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Short Circuit Current Estimator

Estimate the prospective short-circuit current (PSCC) at a point in a low-voltage circuit, from the upstream transformer's rating and impedance plus the cable run to the fault point. A quick planning estimate, not a full fault-current study.

Input

Line-to-line for a three-phase fault, line-to-neutral for a single-phase fault.

Typically 4-6% for distribution transformers — check the nameplate.

Output

Result
MetricValue
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Guides

Prospective short-circuit current (PSCC) is how much fault current would flow at a point in a circuit if a bolted short occurred there right now. It's the number that sets minimum breaking-capacity requirements for the switchgear installed at that point — pick a breaker or fuse rated below it and the device can fail catastrophically during the very fault it's meant to clear. This tool estimates it from the upstream transformer's rating and impedance, plus the resistance of the cable run out to the fault point.

How to use it

  1. Pick the fault type — three-phase (the usual worst case for equipment rating) or single-phase (line-to-neutral).
  2. Enter the voltage at the fault point — line-to-line for a three-phase fault, line-to-neutral for a single-phase one.
  3. Enter the transformer rating (kVA) and its impedance (%Z), both from the nameplate. Distribution transformers are typically 4–6%.
  4. Enter the cable length, material and cross-sectional area for the run from the transformer to the point you're checking.

The formulas

  • Transformer impedance: Zt = V² × (%Z / 100) / (kVA × 1000) — the standard %Z-to-ohms conversion, referred to the fault-point voltage.
  • Cable resistance: R = ρ × length / area, using ρ = 0.017241 Ω·mm²/m for copper and 0.0282 Ω·mm²/m for aluminum (the IEC 60364 20°C constants).
  • Three-phase fault: Isc = VLL / (√3 × (Zt + R))
  • Single-phase fault: Isc = VLN / (Zt + 2R) — the 2R accounts for the full loop, out on the line conductor and back on the neutral.

What this estimate leaves out

This is a resistive-only, first-pass estimate, not a full IEC 60909 fault study:

  • Cable reactance is ignored. For the smaller conductor sizes this tool targets, resistance dominates and reactance is a minor correction — but for very large conductors or long high-voltage runs, a full R+jX calculation will give a somewhat lower current than this tool reports.
  • No motor contribution. Running induction motors briefly feed extra current into a nearby fault; this tool doesn't add it.
  • No upstream utility impedance beyond the transformer. If the transformer itself is fed from a weak grid connection, the real fault level will be lower than this estimate.

Treat the result as a planning figure for a first cut at switchgear rating, then confirm with a proper fault-level study before finalizing protective-device selection.

FAQ

Why does single-phase current come out lower than three-phase at the same transformer?

Two things stack against it: the fault voltage is lower (line-to-neutral instead of line-to-line, no √3 in the denominator to offset), and the cable resistance is doubled because the fault current has to travel out on the line conductor and back on the neutral, not just one way.

My transformer impedance isn't on the nameplate — what should I use?

4% is a common default for smaller distribution transformers, rising toward 6-8% for larger units. Always prefer the actual nameplate figure when it's available — it varies enough between units to matter.

Privacy

All calculations run entirely in your browser — nothing you enter is sent to a server.

short circuit currentprospective fault currentpscpsc cfault currenttransformer impedanceelectricalthree phasesingle phasecalculator

Use it from code

From 3 credits per call

REST API

curl -X POST https://api.iotools.cloud/v1/tool/short-circuit-current-estimator \
  -H "Authorization: Bearer YOUR_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "circuitType": "three",
    "voltage": "400",
    "transformerKva": "500",
    "transformerImpedancePercent": "5",
    "cableLength": "50",
    "conductorMaterial": "copper",
    "conductorSize": "25"
  }'

Swap in your own key from your account. The tool's fields are the body — no wrapper.

Ask an AI agent

Use the IOTools `short-circuit-current-estimator` tool (Short Circuit Current Estimator) on this input:

YOUR_INPUT_HERE

Paste this at any agent connected to the IOTools MCP server, then add your input.

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