Complete Guide to Short Circuit Current
The Short Circuit Current Calculator computes available fault current at transformer secondary. Required for breaker selection per NEC 110.10.
What is SCC?
Maximum current during a bolted fault. I_sc = (kVA × 1000) / (V_sec × √3 × %Z/100). Our Short Circuit Current Calculator handles 1φ & 3φ.
Why Calculate SCC?
Ensures:
- Breaker Rating: AIC > SCC.
- Arc Flash: Label per NFPA 70E.
- Code Compliance: NEC 110.9, 110.10.
- Safety: Prevent explosion.
Use with Breaker Size and Arc Flash.
NEC Requirements
- 110.9: Interrupting rating ≥ SCC.
- 110.10: Impedance coordination.
- 110.24: Label available fault current.
Typical %Z Values
| kVA | %Z Range |
|---|---|
| ≤ 15 | 1.5–3.0% |
| 25–75 | 2.0–4.0% |
| 112.5–300 | 3.0–5.0% |
| ≥ 500 | 4.5–5.75% |
Infinite Bus Assumption
Utility delivers infinite current. SCC limited by transformer impedance only.
Calculation Steps
- Full Load: I_fl = kVA × 1000 / (V × √3).
- SCC: I_sc = I_fl / (%Z/100).
- Round: Conservative.
Example: 500 kVA, 480V, 5.75%Z
3-Phase:
- I_fl = 500 × 1000 / (480 × 1.732) = 601 A
- I_sc = 601 / 0.0575 = 10,452 A
- Breaker: ≥ 10kAIC
1-Phase vs 3-Phase
1φ: I_sc = (kVA × 1000) / (V × %Z/100).
Downstream Faults
Add cable impedance: Z_cable = (ρ × L × 1.732) / (1000 × CM). Reduces SCC.
Practical Tips
- Nameplate: Use actual %Z.
- Label: At service & panels.
- Software: SKM, ETAP for complex.
- Conservative: Use lower %Z.
Common Mistakes
- Ignore %Z: Infinite SCC.
- Wrong Voltage: Line-to-line.
- Forget 1.732: 3-phase.
- No Label: NEC violation.
Advanced
- Motor Contribution: +4× FLA.
- X/R Ratio: Asymmetrical.
- Point-to-Point: Full study.
- Infinite vs Finite: Utility data.
Conclusion
Available fault current is critical for safety. Our Short Circuit Current Calculator ensures NEC 110.10 compliance. Pair with Breaker Size and Construction tools. Stay safe with 1000 Calculators.