Calculator #015 Circuit breaker sizing LV a.c., up to 1000 V IEC 60364-4-43 · IEC 60947-2

LV Circuit Breaker Sizing & Verification — AC ≤ 1 kV — IEC 60364-4-43:2023 / IEC 60947-2

Verify one fixed or adjustable IEC 60947-2 breaker against the circuit data: overload coordination against the cable, ultimate and service breaking capacity, the cable's thermal withstand, clearing at the minimum fault and making capacity. All checks are available free. Ratings and operating times must come from the selected manufacturer's data at the actual voltage and frequency — this tool verifies a product, it does not invent one.

Developed by the ieccalc.com engineering team

Questions and corrections — forum

Circuit and load

Project / circuit

Supply

Design current IB (A)

Corrected cable ampacity Iz (A)

System voltage U (V)

System frequency (Hz)

IB and Iz are final project inputs. Use the Cable Sizing tool for derating. Parallel conductors and neutral/earth-fault protection require separate assessment.

Breaker and prospective fault

PRO — breaker and trip unit from the library

Pick a catalogue record and transfer only the data it actually carries. Missing safety parameters are cleared and have to come from the manufacturer before the check runs.

Long-time setting Ir / In (p.u.)

Instantaneous setting Ii / In (p.u.)

Positive Ii tolerance (%)

Trip unit

Breaker data source / model / revision

Breaker rated current In (A)

Long-time setting Ir (A)

Conventional operating current I2 (A)

Rated operational voltage Ue (V)

Icu at system voltage / frequency (kA RMS)

Ics at system voltage / frequency (kA RMS)

Maximum fault current (kA RMS)

Minimum end-of-circuit fault (kA RMS)

Ue is the operational voltage, not insulation voltage Ui. Icu and Ics must apply to the entered system voltage and frequency; do not reuse a higher capacity from a lower voltage. I2 is an absolute operating current, not a generic multiplier.

Cable short-circuit withstand

PRO — cable from the library

Picking a cable fills the cross-section; the catalogue current becomes Iz only after your own correction factor for the actual installation.

Cable series cross-section (mm²)

Installation the catalogue current is stated for

Project ampacity correction factor (p.u.)

The factor is the engineer's combined correction for actual installation conditions. A catalogue current is not automatically the corrected circuit Iz. Select only a cable suitable for the circuit voltage and installation.

Thermal assessment

k source, material and temperatures

Conductor cross-section S (mm²)

Conductor k (verified input) (A·√s/mm²)

Total clearing time at Ik,max (s)

Maximum total let-through I²t (A²s)

For current-limiting breakers or clearing below 0.1 s, use manufacturer's total let-through energy, including the complete breaking interval. Enter its worst value over the relevant fault-current range.

Minimum fault and making capacity

Upper instantaneous pickup incl. tolerance (A)

Total clearing time at Ik,min (s)

Project maximum disconnection time (s)

Prospective peak current ip (kA peak)

Making capacity Icm (kA peak)

Use the upper pickup bound, including tolerance. Pickup alone does not prove disconnection time. The time limit is a project input; this tool does not determine IEC 60364-4-41 shock-protection requirements.

Verification result

Report package

Compact Word report contains the inputs, method, calculation summary, results, verdict and limits. Full report adds detailed derivation, a data register, glossary and verification examples.

Method, references and limits

IEC 60364-4-43:2023: 431.4.2 overload coordination; 431.5.4 short-circuit protection and Table 2 conductor factors. Schneider Electric Electrical Installation Guide: engineering explanation of the overload and thermal conditions. IEC 60947-2 manufacturer ratings are entered by the engineer; product certification is not assessed here.

Glossary

Built-in validation tests — expand

Independent hand-worked examples applying the IEC equations, not a claim that these numerical examples are printed in the standard. The factor test separately pins the 1.45 coordination coefficient. Sample k=143 is an explicit project input.