What is fault current?

Fault current is the abnormally large current that flows when conductors are inadvertently short-circuited. It is limited primarily by the source voltage and the total system impedance (transformer, utility, cables). Protective devices must interrupt this current without damage.

What is percent impedance (%Z)?

%Z is a nameplate value expressing a transformer's leakage impedance as a percentage of its base impedance. A transformer with 5 %Z will deliver a bolted three-phase fault current equal to 100/5 = 20 times its rated current. Typical values range from 2 % (small distribution transformers) to 10 % (large power transformers).

How is the three-phase fault current calculated?

I_fault_3ph = I_base / %Z_pu, where I_base = MVA_base / (√3 × kV_base) and %Z_pu = %Z / 100. This assumes an infinite bus (zero source impedance). When a finite utility short-circuit MVA is provided, the equivalent impedance is Z_eq = Z_transformer + Z_utility.

What is a single-line-to-ground (SLG) fault?

An SLG fault occurs when one phase conductor contacts ground. For a solidly grounded system with equal positive, negative, and zero sequence impedances, the SLG fault current is approximately 3 × V_phase / (Z1 + Z2 + Z0) ≈ equal to the three-phase fault current for a transformer with balanced sequence impedances.

Does this tool account for motor contribution?

No. Motor contribution (back-fed fault current from rotating machines) is not included. For large industrial plants with significant motor load, multiply the calculated result by a motor contribution factor (typically 1.0 to 1.25) as a conservative estimate.

What does infinite bus assumption mean?

Infinite bus means the utility source is assumed to have zero impedance — it can supply unlimited current. This gives the worst-case (maximum) fault current for protective device sizing. If the utility provides a finite short-circuit MVA figure, enter it to get a more accurate combined impedance.

Fault Current Calculator

Name: Fault Current Calculator
Availability: InStock
Author: Nham Vu

Enter system voltage, transformer MVA, and percent impedance to calculate three-phase and single-line-to-ground fault currents instantly.

System Parameters

System Voltage (line-to-line)

Common: 0.48 kV, 4.16 kV, 13.8 kV, 34.5 kV

Transformer Rating

Transformer Percent Impedance (%Z)

From nameplate. Typical: 2–6% (distribution), 6–10% (power).

Utility Short-Circuit Capacity (optional)

MVA

Leave blank for infinite bus (maximum fault current).

Simplified One-Line Diagram

Results

Enter parameters and click Calculate

Common Transformer Presets

Rating	Voltage	%Z	Application
500 kVA	480 V	3.5%	Small commercial
10 MVA	13.8 kV	5.75%	Industrial substation
30 MVA	34.5 kV	7.0%	Distribution feeder
100 MVA	115 kV	9.0%	Transmission substation

Click a row to load its values.

Summary

Enter system voltage, transformer MVA, and percent impedance to calculate three-phase and single-line-to-ground fault currents instantly.

How it works

Enter the rated line-to-line voltage at the fault point in kilovolts (kV).
Enter the transformer MVA (or kVA) rating and select the unit.
Enter the transformer percent impedance (%Z) from the nameplate data.
Optionally enter a utility source impedance (in MVA short-circuit capacity) to combine with the transformer impedance.
Click Calculate to see the three-phase and SLG fault currents along with base values.
Review the formula breakdown and use the results in breaker or fuse sizing studies.

Use cases

Sizing circuit breakers and fuses for the correct interrupting rating.
Protective relay coordination studies in industrial and utility systems.
Verifying that switchgear interrupting capacity exceeds available fault current.
Determining conductor short-circuit withstand ratings.
Teaching power systems fundamentals with per-unit analysis.
Checking NEC / IEEE 1584 arc flash boundary calculations.
Validating transformer nameplate data against system design requirements.
Preliminary fault analysis before full symmetrical component software runs.

Frequently Asked Questions

Last updated: 2026-06-13 · Reviewed by Nham Vu