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UK voltage drop calculator

Enter the design current, one-way route length and the appropriate tabulated mV/A/m value for the cable and circuit you are checking. The calculator shows the circuit drop, percentage, total allowance including any upstream drop and the maximum route length within the selected budget.

For example, 230 V or 400 V as applicable to the calculation.
Use the design current appropriate to the circuit, not automatically the protective-device rating.
Use the route length once when using a tabulated mV/A/m value.
Enter the value from the cable data/table applicable to your cable, circuit arrangement and design conditions.
The 3% and 5% presets are the BS 7671 Appendix 4 recommended values for public LV supplies; verify the value applicable to your installation.
Optional. Useful where a submain or earlier circuit section has already used part of the total voltage-drop allowance.
The calculator does not choose the cable.The mV/A/m input must come from the cable data or table appropriate to the selected cable, circuit arrangement and design conditions.
Include the whole installation path.If an upstream submain has already used part of the voltage-drop allowance, enter that percentage so the remaining circuit budget is shown.
Use and limitations: This is a design aid, not a cable-selection or compliance tool. It does not apply current-carrying-capacity factors, operating-temperature corrections, reactive components, harmonics, starting conditions, fault protection or equipment-specific voltage requirements. Check the selected mV/A/m value and complete the project-specific design using the current standards and manufacturer information.
Reference: IET Wiring Matters describes the BS 7671 Appendix 4 recommended maximum voltage-drop values of 3% for lighting and 5% for other uses for low-voltage installations supplied directly from a public low-voltage distribution system, and gives the voltage-drop calculation in the form (mV/A/m) × Ib × length. IET: voltage-drop design article · IET: current BS 7671 information
Always verify the edition, amendments, cable data and design assumptions applicable to the project. This calculator is not a substitute for the current standard or a full cable-selection calculation.

How this UK voltage drop calculator works

Enter the one-way circuit route length, design current and the mV/A/m value that applies to the cable and circuit arrangement you are checking. The tool calculates the circuit voltage drop in volts and percent, then adds any upstream percentage you enter so you can see how much of the selected design budget remains.

Why the mV/A/m value is entered manually

The calculator deliberately does not guess a cable type or size. The designer selects the appropriate tabulated or manufacturer value for the cable, installation arrangement and design conditions. That keeps the calculation transparent and avoids presenting a partial cable-selection check as a complete compliance result.

Use the result as one part of the electrical design

Voltage drop is only one design consideration. Current-carrying capacity, protective-device operation, fault protection, thermal conditions, starting behaviour, harmonics and equipment requirements may also affect the final cable selection.

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