Fault Current Calculation per NEC 110.10
The Fault Current Calculator determines available short circuit current at the service entrance or downstream panel. Required for proper breaker selection, equipment ratings, and arc flash labeling.
Formula
I_SC = (kVA × 1000) / (√3 × V × %Z/100) (3φ)
I_SC = (kVA × 1000) / (V × %Z/100) (1φ)
Typical Transformer %Z
| kVA | %Z Range | Typical |
|---|---|---|
| ≤ 75 | 1.5–4.0% | 2.5% |
| 112.5–300 | 1.5–5.75% | 4.5% |
| 500–1000 | 5.0–5.75% | 5.75% |
| ≥ 1500 | 5.75–7.0% | 6.0% |
Why It Matters
- Equipment Rating: Breakers must withstand I_SC
- Arc Flash: Higher kA = higher incident energy
- Selective Coordination: Avoid nuisance trips
- NEC 110.10: Must calculate and label
Examples
- 500 kVA, 480V 3φ, 5.75%Z → 8.7 kA
- 1500 kVA, 208V 3φ, 5.75%Z → 72.3 kA
Cable Length Adjustment
Long service conductors reduce fault current due to impedance. Use with Voltage Drop tool.
Common Mistakes
- Assuming infinite bus (no %Z)
- Using primary voltage
- Ignoring motor contribution
- No labeling on panels
Best Practices
- Label all panels with I_SC and date
- Recalculate after upgrades
- Use software for complex systems
- Verify with utility data
Conclusion
Accurate fault current ensures safety and compliance. Use with Arc Flash and Breaker Size tools.