What is the maximum voltage drop allowed under BS 7671?

Regulation 525 limits voltage drop to 3% for lighting and 5% for other uses from origin to load, with an overall 5% limit — confirm the circuit type.

How do I calculate voltage drop for a cable run?

Enter cable CSA, length, load current, and supply voltage. The calculator applies the appropriate mV/A/m factor for copper or aluminium conductors.

Does power factor affect voltage drop?

Yes for AC circuits. Enter the load power factor where known — unity PF is a conservative simplification for resistive loads.

BuildFlo Design Guide · Electrical · BS 7671

Voltage drop calculator for cable sizing

Calculate cable voltage drop and check conductor CSA per BS 7671. Free UK voltage drop calculator for single and three-phase circuits.

What this calculator does

Voltage drop helps UK building services engineers with minimum csa for voltage-drop limits, or check a proposed cable. The calculation follows BS 7671 methods used in everyday MEP design — enter your project inputs below for a first-pass result.

Whether you searched for "voltage drop", "voltage drop calculator", or "how to voltage drop", this page gives you a working tool plus the context to use it on a real job.

When to use it

Use this for early-stage sizing and sanity checks on UK building services work. It is not a substitute for issued calculations — verify against the project specification, drawings, and relevant standard before sign-off.

How to use it

1. Fill in the inputs for your scenario (defaults are starting points only). 2. Click Calculate — results include the method and assumptions where applicable. 3. Verify against the project specification and relevant standard before issuing.

Typical output unit: %.

Tips

Defaults are indicative — replace with project-specific values from the architect, structural, or MEP spec.
For issued calculations, save results in your BuildFlo project and attach to the calculation folder.
Need the regulation citation? Open BuildFlo chat in the app and ask about voltage drop under UK standards.

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Frequently asked questions

What is the maximum voltage drop allowed under BS 7671?: Regulation 525 limits voltage drop to 3% for lighting and 5% for other uses from origin to load, with an overall 5% limit — confirm the circuit type.
How do I calculate voltage drop for a cable run?: Enter cable CSA, length, load current, and supply voltage. The calculator applies the appropriate mV/A/m factor for copper or aluminium conductors.
Does power factor affect voltage drop?: Yes for AC circuits. Enter the load power factor where known — unity PF is a conservative simplification for resistive loads.