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Voltage Drop Calculator
BS 7671:2018+A4:2026Voltage drop in final circuits and distribution systems — Appendix 4, IET Design Guide Section 5.
Circuit
Using tabulated mV/A/m per IET Design Guide Section 5.4
Apply when the designer controls the supply voltage — e.g. a private transformer. Compliance is then judged on the actual voltage at the load against a 216.2 V floor.
Cable Selection
Installation Conditions
Maximum Permitted
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Limit (Table 4Ab (a))
5%
Voltage Drop
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Enter values to calculate
Complete the circuit and cable details to calculate voltage drop.
Worked example: 32 A over 28 m of 6 mm²
A single phase 230 V circuit from a public supply, design current 32 A at power factor 0.85, in 6 mm² copper multicore cable with 70 °C thermoplastic insulation, clipped direct (installation method 20, reference method C), 28 m long, at 35 °C ambient.
| Step | Source | Value |
|---|---|---|
| Voltage drop per amp per metre | Table 4D2B | 7.3 mV/A/m |
| Tabulated current It | Table 4D2A, reference method C | 46 A |
| Ambient correction Ca | Table 4B1, 35 °C | 0.94 |
| Temperature correction Ct | BS 7671 Appendix 4 Equation 6: (230 + 70 − (0.94² − (32/46)²) × (70 − 30)) / (230 + 70) | 0.947 |
| Voltage drop | Eq 12: 28 × 32 × 7.3 × 0.85 × 0.947 / 1000 | 5.263 V |
| As a percentage of 230 V | 2.29% | |
| Limit, public supply, other uses | Table 4Ab: 5% of 230 V | 11.50 V |
5.263 V is within 11.50 V, so the circuit complies. Lengthen the same run to 70 m and the drop becomes 13.159 V, 5.72%, which fails.
Questions
›What are the voltage drop limits in BS 7671?
Table 4Ab in Appendix 4 sets them. For an installation supplied directly from a public low voltage distribution system: 3% for lighting and 5% for other uses. From a private LV supply: 6% and 8%, though the drop within each final circuit should still not exceed the public supply figures. Where the wiring system is longer than 100 m, the values may be increased by 0.005% per metre beyond 100 m, up to 0.5%.
›When does the temperature correction factor Ct apply?
Ct, from BS 7671 Appendix 4 Equation 6, accounts for a conductor running below its maximum operating temperature because the design current is below the cable rating. A cooler conductor has lower resistance, so the voltage drop is smaller. The calculator applies it when the tabulated current It is known and the design current is below It × Ca × Cg × Cs × Cd. It is set to 1.0 where the protective device is a BS 3036 semi-enclosed fuse.
›Which voltage drop equation does the calculator use?
It depends on the conductor size and what you know. Up to 16 mm², the tables give a single mV/A/m value: Eq 6 without power factor or Ct, Eq 8 with power factor, Eq 10 with Ct, and Eq 12 with both. From 25 mm², the resistive and reactive components are treated separately: the tabulated z value, Eq 9 with power factor, Eq 11 with Ct, and Eq 13 with both. The result always names the equation used.
›Does the 100 m relaxation use the whole installation length?
Table 4Ab bases the relaxation on the length of the wiring system of the installation. The standalone Volt Drop calculator has no upstream distribution to measure, so it uses the circuit’s own cable length. That is exact for a circuit fed straight from the origin and conservative anywhere else. The Infrastructure calculator uses the full route length from the origin.