Available fault current from transformer kVA and impedance, checked against breaker interrupting rating and switchboard SCCR per NEC 110.9 and 110.10.
| Transformer | 2500 kVA, 480 V secondary, 5.75% impedance |
| Main breaker interrupting rating | 65 kA |
| Switchboard SCCR (nameplate) | 50 kA |
| Assumption | Infinite utility source; no cable impedance (screening value) |
1.8 is a conservative screening factor. A full IEC 60909 calculation uses κ from the actual X/R ratio.
Breaker: 65 kA ≥ 52.3 kA, adequate. Switchboard SCCR: 50 kA < 52.3 kA, not adequate.
| Check | Requirement | Actual | Status |
|---|---|---|---|
| Breaker interrupting rating (NEC 110.9) | ≥ 52.3 kA | 65 kA | ✓ PASS |
| Switchboard SCCR (NEC 110.10) | ≥ 52.3 kA | 50 kA | ✗ FAIL |
Key insight: Breaker interrupting rating and equipment SCCR are two separate checks. A high-rated breaker inside a lower-rated switchboard does not raise the switchboard's rating unless the combination is tested and listed as a series rating.
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Open Switchgear Short-Circuit Rating calculator →It ignores the utility source impedance and cables, both of which reduce the fault current. Motor contribution, which increases it, is also ignored, so a full study is needed when the margin is small.
A tested combination in which an upstream device protects a downstream device with a lower interrupting rating. It must be listed and marked; it cannot be assumed by calculation.