

Voltage Drop

2.76V22.99%

End Voltage

9.24V

Power Loss

27.58 W

Conductor Size

2.5 mm²

Status

Optimal

Calculations are based on resistance data and do not account for reactance in long AC runs. Always verify with local electrical codes.

Note: this tool sizes cable for VOLTAGE DROP only. It does not perform full current-carrying-capacity sizing, which must also account for installation method, grouping, ambient temperature and other correction factors under BS 7671. Always confirm the final cable size against the full BS 7671 requirements or with a qualified electrician.

How this calculator works

This calculator estimates the conductor resistance over the cable run and multiplies it by the circuit current to give the voltage drop. The tool uses resistance data based on conductor material and size, and applies a temperature correction to account for the ambient temperature entered. For DC and single-phase AC circuits, the one-way cable length is doubled internally to account for the return path. For three-phase circuits, a √3 factor is used instead. Power factor is applied for AC calculations. The tool supports copper and aluminium conductors in both metric mm² sizes and AWG/kcmil, and also provides an auto-size mode that finds the smallest conductor meeting a target drop percentage.

Formulas used

DC circuit

voltage drop = 2 × current × resistance per unit length × one-way length

Single-phase AC circuit

voltage drop = 2 × current × resistance per unit length × one-way length × power factor

Three-phase AC circuit

voltage drop = √3 × current × resistance per unit length × one-way length × power factor

Derived outputs

voltage drop % = (voltage drop ÷ supply voltage) × 100

end voltage = supply voltage − voltage drop

power loss (W) = current² × total resistance

Resistance is derived from conductor size (metric mm² or AWG/kcmil) and corrected for ambient temperature. The calculator uses resistance data only and does not account for inductive reactance in long AC runs.

Worked example

For a DC circuit, the tool takes the one-way cable length, load current, conductor material, and conductor size, then estimates the circuit resistance including the return path (2× one-way length). The resulting voltage drop is the current multiplied by that total resistance. The tool then shows voltage drop in volts, the drop as a percentage of the supply voltage, the end voltage at the load, and the power lost in the cable as heat. If the auto-size mode is enabled, the calculator works through available conductor sizes from smallest to largest and selects the first one that keeps the drop within the target percentage. The exact values depend on the conductor size selected, material resistivity, and temperature entered.

Why voltage drop matters

Voltage drop matters because excessive loss along the cable run can reduce equipment performance and affect system efficiency. Long cable runs, high current, and undersized conductors increase the problem, so an early estimate helps with cable selection and layout planning. This tool is for estimation and planning only and does not replace detailed electrical design or compliance checks.

Frequently asked questions

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Disclaimer: This calculator is for planning and estimating only. Actual voltage drop depends on conductor size, material, route length, current, system type, temperature, and calculation method. Always confirm final requirements before installation.