Electrical calculations
Voltage drop calculator
Calculate voltage drop in volts and percent from cable length, size, material and load. The tool supports single phase, balanced three phase and DC, using power or current. Results apply to the entered run; the applicable allowance depends on its origin, use and the rest of the circuit.
Effika
Voltage drop calculator
Use a decimal point or comma, without thousands separators. Enter one-way length and total three-phase power. The drop allowance applies to this run, not necessarily the entire installation.
Approximate calculation for an end-of-line load, equal outgoing and return conductors and, on AC, a sinusoidal load with lagging or unity power factor. ρ(T) = ρ₂₀·[1 + α·(T−20)]. Adopted from Guía-BT Annex 2 (February 2026): copper ρ₂₀ = 0.018 Ω·mm²/m and α = 0.00392/°C; aluminium 0.029 and 0.00403. R′ = c·ρ(T)/S, with c = 1.02 on AC and 1 on DC; X′ = x/1000 Ω/m. ΔU = 2·L·I·(R′·cos φ + X′·sin φ) for single phase; replace 2 with √3 for three phase; use 2·L·I·ρ(T)/S for DC. Cable sizing solves this expression for the selected drop limit. The initial reactance of 0.1 Ω/km is an editable estimate. Current capacity, short-circuit withstand, protection and regulatory minimum sizes are not verified.
Applicable limit
ITC-BT-19 generally requires less than 3% voltage drop in internal dwelling installations, and less than 3% for lighting or 5% for other uses in other internal installations. Allocation with the individual feeder and applicable special conditions must also be considered; each run does not automatically receive the full allowance.
The regulatory origin and load type determine the allowance. For industrial installations supplied directly from their own transformer, the REBT allows 4.5% for lighting and 6.5% for other loads.
Example
For 25 A, a 20 m one-way run, 6 mm² copper at 70 °C, unity power factor and 230 V single phase, the calculator gives 3.66 V and 1.59% using its stated constants and AC resistance factor. Results may differ from a tool assuming 20 °C or another resistivity.
Formula used by the calculator
The calculator uses ΔU = 2·L·I·(R·cosφ + X·sinφ) for single phase and ΔU = √3·L·I·(R·cosφ + X·sinφ) for three phase. R uses the material resistivity corrected to the selected temperature; X is the entered reactance.
For the next part of the decision, see the guides to Cable sizing and Power and current.
How to enter values and interpret voltage drop
Enter the one-way distance from the supply to the load. Do not double the cable length for single-phase or DC circuits: the voltage drop calculator already includes the return path. Three-phase mode uses line-to-line voltage and assumes a balanced load. You can start with current or active power; 5.75 kW means 5,750 W.
Temperature means conductor temperature, not room temperature. A hot cable has greater resistance than the same cable at 20 °C. Reactance is entered per unit length and can be adjusted using the cable manufacturer’s data. In DC mode, power factor and reactance are disabled because neither enters this resistive calculation.
The percentage applies to the entered cable run, and the receiving voltage is an estimate under the stated assumptions. An upstream feeder uses part of the available voltage-drop allowance too. Increasing cross-section can reduce voltage drop, but does not establish that the breaker rating, current capacity or short-circuit protection is adequate.
Frequently asked questions
Can voltage drop exceed 3%?
It depends on use and reference point. Outside dwellings, the general internal limits are 3% for lighting and 5% for other loads.
Is length one way?
Yes in the displayed single-phase formula; the factor 2 accounts for the return path.
Regulatory basis and official sources
- Spanish Low Voltage Regulation (REBT) and technical instructions — BOE
- Voltage-drop calculations — official Guía-BT Annex 2, February 2026
Always check the current version of the rule and the administrative procedure that applies to the specific case.