Using the classic Base kVA Method to estimate available short-circuit current at two points down a radial LV network, and checking a downstream breaker's interrupting rating against it.
| Base | 1000 kVA |
| System voltage | 415 V line-to-line |
| Utility source fault level | 250 MVA |
| Transformer | 1000 kVA, 6% impedance (own base) |
| Cable, transformer to sub-panel | 1.5% impedance (on the 1000 kVA base) |
| Sub-panel main breaker | Example: rated 22 kA interrupting capacity |
| Check | Requirement | Actual | Status |
|---|---|---|---|
| Fault current at transformer secondary bus | n/a (informational) | 21.7 kA | ✓ PASS |
| Fault current at sub-panel | n/a (informational) | 17.6 kA | ✓ PASS |
| Sub-panel breaker interrupting rating | ≥ 17.6 kA | 22 kA rated | ✓ PASS |
Key insight: This method sums %Z values arithmetically (scalar addition) rather than resolving each element into resistance and reactance and combining them vectorially. That's the classic quick-estimate version of the method — it slightly overstates total impedance and therefore understates fault current relative to a full R+jX study, so it should never be used to finalize a protective device's interrupting rating on its own; treat the result as a first screening figure, then verify with a full short-circuit study before specifying equipment.
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Open Fault Current Propagation (Base kVA Method) calculator →Every element between the source and a given point — the transformer, cables, busbars — adds its own impedance to the total fault-current path. More cumulative impedance means less current can flow for a given system voltage, so available fault current is always highest right at (or upstream of) the source and decreases at each downstream point.
Percentage or per-unit impedance values are only meaningful relative to a stated power base — a transformer's 6% impedance is 6% of its own rated kVA. To add impedances from different equipment together, they must first be converted onto one common base, which is exactly what the elementBaseKva field on each element does before summing.