When to use: Required at every stage of electrical system design. NEC 110.9 requires that every overcurrent protective device (breaker, fuse) have an interrupting rating at least equal to the available fault current. Undersized OCPDs can explode during a fault, causing fires, equipment destruction, and injury. Use this calculator when selecting circuit breakers and fuses, and when performing equipment labeling for arc flash studies.
Available fault current determines the minimum interrupting rating required for overcurrent protective devices (OCPDs) per NEC 110.9. This calculator determines maximum available fault current at the transformer secondary based on kVA, %Z, and voltage — helping engineers select properly rated breakers and fuses.
NEC 110.9 requires that every OCPD must have an interrupting rating equal to or greater than the available fault current at the point of installation. An under-rated breaker or fuse can fail violently during a fault — rupturing the enclosure and creating an arc flash hazard. Standard molded-case breakers are rated 10 kAIC; higher interrupting ratings (22, 42, 65, 100 kAIC) are required at high-fault-current locations near large transformers.
The simplified method uses transformer kVA, percentage impedance (%Z), and secondary voltage:
Three-phase FLA = kVA × 1000 / (√3 × V_secondary) Available fault current = FLA / (%Z / 100)
This gives the bolted three-phase fault current at the transformer secondary terminals. Downstream impedance (conductors, buses) reduces the available fault current at panels further from the transformer. This calculator also accepts optional utility fault contribution at the primary.
ANSI/IEEE standard transformers have impedance values that vary by kVA rating. Smaller units (under 75 kVA) typically have 2–3% Z; standard 112.5–500 kVA distribution transformers have 3–5% Z; large units (1000–2500 kVA) typically have 5.75% Z. Always use the actual nameplate %Z for calculations — field measurements can vary ±10% from the nominal value.
Select transformer kVA, percentage impedance, secondary voltage, and phase configuration. The calculator outputs transformer full-load amperes (FLA) and available fault current in kA at the secondary terminals. Use this value to verify OCPD interrupting rating compliance per NEC 110.9. Add conductor impedance for more accurate downstream panel fault current values.
Available fault current (short circuit current) is the maximum current that can flow at a given point during a bolted three-phase fault. It is used to verify that OCPDs have adequate interrupting ratings per NEC 110.9. Under-rated devices can fail catastrophically, creating fire and arc flash hazards.
Transformer percentage impedance (%Z) is printed on the transformer nameplate. Standard ANSI/IEEE values range from 2% for small single-phase units to 5.75% for 1000–2500 kVA distribution transformers. The actual nameplate value must be used for arc flash and short circuit studies — nominal values are not always accurate.
Interrupting capacity (kAIC — kiloamperes interrupting capacity) is the maximum fault current a breaker or fuse can safely clear. Standard molded-case breakers are rated 10 kAIC; high-interrupting-capacity devices are rated 22, 42, 65, or 100 kAIC. The device kAIC must meet or exceed the available fault current at its location (NEC 110.9).
Yes. Conductor impedance between the transformer and downstream panels reduces available fault current. This calculator computes the maximum value at the transformer secondary terminals. For precise downstream values, use IEEE methods to add conductor impedance based on wire size, length, and material.
Symmetrical (RMS) fault current is the steady-state value used for OCPD interrupting rating. Asymmetrical (peak) fault current accounts for the DC offset in the first cycle and is approximately 1.6× the symmetrical value. Most OCPD interrupting ratings are specified in symmetrical kA; momentary ratings for bus bars and switchgear use asymmetrical (crest) values.
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