Power Systems FundamentalsSafety-sensitive
Symmetrical Fault Current Calculator
Three-phase bolted fault current at a transformer secondary, from source and transformer impedance.
About this calculator
A three-phase bolted fault draws current limited only by the impedance between the source and the fault point. For a service entrance or unit substation, that impedance is the utility's source impedance in series with the step-down transformer's impedance, both expressed in per-unit on the transformer's own kVA base: Z_source,pu = S_base / S_fault,source, and Z_xfmr,pu = %Z / 100. The available symmetrical fault MVA at the secondary is then S_base / Z_total,pu, converted to amperes with the secondary line-to-line voltage.
This calculator is the standard hand-calculation utilities and consulting engineers use to size switchgear interrupting ratings and verify equipment short-circuit withstand before running a full software study. It also reports the transformer-only (infinite-bus) fault current alongside the source-limited value, showing how much the utility's source impedance actually reduces available fault current — useful for sanity-checking whether a full study is likely to change the equipment rating decision.
This is a single-transformer, radial, three-phase bolted-fault approximation. It does not include motor contribution (induction and synchronous motors add several cycles of fault current through subtransient reactance), does not compute asymmetrical (X/R-adjusted) peak current, and does not model parallel sources, cable impedance between the transformer and the fault point, or ground-fault current. Use a full short-circuit study for equipment ratings on a system with significant motor load or multiple sources.
Assumptions
- Single transformer, radial system, bolted three-phase fault at the transformer secondary terminals.
- No motor contribution to fault current is included.
- Cable/busway impedance between the transformer and the fault point is neglected.
- Symmetrical RMS fault current only — asymmetrical peak current (X/R-adjusted) is not computed.
When to use this calculator
Appropriate for
- A first estimate of bolted three-phase symmetrical fault current at a transformer secondary from nameplate %Z and source strength
- Sanity-checking the order of magnitude of an available fault current for early equipment interrupting-rating selection
- Teaching how transformer impedance and source impedance combine to limit fault current
Not suitable for
- A formal short-circuit study of record — use a full study with motor contribution, cable and reactor impedances, and X/R-based asymmetry
- Line-to-ground, line-to-line, or double-line-to-ground faults, which need symmetrical-component analysis
- Selecting or verifying protective-device interrupting ratings or arc-flash energy for a real installation
What this calculator does not cover
- Three-phase bolted symmetrical faults only — line-to-ground and line-to-line faults require sequence-component analysis, and arc impedance is not modeled.
- No motor contribution — running motors feed the fault during the first cycles and can raise the interrupting and momentary duty meaningfully.
- Impedances combine as magnitudes (not complex phasors), and X/R is not tracked — this is not a protective-device duty evaluation per the IEEE C37 series.
- As with every calculator on this site: results are preliminary and educational, are not verified for any specific installation, and must be reviewed against the applicable code edition and stamped by a licensed Professional Engineer before real-world use.
Frequently asked questions
What is a symmetrical bolted fault?
A bolted fault assumes zero fault impedance — a direct short between all three phases. It's symmetrical because the resulting current is a balanced three-phase quantity, used as the baseline before considering asymmetry from DC offset in the first few cycles.
Why does the source impedance matter if the utility system is so much larger?
Even a large utility system has finite fault MVA. Ignoring it (infinite-bus assumption) overstates available fault current, sometimes significantly for smaller transformers fed from a weaker point on the utility system.
Does this include motor contribution?
No. Induction and synchronous motors add fault current through their subtransient reactance for the first several cycles. For facilities with significant motor load, add motor contribution separately or run a full short-circuit study.
References
- IEEE Std 141 (Red Book), Ch. 2 — Short-Circuit Current Calculations
- IEEE Std 242 (Buff Book), Ch. 2
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