Assumptions & Limitations

Fault current calculations are based on the point-to-point method, together with the assumptions described below. Center-tapped systems estimate L-N fault current at 1.5× the L-L value at the transformer terminals, and L-N branches model the neutral at phase-conductor impedance. Single-phase branches from three-phase systems use bolted-fault ratios: L-L legs at 86.6% of the three-phase symmetrical value, L-N legs at 100% per the near-transformer assumption; actual L-N bolted faults normally fall between 25% and 125% of the three-phase value. Calculations assume nominal utility voltage and ungrounded delta windings; corner-grounded delta is not modeled, and motor contribution is excluded. Where source available fault current is calculated rather than user-specified, the utility source is assumed to have zero source impedance (an infinite bus).

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