What is bolted fault current?

Bolted fault current (also called available short-circuit current) is the maximum symmetrical RMS current that flows when all phase conductors are connected together with zero impedance at a point in the system. It represents the worst-case fault and is used to size protective devices.

Where do I find the percent impedance (%Z)?

The percent impedance is printed on the transformer nameplate. Common values are 2% for small distribution transformers, 4–6% for medium power transformers, and up to 8–10% for large substation transformers.

What is the formula used?

For three-phase: I_sc = kVA × 1000 / (√3 × V_LL × %Z/100). For single-phase: I_sc = kVA × 1000 / (V × %Z/100). When source impedance is included, it is added to the transformer impedance in per-unit before dividing.

Why does source impedance matter?

The utility source has its own impedance that limits fault current. For large transformers or weak utility feeds, the source impedance can significantly reduce the available fault current below the infinite-bus assumption.

What is the difference between symmetrical and asymmetrical fault current?

The symmetrical (RMS) value is the steady-state AC component. The asymmetrical (peak) value includes a DC offset that decays over the first few cycles. For device sizing, standards typically use a multiplying factor of 1.6–2.0× the symmetrical value for the asymmetrical peak.

Does this account for conductor impedance?

No. This calculator gives the fault current at the transformer secondary terminals. To find the fault current at a downstream panel, you must subtract the conductor impedance between the transformer and that panel.

Short Circuit Current Calculator

Name: Short Circuit Current Calculator
Availability: InStock
Author: Nham Vu

Calculate the available bolted short-circuit (fault) current at a transformer secondary given kVA rating, primary voltage, and percent impedance.

System Parameters

System Type

Three-Phase Single-Phase

Transformer Rating (kVA)

Secondary Voltage (V) line-to-line

Transformer Impedance (%Z)

Typically 2–6%; found on transformer nameplate.

Utility Source MVA (optional — leave blank for infinite bus)

Short-circuit MVA of the utility at the primary bus.

Enter system parameters and click Calculate to see results.

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Summary

Calculate the available bolted short-circuit (fault) current at a transformer secondary given kVA rating, primary voltage, and percent impedance.

How it works

Enter the transformer kVA rating (e.g. 500 kVA for a 500 kVA transformer).
Enter the secondary line-to-line voltage in volts (e.g. 480 V for a typical low-voltage system).
Select the system type: single-phase or three-phase.
Enter the transformer percent impedance (%Z), typically 2–6% from the nameplate.
Optionally enter the source impedance at the primary side to account for utility contribution.
Click Calculate to see the available bolted fault current in kA and A.

Use cases

Size circuit breakers and fuses for an electrical panel.
Verify that switchgear interrupting ratings exceed the available fault current.
Perform NEC and IEC arc-flash hazard assessments.
Validate transformer protection coordination studies.
Check bus ratings in motor control centers (MCCs).
Estimate fault current for new distribution transformer installations.
Design low-voltage power distribution for commercial buildings.
Compare fault levels before and after adding a second parallel transformer.

Frequently Asked Questions

Last updated: 2026-06-11 · Reviewed by Nham Vu