⚡ Cable Size & Voltage Drop Calculator
Pick a circuit or enter a load: the tool gives the breaker, the smallest cross-section that meets both ampacity and voltage drop, and the rules of the French wiring standard NF C 15-100.
📋 Dwelling circuits (NF C 15-100)
| Circuit | Section | Breaker | Limit |
|---|
Copper conductors, per the French standard NF C 15-100 revised on 23 August 2024 (Legrand “NF C 15-100, 2024 revision” guide; Hager). Water heater, ventilation and EV chargers: also check the appliance’s instructions.
⚠️ Indicative calculation. Electrical work must follow the wiring rules that apply where you live and be checked by a qualified electrician; in France a new or fully renovated installation needs a Consuel certificate of conformity before it is energised.
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About the Cable Size Calculator
A cable that is too small heats up and loses voltage at the end of the line; one that is too large costs more and is harder to run. This tool checks the two criteria of IEC 60364 and the French standard NF C 15-100: the cable’s ampacity for its installation method must exceed the breaker rating, and the voltage drop must stay under the limit. For common dwelling circuits it also shows the section and breaker set by NF C 15-100.
How to Use the Tool
- Pick a circuit type (lighting, sockets, oven, heating…) or “Other circuit” for a free calculation.
- Enter the power or current, the supply (single- or three-phase), the power factor and the cable length.
- Describe the cable and its installation: copper or aluminium, insulation, method, temperature and number of grouped circuits.
- Read the recommended section and the table of sections, then copy the result.
Formulas and Tables Used
- Design current: Ib = P ÷ (230 × cos φ) single-phase, P ÷ (√3 × 400 × cos φ) three-phase.
- Protection: Ib ≤ In ≤ Iz, In being the breaker rating and Iz the corrected ampacity.
- Ampacity: NF C 15-100 tables 52H (methods B, C, E) and 52J (buried cables, method D), equivalent to IEC 60364-5-52 tables B.52, at 30 °C ambient (20 °C in the ground).
- Corrections: Iz = table value × K1 (method) × K2 (grouping, IEC 60364-5-52 B.52.17) × K3 (temperature, B.52.14 and B.52.15).
- Voltage drop: ΔU = b × (ρ × L ÷ S × cos φ + λ × L × sin φ) × Ib, with b = 2 single-phase and 1 three-phase, ρ = 0.0225 Ω·mm²/m for copper and 0.036 for aluminium, λ = 0.08 mΩ/m; ΔU% = 100 × ΔU ÷ 230 V.
- Limits: 3% for lighting and 5% for other uses when the installation is supplied by the public low-voltage network.
Tips
- Long runs (garden shed, gate, EV charger): voltage drop often requires a larger section than the current does.
- Grouped cables in the same conduit or trunking run hotter: enter how many there are.
- Buried: use an armoured or ground-rated cable in a duct with a warning tape.
- EV charger: follow the manufacturer’s instructions and have it installed by a qualified installer.
Common Mistakes and Limits
- Counting the return length: enter the one-way length; factor b accounts for the return.
- Forgetting the power factor of motors (washing machine, pump, fan), lower than 1.
- Ignoring dwelling circuit rules: even when the calculation allows 1.5 mm², some circuits require 2.5 or 6 mm².
- Out of scope: discrimination, short-circuit currents, protection against electric shock and earthing systems are not checked.
Privacy & Security
- 100% in your browser: no data is sent to a server.
- No sign-up: free and instant.
- Works offline once the page has loaded.