Available Fault Current
Available fault current (AFC), also called maximum available fault current or available short-circuit current, is the largest amount of current capable of being delivered at a specific point on an electrical system during a short-circuit condition, as defined in NFPA 70 (NEC) Article 100. It assumes a bolted fault of negligible impedance and varies with source size, impedance, conductor length, transformer kVA, and network configuration. AFC is a system property at a location, not a device rating.
On a shop floor, available fault current drives three main safety and compliance activities: selecting and verifying interrupting ratings of protective devices (e.g., fuses, circuit breakers) per NEC 110.9, ensuring equipment short-circuit current rating (SCCR) exceeds AFC per NEC 110.10, and field-marking AFC at service equipment per NEC 110.24. In day-to-day workflows, engineers perform short-circuit studies to calculate AFC at service entrances and distribution panels, using utility data and transformer parameters. Results inform one-line diagrams, device selection, and arc-flash hazard analysis. Labels must be durable and updated after system modifications. WorkSafeBC expects compliance with applicable codes, implicitly relying on correct AFC calculations for equipment ratings and worker safety.
How is available fault current calculated at a service entrance in an industrial facility?
AFC at the service is determined using utility-provided fault current at the transformer secondary, transformer kVA, secondary voltage, and percent impedance (%Z). The approximate three-phase symmetrical fault current is I_sc ≈ (kVA × 1000) / (√3 × V_LL) × (100 / Z%). Downstream conductor impedance is applied to calculate AFC at the service disconnect and further points. Software like SKM or ETAP automates this, but the NEC Article 100 definition remains the same.
How is 'fault current' different from 'available fault current' in technical use?
Fault current is the actual current flowing during a specific fault event, which may be less than the maximum due to partial impedance, arcing, or motor contributions. Available fault current is the theoretical maximum current that could flow during a bolted short-circuit condition, considering system configuration and source capabilities. AFC is used as the design value for verifying interrupting ratings and SCCR, while actual fault current may be lower in practice.
How does available fault current relate to arc-flash incident energy calculations?
Arc-flash incident energy analysis per IEEE 1584/NFPA 70E requires system short-circuit current at the fault point as an input. The bolted fault current magnitude (close to AFC) is used to estimate arc-flash current, but maximum AFC does not always produce the highest incident energy because lower fault current can lead to slower protective device clearing times, increasing energy. AFC must be known for proper arc-flash modeling, and AFC labels are separate from arc-flash labels.