Voltage drop is the reduction in electrical potential between the supply and the equipment at the end of a cable run. A small drop is unavoidable, but an excessive drop can affect performance and waste energy as heat.
This guide explains the basic calculation for common UK circuits. It is intended as a design aid, not a substitute for a complete cable-sizing assessment.
The information you need
Before calculating voltage drop, collect the circuit voltage, design current, one-way route length, conductor material and cable cross-sectional area.
- Supply voltage in volts
- Load or design current in amperes
- One-way cable length in metres
- Cable cross-sectional area in mm²
- Circuit type: DC, single-phase or three-phase
Apply the correct circuit formula
For DC and single-phase circuits, the current travels through an outgoing and return conductor, so the one-way length is multiplied by two. Three-phase circuits use the square-root-of-three relationship.
Cable resistance rises with temperature. For a conservative estimate, use the likely operating temperature rather than assuming the conductor remains at 20°C.
Check the percentage result
Divide the calculated voltage loss by the nominal supply voltage and multiply by 100. This makes it easy to compare circuits operating at different voltages.
BS 7671 guidance commonly uses 3% for lighting and 5% for other circuits supplied directly from a public low-voltage system. Equipment requirements can justify a lower design target.
Do not stop at voltage drop
A cable that passes the voltage-drop check may still be unsuitable. Confirm current-carrying capacity, installation method, grouping, ambient temperature, fault protection and the requirements of the connected equipment before making a final selection.
This article provides general information only. Electrical installation design and verification should be completed by a competent person using the current regulations and manufacturer data.