Estimates the prospective bolted symmetrical fault current at a transformer's secondary terminals from its kVA and nameplate %impedance, an approximate peak asymmetrical fault current, and checks that the downstream OCPD's AIC rating and the switchgear/panelboard's SCCR nameplate rating both meet or exceed that available fault current, per NEC 110.9 (interrupting rating) and 110.10 (component protection/withstand rating).
Standards followed
Transformer %Z fault current method: I_fault = I_FLA / (%Z/100)NEC 110.9 â equipment interrupting ratingNEC 110.10 â circuit impedance, short-circuit current ratings, and other characteristics
What it can do
- Symmetrical RMS fault current from transformer kVA, secondary voltage and %Z.
- Approximate peak asymmetrical fault current using a conservative worst-case multiplier.
- Pass/fail check of both the OCPD AIC rating and the equipment SCCR nameplate rating.
Example problem
1500kVA transformer, 480V secondary, 5.75% impedance, 42kA-rated OCPD and switchgear.
- I_FLA = 1500Ă1000/(â3Ă480) â 1804.3A
- I_fault = 1804.3/(5.75/100) â 31,380A
- Both the 42kA OCPD and 42kA switchgear SCCR exceed 31,380A â adequate.
â31.4kA available fault current â both the OCPD and switchgear ratings are adequate.
This is a single-transformer screening calculation only â it ignores upstream utility/source impedance (which would reduce the fault current somewhat) and downstream cable impedance (which would reduce it further at points away from the transformer), so it reads conservatively high versus a full let-through study. The peak figure uses a conservative fixed 1.8Ă worst-case asymmetry multiplier rather than a system-specific IEC 60909 Îș factor. For coordinated multi-point fault studies, see the Fault Current Propagation (Base kVA Method) calculator.