Standard explained
Voltage Drop Explained
Voltage drop is the fall in voltage along a conductor under load. BS 7671 limits it so equipment receives adequate voltage to operate safely and correctly.
Voltage drop is the reduction in voltage that occurs along a cable as current flows through it. Every conductor has resistance, so the voltage at the far end of a long, loaded circuit is lower than at its origin.
Why it is limited
If voltage drop is too great, equipment at the end of the circuit may not operate correctly — motors run poorly, lighting dims, and sensitive equipment misbehaves. BS 7671 gives recommended limits (expressed as a percentage of nominal voltage) for lighting and other uses.
What affects it
- Cable length — longer runs drop more voltage.
- Conductor size — larger conductors drop less.
- Load current — higher current increases the drop.
Where it matters most
Voltage drop is a key consideration for long circuits — large commercial and industrial sites, outbuildings, and runs to remote equipment such as EV chargers or car-park lighting. It is closely tied to cable selection and sizing: sometimes a larger cable is chosen specifically to keep voltage drop within limits.
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