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Breaker size and short circuit

From load current, conductor and transformer, get the breaker rating, trip characteristic, fault current at the panel and at the end of the circuit, and the interrupting rating you need.

Pre-sizing and estimating tool: results do not replace design, calculations or reports signed by a licensed professional, nor verification against the applicable codes and standards.

Pre-sizing
Load
Circuit cable
Source and feeder

Parallel sets: add the areas (2 × 500 kcmil = 2 × 253 = 506 mm²; 1 kcmil = 0.5067 mm²).

—Recommended breaker
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Fault current along the circuit

Maximum three-phase and minimum line-to-ground fault versus distance from the panel, with the breaker instantaneous trip.

Tap the chart to see the values.

Load, breaker and cable coordination

Design current must be below breaker rating, and breaker rating below cable ampacity.

Tap the chart to see the values.

The report gathers this page's inputs, results, equations, assumptions and references, with fields for project, engineer and revision. In the print dialog, choose Save as PDF.

Method, assumptions and references

Equations used

NEC: breaker ≥ 125% of continuous load (210.20, 215.3); conductor protected per 240.4, with next size up allowed to 800 A; motors per 430.52
Maximum three-phase fault (IEC 60909 method): Ik3 = c_max · U ÷ (√3 · |Zutility + Ztransformer + Zfeeder + Zcable|), c_max = 1.05
Zutility = 1.1 U² ÷ Ssc; Ztransformer = (Z% ÷ 100) · U² ÷ Sn, with X ≈ 0.98 Z and R ≈ 0.2 Z; Zcable = ρ L ÷ S + j 0.08 Ω/km · L
Minimum line-to-ground fault: Ik1 = 0.95 U0 ÷ |Zsource + 2 Zcable|, cable at 80 °C and equipment grounding conductor of the same size
Conductor withstand: S ≥ Ik · √t ÷ k (k = 115 Cu/PVC, 143 Cu/XLPE, 76 Al/PVC, 94 Al/XLPE)

Assumptions and limits

  • Preliminary estimate: it does not replace a short-circuit and coordination study with actual utility, transformer and breaker data (IEEE 551, NEC 110.24 labeling).
  • Motor fault contribution neglected; with many large motors the actual fault current is higher.
  • Solidly grounded system with the grounding conductor run with the circuit. Other systems require ground-fault protection or insulation monitoring.

Standards and technical documents

  • IEC 60909-0:2016: Short-circuit currents in three-phase a.c. systems — Calculation of currents
  • IEC 60898-1 e IEC 60947-2: disjuntores para instalações domésticas e industriais
  • IEC 60364-4-41 e 4-43: proteção contra choques e contra sobrecorrentes; ABNT NBR 5410:2004
  • NFPA 70 (NEC) 2023: 110.9, 110.24, 210.20, 240.4, 240.6, 430.52
  • IEEE Std 551 (Violet Book): Calculating Short-Circuit Currents in Industrial and Commercial Power Systems

U.S. regulations

  • NFPA 70 (NEC) as adopted locally; 110.24 requires marking of available fault current at service equipment
  • OSHA 29 CFR 1910.303 and 1910.304: equipment interrupting rating and overcurrent protection
  • NFPA 70E: arc flash risk assessment uses the available fault current

Frequently asked questions

How do I size a circuit breaker?

Under the NEC, the breaker must be at least 125% of the continuous load plus 100% of the noncontinuous load, and it must protect the conductor (240.4). Round up to the next standard size in 240.6(A).

What is available fault current?

The maximum current a bolted fault would draw at a point, set by the utility, the transformer and the conductors. NEC 110.24 requires it to be marked at service equipment, and breakers must be rated for it (110.9).

How are motor circuits different?

Motor branch circuits use NEC 430: the breaker protects against short circuit (up to 250% of FLC for inverse-time breakers) and a separate overload relay protects the motor.