Three-Phase Bolted Fault & Short Circuit Calculator

Determine maximum available symmetrical fault current at transformer terminals and downstream panelboard buses using the IEEE point-to-point method.

Transformer Source Parameters

Transformer Rating (kVA):

Secondary Voltage (V L-L):

Transformer Impedance (%Z):

Utility Primary Fault (MVA):

Feeder Run to Sub-Panel

One-Way Feeder Length (ft):

Conductors per Phase (n):

Wire Size:

Conductor & Conduit Type:

Available Short-Circuit Fault Results

Downstream Bus Fault Current (Isc)

-- kA

-- RMS Symmetrical Amperes

Transformer Terminal Isc

-- kA

-- A FLA

Min Panel AIC Rating

-- kA AIC

NEC 110.9 compliant

Feeder f-Factor: --

Multiplier M [1 / (1 + f)]: --

Estimated Asymmetrical Peak (Crest): -- kA peak

NEC 110.9 requires all overcurrent devices to have an interrupting rating equal to or exceeding available short-circuit current.

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Frequently Asked Questions

What is the point-to-point short-circuit calculation method?

The point-to-point method is a straightforward, widely used IEEE/Bussmann formula for calculating available 3-phase symmetrical short-circuit current from a transformer through successive feeder runs. It converts wire impedance into an f-factor: f = (1.732 × L × I) / (C × n × V), reducing fault current at each downstream bus by multiplier M = 1 / (1 + f).

Why assume an infinite bus on the transformer primary?

Assuming an infinite utility bus presumes the utility supply has zero source impedance and infinite fault capacity. This represents the worst-case, most conservative calculation possible, ensuring switchgear interrupting ratings will never be undersized if the utility upgrades nearby substation capacity.

What is the difference between symmetrical kA and asymmetrical peak kA?

Symmetrical kA is the root-mean-square (RMS) value of the AC component of the fault current. Asymmetrical peak kA accounts for the DC offset that occurs when a short circuit happens at a non-zero voltage angle, producing an initial crest current up to 1.7 to 2.2 times higher during the first half-cycle.