Electrical calculations
Cable size calculator
Cable size cannot be selected from kW alone. Current capacity, voltage drop, short-circuit withstand, protection and minimum size must all be checked. The calculator below solves only a simplified voltage-drop condition.
Effika
Cable size 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.
A cable size must pass four separate checks
- Installation method and grouping change current capacity.
- Material and temperature alter resistance and drop.
- Protection must prevent overload and short-circuit damage.
- The final standard size must pass every condition.
Four checks
Determine design current, apply tabulated capacities and correction factors, check cumulative voltage drop, then validate thermal short-circuit withstand and coordination with the circuit breaker or fuse.
Why a calculator is incomplete
The calculator uses your entered conductor temperature, but does not verify heat dissipation for the installation: conduit, tray, grouping and insulation still require a separate current-capacity check.
What the calculator does — and does not — size
The result is the next standard cross-section that satisfies the selected voltage-drop percentage under the entered assumptions. It does not verify current-carrying capacity, grouping, short-circuit withstand, protection or any minimum size imposed by the applicable ITC.
For the next part of the decision, see the guides to Voltage drop and Power and current.
From the calculated cross-section to a suitable cable
The cable size calculator finds the theoretical minimum for voltage drop and selects the first size in its series that meets it. If the calculation requires 3.2 mm², it suggests 4 mm² rather than 2.5 mm². This meets only the criterion calculated: installation method and ambient temperature may still require a larger conductor.
The same cable dissipates heat differently in conduit, on a tray or alongside other loaded circuits. There is therefore no universal current-per-size table suitable for every installation. Current-carrying capacity must be checked using the insulation type, loaded conductors and relevant correction factors, in coordination with overcurrent protection.
If the result exceeds the largest offered size, or reactance alone uses up the allowed voltage drop, the tool reports that limitation instead of inventing a commercial size. Review the design, manufacturer’s data and alternative supply arrangements. The neutral and protective conductors are not sized here; they need their own checks.
Example: passing the voltage-drop check is not enough
Imagine a long cable run to a pump. The calculator might suggest a standard size that stays within the entered voltage-drop percentage. That is only one filter: a hot installation location, grouped cables in conduit or unsuitable protection could require a different final size. Motor starting and short-circuit performance may also matter.
Before ordering cable, record single- or three-phase supply, design power or current, one-way route length, conductor material, installation method and proposed protection. The tool addresses voltage drop for the entered data; current-rating tables and correction factors require separate checks. Its output is an indicative size, not an installation instruction.
Frequently asked questions
What cable for 5.75 kW?
Voltage, phase, length, method, temperature, protection and load type are still needed.
Is a larger cable always better?
It lowers resistance, but connections, protection, space, cost and terminal capacity still matter.
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.