Formula
This simplified method ignores cable reactance and power factor and uses conductor resistance only. Length L is in m, current I in A and cross-section A in mm².
Single-phase 2-wire e = 35.6 · L · I / (1000 · A) Three-phase 3-wire e = 30.8 · L · I / (1000 · A) Single-phase 3-wire, three-phase 4-wire e = 17.8 · L · I / (1000 · A) Voltage drop % = e / reference voltage × 100
The coefficients come from the resistivity of annealed copper (1/58 Ω·mm²/m) with conductivity derated to 97 % for practical conductors: 1,000 ÷ (58 × 0.97) ≈ 17.8. A single-phase 2-wire circuit has two conductors in the loop, which gives 35.6, and a three-phase 3-wire circuit uses √3, which gives 30.8. The calculator derives the coefficients the same way, and automated tests check that they match these three values.
For single-phase 3-wire and three-phase 4-wire systems, e is the drop between one line and neutral, so the percentage uses the line-to-neutral voltage. For a 380/220 V three-phase 4-wire system, use 220 V.
Permitted voltage drop
IEC 60364-5-52 Annex G recommends limiting the voltage drop between the origin of the installation and the equipment to the values below. Korea adopts the same values in KEC 232.3.9.
| Installation type | Lighting (%) | Other uses (%) |
|---|---|---|
| A: supplied directly from a public low-voltage network | 3 | 5 |
| B: supplied from a private supply (own transformer) | 6 | 8 |
Where the wiring is longer than 100 m, the limits may be increased by 0.005 % for each metre beyond 100 m, up to 0.5 %. The calculator applies this automatically. National wiring rules can differ, so check the rules where you work.
Worked example
Single-phase 220 V, 20 A load, 50 m run, 4 mm² copper, lighting circuit on a public low-voltage supply:
- e = 35.6 × 50 × 20 / (1000 × 4) ≈ 8.89 V
- Voltage drop ≈ 4.04 %, which exceeds the 3 % limit
- Required cross-section ≈ 5.39 mm², so choose the standard 6 mm² size
The calculator opens with these values.
Limitations
- Do not choose a cable size from voltage drop alone. It must also meet current-carrying capacity (installation method, ambient temperature, grouping), protective device coordination and short-circuit withstand.
- For aluminium conductors, or large cables (roughly 50 mm² and up) feeding low power-factor loads, reactance becomes significant and this method can underestimate the drop.
- Electrical work must be designed, installed and inspected by qualified people. Use these results as a reference for review.
References
- IEC 60364-5-52, Low-voltage electrical installations, Part 5-52: Selection and erection of electrical equipment, Wiring systems, Annex G (Voltage drop in consumers' installations)
- Korea Electrotechnical Code (KEC) 232.3.9, Voltage drop in consumer installations
- IEC 60228, Conductors of insulated cables (nominal cross-sections 1.5 to 300 mm²)