Voltage Drop Calculator
Work out the voltage drop along a cable run.
Voltage drop
7.27V
- Percentage drop
- 3.16 %
- Voltage at load
- 222.73 V
Single-phase, counting both the line and neutral conductors.
v1.0 · Reviewed September 2026
How this is worked out
Drop is 2 × current × one-way length × resistance per metre. The factor of two accounts for the current travelling out along the line conductor and back along the neutral.
Worked example
- 20 A
- 25 m run
- 0.00727 Ω/m
- 230 V
2 × 20 × 25 × 0.00727
7.27 V, a 3.16% drop, leaving 222.73 V
Assumptions and limits
- Single phase, using the one-way route length. Enter the cable run, not double it.
- Resistance per metre depends on conductor size and temperature. Published mV/A/m figures for the cable and its operating temperature are the reliable source.
- BS 7671 gives recommended limits of 3% of nominal voltage for lighting and 5% for other uses on a public supply. This calculator reports the percentage but does not judge compliance, and it is not a substitute for design by a competent electrician.