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Cable Sizing

Preliminary section from current capacity and voltage drop.

Calculation inputs

Illustrative example values — replace them before any engineering decision.

Engineering results

Design current56.6 A
Recommended standard section16 mm²
Screening current capacity64 A
Current margin7.4 A
Voltage drop5.66 V
Voltage drop1.41 %

Method and traceability

Current follows P/(V·η) for DC, P/(V·cosφ·η) for single phase, and P/(√3·V·cosφ·η) for balanced three phase.

Iz = section × entered current density × ambient factor × grouping factor. Installation method and ambient temperature are documented; their effect is controlled by the visible factors.

R=ρ₂₀·[1+α·(T−20)]/S. AC drop includes entered reactance and cosφ; length is one way. DC and single phase include the return path; balanced three phase uses √3.

Visible assumptions: copper ρ₂₀ 0.0175 and aluminium 0.0282 Ω·mm²/m; copper α 0.00393 and aluminium 0.00403/°C. Reactance, density, factors and thresholds remain editable.

Screening lists — sections: 1.5, 2.5, 4, 6, 10, 16, 25, 35, 50, 70, 95, 120, 150, 185, 240, 300, 400, 500, 630 mm²; breakers: 2, 4, 6, 10, 16, 20, 25, 32, 40, 50, 63, 80, 100, 125, 160, 200, 250, 315, 400, 500, 630, 800, 1000, 1250, 1600 A; transformers: 25, 50, 100, 160, 250, 315, 400, 500, 630, 800, 1000, 1250, 1600, 2000, 2500, 3150 kVA.

Final design must verify applicable IEC 60364 requirements, actual thermal conditions, short circuit, shock protection, selectivity, starting, harmonics and local rules.

IEC 60364-5-52 ↗Guide — voltage drop ↗

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Engineering reference

Electrical Cable Sizing Calculator

Estimate a preliminary conductor section from load current, screening ampacity and voltage-drop criteria.

Calculation method

The engine calculates design current when power is supplied, then tests standard sections in ascending order. A section is retained only when its corrected screening ampacity covers the current and its calculated voltage drop remains below the entered limit.

Formulas used

  • Three-phase current: I = P × 1,000 / (√3 × V × cosφ × η)
  • Screening ampacity: Iz = S × J × kamb × kgroup

Worked example

Example: a 30 kW, 400 V, cosφ 0.85 and 90% efficient three-phase load gives about 56.6 A before section and voltage-drop checks.

How to interpret the result

The returned section is the first available section satisfying both configured checks. It is a preliminary engineering result, not a regulatory ampacity table selection.

Common mistakes

  • Entering round-trip length instead of one-way length.
  • Checking ampacity without checking voltage drop.
  • Treating the screening current density as a wiring-code table.

Scope and limitations

  • Final selection must verify the applicable code, installation method, temperature, grouping, harmonics, short-circuit withstand and protective device.

Frequently asked questions

How is three-phase cable current calculated?

From active power, line voltage, power factor and efficiency using the displayed √3 relationship, unless current is entered directly.

Does the calculator choose a regulatory cable size?

No. It performs an explicit preliminary screening and identifies the checks still required for final design.

Why can voltage drop govern the section?

Long circuits can satisfy current capacity yet exceed the permitted receiving-voltage drop.