Voltage drop calculator
Calculate the voltage drop of an electrical line in volts and percent, the power lost in the cable, and the smallest standard cross-section that would meet the limit you set. Valid for single-phase and three-phase AC circuits with copper or aluminium conductors.
Result
–
Voltage drop
–
Drop in percent
| Voltage at the load end | |
|---|---|
| Estimated cable loss | |
| Conductor resistance | |
| Smallest section meeting the limit |
Entered values
Calculation assumptions
- Resistivity corrected to the selected conductor temperature (20 °C reference).
- Inductive reactance estimated at 0.08 Ω/km, a typical value for installed cables.
- Three-phase assumes a balanced load with no neutral current.
- Steady-state sinusoidal conditions, no harmonics.
What would you like to do next?
Choose the option that best fits your situation.
Download calculation report
Save or print the result for your records.
Calculate another circuit
Quickly start another calculation.
What this tool calculates
The calculator obtains the voltage drop between the origin of a line and its load, in steady state and with a balanced load, from the cable length, current, cross-section, conductor material, power factor and conductor service temperature.
It also estimates the power lost in the cable (Joule heating) and the smallest standard cross-section that would meet the drop limit you set.
What it does not do: it does not check the cable’s current-carrying capacity, the protective devices or short-circuit behaviour. A line can meet the voltage-drop limit and still be undersized.
Formulas used
For single-phase circuits:
e = 2 · L · I · (R′ · cos φ + X′ · sin φ)
For balanced three-phase circuits:
e = √3 · L · I · (R′ · cos φ + X′ · sin φ)
where:
| Symbol | Meaning | Unit |
|---|---|---|
| e | voltage drop | V |
| L | one-way route length | m |
| I | line current | A |
| R′ | conductor resistance per metre | Ω/m |
| X′ | conductor reactance per metre (0.08 Ω/km) | Ω/m |
| cos φ | load power factor | — |
Resistance per metre comes from the temperature-corrected resistivity:
R′ = ρ₂₀ · [1 + α · (T − 20)] / S
with ρ₂₀ = 0.017241 Ω·mm²/m for copper and 0.028264 Ω·mm²/m for aluminium (IEC 60228 basis), α ≈ 0.004 K⁻¹ and S the cross-section in mm².
As a percentage, the drop is referred to the nominal line voltage: e(%) = 100 · e / U. For three-phase, U is the line-to-line voltage (400 V).
Worked example: single-phase circuit
Household socket circuit: 230 V, 16 A, 25 m of 2.5 mm² copper, cos φ = 1, conductor at 70 °C.
R′ = 0.017241 · [1 + 0.00393 · 50] / 2.5 = 0.00825 Ω/m
e = 2 · 25 · 16 · 0.00825 = 6.60 V → 6.60 / 230 = 2.87 %
Against the 3 % limit the Spanish REBT sets for any interior circuit of a dwelling, this is marginal (above 90 % of the margin); in non-residential installations, against the 5 % “other loads” limit it passes comfortably. The cable dissipates about 106 W while carrying 16 A.
Worked example: three-phase line
Three-phase pump: 400 V, 25 A, 35 m of 6 mm² copper, cos φ = 1, conductor at 70 °C.
R′ = 0.020629 / 6 = 0.00344 Ω/m
e = √3 · 35 · 25 · 0.00344 = 5.21 V → 5.21 / 400 = 1.30 %
Passes the 5 % limit with a wide margin.
Common mistakes
- Doubling the length. The formula already includes the return conductor; entering the out-and-back length doubles the result.
- Using 230 V for three-phase lines. In three-phase the percentage drop refers to the line-to-line voltage (400 V), and the formula uses √3, not 2.
- Ignoring temperature. Using 20 °C resistivity underestimates the drop by about 20 % compared with a cable running at 70 °C.
- Mis-adding sections. Regulatory limits apply from the origin of the installation to the point of use: if you calculate one section, remember the rest of the run.
- Sizing by voltage drop alone. The cable’s current-carrying capacity may demand a larger section than this calculation suggests.
Reference limits (Spanish REBT)
The most common values in Spain, from the Low-Voltage Electrotechnical Regulation:
| Section | Limit |
|---|---|
| Dwelling, any interior circuit (ITC-BT-19) | 3 % |
| Other interior installations, lighting (ITC-BT-19) | 3 % |
| Other interior installations, other loads (ITC-BT-19) | 5 % |
| Industry with own transformer, lighting | 4.5 % |
| Industry with own transformer, other loads | 6.5 % |
| General supply line (ITC-BT-14) | 0.5–1 % |
| Individual supply line (ITC-BT-15) | 0.5–1.5 % |
ITC-BT-19 also allows compensating the individual-supply-line drop against the interior drop, as long as the total stays below the sum of both limits.
This table is indicative only: the calculator applies exactly the limit you set and draws no regulatory-compliance conclusions. Always verify your specific case against the current text of the applicable regulation.