Short-Circuit Current Calculator

Before a fault exists, you must already know what it will look like. Every breaker and fuse in the panel is specified against that number - the short-circuit current that will arrive the moment something lets go.

Short-Circuit Current Calculator estimates the prospective fault current from the source (a transformer or the utility) and tracks how a feeder reduces it at the load point.

It is a study aid. Real fault studies account for motors, cables, and the exact impedance data from the network operator.

• Transformer source: kVA and %Z give terminal fault current.
• Utility source: Enter the available short-circuit MVA.
• Feeder effect: Length and impedance reduce the fault level.
• 1-phase or 3-phase: Formula follows the system.
• At source and at load: Both levels reported.
• kA output: Directly comparable to device ratings.
• Export: Copy or download as text, CSV or JSON.

1. Choose the source type (transformer or utility MVA).
2. Enter transformer kVA and %Z or the utility fault MVA.
3. Set the system voltage and phase.
4. Optional: add a feeder length and impedance.
5. Read the short-circuit current at source and at load.
6. Compare with the device interrupting rating.

Example 1 — Transformer. 1000 kVA, 5%Z, 400 V 3-phase: 28.9 kA at the terminals.

Example 2 — Utility. 250 MVA at 11 kV: 13.1 kA; at 400 V after the transformer the level is set by the transformer impedance.

Example 3 — Feeder. A 30 m, 120 mm² run from a 28.9 kA source drops the fault to roughly 22 kA at the far end.

• Device ratings: Compare the kA figure with breaker ratings.
• Source flexibility: Transformer or utility input.
• Feeder aware: See the drop across the cable.
• Fast and free: Instant result, no sign-up, no upload.
• Private: Nothing is stored or logged.