Estimates available short-circuit current at successive points down a radial network by converting each element's %impedance to a common kVA base and accumulating it along the path — the classic textbook 'Base kVA Method' quick-estimate technique.
Standards followed
Base kVA Method (standard percentage-impedance short-circuit estimation technique)
What it can do
- Source impedance derived from utility fault MVA.
- Any number of series elements (transformers, cables), each entered as %Z at its own rated kVA and auto-converted to the common base.
- Fault MVA and fault current (kA) reported progressively at each point down the chain.
Example problem
Base 1000kVA, 415V system, 250MVA source, 1000kVA transformer at 6%Z, cable at 1.5%Z (same base).
- Source %Z = (1/250)×100 = 0.4%.
- After transformer: cumulative %Z = 0.4+6 = 6.4% → fault MVA = 100/6.4 = 15.6 MVA.
- After cable: cumulative %Z = 6.4+1.5 = 7.9% → fault MVA = 100/7.9 = 12.66 MVA → ≈17.6kA at 415V.
≈17.6kA available fault current at the sub-panel.
Want the full step-by-step version?
IMPORTANT SIMPLIFICATION: this combines %Z magnitudes arithmetically rather than resolving each element into R and X and summing vectorially. Arithmetic summation slightly OVERSTATES total impedance and therefore UNDERSTATES fault current relative to a full R+jX study. Use this for quick estimates only — never finalize protective device interrupting-rating selections from this figure alone; use a full R+X or dedicated short-circuit study for that.