Fault Current Available Calculator

Available fault current is the current that would flow if a bolted short circuit occurred at a given point, and it is set by the source voltage divided by the total impedance between source and fault. For a three-phase system the symmetrical RMS value is the voltage divided by the product of the square root of 3 and the total impedance. The dominant impedance in most building systems is the service transformer, whose contribution is derived from its nameplate percent impedance: transformer ohms equal percent impedance divided by 100, times voltage squared, divided by the transformer volt-ampere rating. A 1,000 kVA transformer at 480 volts with 5.75 percent impedance contributes about 0.0133 ohms, producing roughly 21,000 amperes of symmetrical fault current at its secondary terminals. The symmetrical figure captures only the AC component. At the instant of a fault the waveform can be offset by a decaying DC component whose magnitude depends on the circuit X over R ratio and on the point of the voltage wave at which the fault initiates, so the asymmetrical value commonly runs 1.2 to 1.7 times the symmetrical value. Protective devices carry an interrupting rating, typically 10 kA, 22 kA, 42 kA, or 65 kA for molded-case breakers, which must equal or exceed the available fault current where they are applied. Conductor impedance reduces fault current with distance, so the worst case is normally at the service entrance. Contribution from running motors during the first few cycles adds to the total and is often neglected in simplified hand methods.

Rigorous short-circuit analysis of polyphase systems became possible in 1918, when Charles LeGeyt Fortescue presented his method of symmetrical coordinates to the American Institute of Electrical Engineers. By resolving unbalanced three-phase quantities into balanced positive, negative, and zero sequence components, Fortescue turned unbalanced faults into tractable single-phase problems, and the technique remains the foundation of every fault study performed today. The per-unit system, which normalizes impedances to a common base and makes transformer percent impedance directly usable, spread through the same period as utility interconnection grew and hand calculation of large networks became unavoidable. As distribution equipment grew in capacity, the destructive potential of a fault grew with it, and industry practice consolidated the calculation into repeatable procedures. IEEE Std 141, the Red Book on electric power distribution for industrial plants, and the companion IEEE Std 241 for commercial buildings, documented the accepted methods, while equipment makers such as Bussmann popularized the point-to-point method that lets an installer estimate fault current at successive points along a feeder using only transformer data and conductor lengths. Arc-flash research from the 1980s onward, later codified in NFPA 70E and IEEE Std 1584, made available fault current a worker safety parameter and not merely an equipment rating question. The National Electrical Code added 110.24, requiring service equipment in other than dwelling units to be field marked with the maximum available fault current and the date of the calculation, in its 2011 edition.