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Circuit Breaker Sizing

Preliminary load–breaker–cable consistency check.

Calculation inputs

Illustrative example values — replace them before any engineering decision.

Engineering results

Design current37.5 A
Preliminary breaker rating40 A
Entered cable capacity50 A
Cable margin above breaker10 A

Method and traceability

The next listed rating at or above load current × entered design factor is compared with entered cable ampacity.

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

Circuit Breaker Sizing Calculator

Perform a preliminary load–breaker–cable consistency check from current, design factor and cable ampacity.

Calculation method

The design current is the load current multiplied by the entered factor. The engine selects the first configured standard breaker rating not below that value, then checks it against cable ampacity.

Formulas used

  • Design current = load current × design factor
  • Preliminary consistency: load current ≤ breaker rating ≤ cable ampacity

Worked example

Example: 30 A with a 1.25 factor requires at least 37.5 A before selection of the next configured rating.

How to interpret the result

A selected rating is only a current-level screening result. Breaking capacity, trip curve, fault loop and discrimination remain separate decisions.

Common mistakes

  • Selecting from load current without a documented design factor.
  • Ignoring cable ampacity or motor protection needs.

Scope and limitations

  • No short-circuit, earth-fault, selectivity or coordination calculation is performed.

Frequently asked questions

Is breaker rating the same as breaking capacity?

No. Rating is continuous current; breaking capacity is the fault current the device can interrupt.

Why compare the breaker with cable ampacity?

A protective rating above the cable capacity can leave the conductor inadequately protected.

Does a motor need a special check?

Yes. Starting current, overload protection and coordination require motor-specific data.