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Fault Current Calculator

Infinite-bus available fault current at transformer secondaries.

Inputs

Transformer

Transformer nameplate kVA.

Secondary line-to-line voltage: 208, 240, 480, 600.

Percent impedance from the transformer nameplate. Lower impedance means higher available fault current.

Primary line-to-line voltage, used for the primary-side fault current.

Fault path

Running motors add fault current. A common screening allowance is four times the motor full-load current, which for a motor-heavy plant can approach or exceed the transformer full-load amps.

Multiplier from RMS symmetrical to peak asymmetrical current. About 1.6 for a typical X/R ratio, and higher for large transformers close to generation.

Result

Transformer-only symmetrical current
20,818 A
Available fault current
20.82 kA
Symmetrical fault current with motors
20,818 A
Peak asymmetrical current
33,309 A
Primary-side fault current
347 A
Motor contribution
0 A
Secondary full-load current
416.4 A

What to check

  • Transformer kVA
  • Secondary volts
  • Nameplate impedance

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Related guides

Screening tool. Verify against the NEC as adopted locally, the AHJ, nameplate data, and metered conditions before acting.

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