Short Circuit Estimator

Short circuit. Each part of the supply path becomes an impedance in ohms with its X/R ratio. The utility comes from its fault MVA or kA, the transformer from its %Z on its own kVA base, and low-voltage cables from NEC Chapter 9 Table 9 (600 V cable, 75 °C, 60 Hz, three single conductors in conduit). Cable R and X use circuit length divided by the number of identical parallel sets. Above 600 V through 15 kV, EPR estimates use representative Southwire 15 kV, 133% insulation-level, tape-shield cable data at 90 °C with touching conductors (SPEC 46311 aluminum / 46302 copper). Other cables and arrangements use custom manufacturer positive-sequence R and X. The neutral does not enter this three-phase calculation. Cable assumptions are THWN for low voltage and EPR for medium voltage, with three phase conductors on the primary and three phases plus neutral on the secondary. Conductor material is selectable; defaults are aluminum on the primary and copper on the secondary. The neutral is excluded from fault-current impedance; parallel sets are identical phase-conductor sets. Calculation temperatures remain 75 °C for LV and 90 °C for the representative MV data. Impedances upstream of the transformer are referred to the secondary by the voltage ratio squared. The supply's bolted three-phase fault current is the nominal line-to-neutral voltage divided by the total impedance. Grouped motor hp or kW adds a simplified initial motor contribution, without requiring individual motor data.

Peak estimate. With motors excluded, the single-source series-RL estimate uses √2 × I × (1.02 + 0.98e^−3R/X), the peak-factor approximation in IEC 60909-0:2026 §8.1. This is not a full IEC short-circuit calculation: voltage and transformer correction factors, network operating scenarios and equipment-duty adjustments are not applied. No combined peak is shown with motors.

Arc flash. The IEEE 1584-2018 model takes the bolted fault current, conductor gap and electrode configuration to get the arcing current, then a reduced arcing current using the standard's variation factor. It applies the enclosure size correction and calculates both cases at their entered durations. Incident energy and the arc flash boundary are maximized independently. The boundary is the distance where energy falls to 1.2 cal/cm². Above 600 V the model interpolates between the 600 V, 2.7 kV and 14.3 kV equations.

Equipment defaults. Typical gaps and enclosure classes follow IEEE 1584-2018 Table 8; working distances follow Table 10. Where Table 8 specifies a depth class, 100 mm represents shallow enclosures (≤8 in) and 250 mm represents deep enclosures (>8 in). These are model inputs for the selected class, not dimensions verified for installed equipment.

What it leaves out. One fault point and one source path only: no generators, no multiple utility feeds, no ties, no motor decay over time, and no automatic device curves. Medium-voltage cable construction, spacing and shield bonding are not inferred from the low-voltage table. It doesn't check equipment ratings. The two-second exposure assumption follows the engineering-judgment discussion in IEEE 1584-2018 §6.9.1; it is not a predicted device clearing time or a guaranteed upper bound. The default compares VCB, VCBB and HCB at the entered fault level and duration, and reports the highest incident energy and the largest boundary separately. It does not search every source condition. A real study models the whole system, every operating scenario and the actual protective device settings.