What the calculator does
It computes the voltage lost along a cable run due to conductor resistance, the percentage of supply voltage that represents, the voltage actually available at the load, and the power dissipated in the cable. It then checks the result against a 3 %, 4 % or 5 % limit and reports the smallest standard conductor size that would meet it with the same inputs.
Enter the one-way route length. The formula accounts for current flowing out and back (two conductors) for DC and single-phase circuits, and for the √3 relationship in a balanced three-phase circuit.
Formula
Three-phase (balanced): Vd = √3 × I × L × Rc × cos φ
Percentage drop = Vd ÷ Vsupply × 100
where I is the load current in amperes, L the one-way route length in metres, Rc the conductor resistance in Ω/m at operating temperature, and cos φ the power factor (1 for DC). Resistance at temperature T is derived from the 20 °C value:
Cable power loss is 2 × I² × R for DC and single-phase, and 3 × I² × R per conductor for three-phase, where R = Rc × L.
Worked example
A 230 V single-phase circuit supplies a 20 A load 30 m away through 2.5 mm² copper at 70 °C, power factor 1.
- R20 for 2.5 mm² copper (IEC 60228) = 7.41 Ω/km = 0.00741 Ω/m.
- At 70 °C: 0.00741 × (1 + 0.00393 × 50) = 0.00887 Ω/m.
- Vd = 2 × 20 × 30 × 0.00887 = 10.6 V.
- Percentage: 10.6 ÷ 230 × 100 = 4.6 %. Voltage at the load ≈ 219.4 V.
That passes a 5 % limit but fails 3 %. Repeating with 4 mm² (4.61 Ω/km) gives 6.6 V, or 2.9 %, so 4 mm² is the smallest standard size for a 3 % limit on this run.
Conductor resistance used (IEC 60228, class 2 stranded, maximum at 20 °C)
| Size (mm²) | Copper (Ω/km) | Aluminium (Ω/km) |
|---|---|---|
| 1 | 18.1 | — |
| 1.5 | 12.1 | — |
| 2.5 | 7.41 | — |
| 4 | 4.61 | — |
| 6 | 3.08 | — |
| 10 | 1.83 | — |
| 16 | 1.15 | 1.91 |
| 25 | 0.727 | 1.20 |
| 35 | 0.524 | 0.868 |
| 50 | 0.387 | 0.641 |
| 70 | 0.268 | 0.443 |
| 95 | 0.193 | 0.320 |
| 120 | 0.153 | 0.253 |
| 150 | 0.124 | 0.206 |
| 185 | 0.0991 | 0.164 |
| 240 | 0.0754 | 0.125 |
| 300 | 0.0601 | 0.100 |
Assumptions and limitations
- Resistance only. Cable reactance is ignored. For conductors up to about 25 mm² at 50/60 Hz the error is small; for large conductors and low power factors, reactance adds to the drop and the tabulated mV/A/m values in your wiring standard should be used instead.
- Balanced three-phase. The three-phase formula assumes equal loading on all phases with no neutral current.
- Conductor temperature is your input. A lightly loaded cable runs cooler than its rated temperature and drops less voltage; the 70 °C and 90 °C options represent fully loaded PVC and XLPE cables.
- Limits are typical, not universal. BS 7671 Appendix 12 uses 3 % for lighting and 5 % for other uses from the origin of a public supply; the NEC recommends (informationally) 3 % for a branch circuit or feeder and 5 % combined. Check the standard and any equipment tolerance that applies to you.
- Not a current-carrying-capacity check. A cable that meets the voltage-drop limit must separately be sized for its current rating, installation method, grouping and ambient temperature, and for protective-device coordination.
Frequently asked questions
Should I enter the one-way length or the total cable length?
The one-way route length. The calculator doubles it for DC and single-phase circuits (current flows out and back) and applies √3 for three-phase.
Why is my result a little lower than the mV/A/m table in the wiring regulations?
Tabulated values include reactance and are rounded conservatively. For small conductors the difference is a few percent; for large conductors or low power factor, use the table.
What voltage should I enter for three-phase?
The line-to-line voltage, for example 400 V (or 380/415 V depending on your supply). The percentage is then relative to that line-to-line value.
Does a long run mean I need a bigger cable even if the current is small?
Often, yes. Voltage drop is proportional to length, so long runs to pumps, gates, outbuildings or solar arrays are frequently sized by voltage drop rather than by current rating.
References
- IEC 60228:2004, Conductors of insulated cables — maximum conductor resistance at 20 °C by class and size
- BS 7671:2018+A2:2022, Requirements for Electrical Installations, Appendix 12 — voltage drop limits of 3 % (lighting) and 5 % (other uses) for public low-voltage supplies
- NFPA 70 (NEC) 2023, 210.19(A) Informational Note No. 4 and 215.2(A) Informational Note No. 2 — recommended 3 % / 5 % voltage drop for reasonable efficiency of operation