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A short-circuit study calculates the maximum available fault current (in amperes) at each point in an electrical distribution system. The results determine whether installed circuit breakers and fuses have sufficient interrupting capacity (AIC rating) and provide the fault current input required for arc flash incident energy analysis per IEEE 1584-2018.
The point-to-point method is the most common simplified approach for short-circuit analysis in commercial and industrial distribution systems. The method calculates fault current by summing impedances from the utility source through each transformer, feeder conductor, and branch circuit to the fault location. The available fault current decreases with distance from the source due to the increasing impedance of conductors.
Starting point: the utility provides the available fault current (AFC) at the service entrance transformer primary, typically expressed in kVA short-circuit or amperes. For utility-supplied systems in the US, this is the interrupting duty at the point of common coupling (PCC).
For transformers: the transformer percent impedance (%Z) is the primary variable. A 75 kVA, 480V transformer with 2% impedance has an available secondary fault current of approximately: (75,000 VA / (1.732 × 480V)) / 0.02 = 4,505A maximum fault current — before considering conductor impedance. Higher %Z transformers (5-6%) reduce fault current significantly.
NEC 110.9 requires that all electrical equipment be rated to safely interrupt (or withstand) the maximum available fault current at their location. Equipment with insufficient AIC rating can fail catastrophically under fault conditions. The point-to-point calculation verifies AIC compliance for every protective device.
SparkShift's short-circuit calculator performs the point-to-point analysis, generates fault current at each bus, and outputs results formatted for arc flash study input and breaker AIC verification.
Open the short circuit calculator. Results include NEC citations and can be saved to a project for PDF export.
The point-to-point method calculates available fault current by summing impedances from the utility source through transformers and conductors to each fault location. It is a simplified method suitable for commercial and industrial distribution systems and produces conservative (higher) fault current estimates.
NEC 110.9 requires that all equipment intended to interrupt fault current be rated for the maximum available fault current at its location. Equipment with insufficient AIC rating can fail catastrophically — producing explosive arc flash energy rather than safely interrupting the fault.
Transformer percent impedance (%Z) directly limits the maximum fault current on the secondary. Higher %Z transformers produce lower fault currents. A 5% impedance transformer produces approximately half the fault current of a 2.5% impedance transformer of the same kVA rating.
The utility's available fault current (AFC) at the service entrance transformer primary must be obtained from the serving utility. US utilities typically provide this as a short-circuit kVA or ampere value at the point of common coupling. This value is the starting point for the point-to-point calculation.
Disclaimer: SparkShift is a calculation tool for electricians and engineers. It is not a substitute for a professional power systems engineering study. SparkShift is not approved by NCEES, IEEE, NFPA, or any professional organization. For stamped engineering documents, engage a licensed Professional Engineer in your jurisdiction.