Maximum Short-Circuit Current Study

Source / Point of InterconnectionUtility or upstream system data

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Study / equipment tag System voltage at POI (V, line-to-line)

600 V

4160 V

12.47 kV

13.8 kV

25 kV

34.5 kV

Max 3φ sym. Isc at POI Unit

X/R ratio at POI

If the utility gives only Isc with no X/R, IEEE Std 242 notes a common conservative assumption of 12–15 for transmission-fed sources.

Primary Cable (optional)Between POI and transformer primary

Length Unit

Conductor Raceway

Conductor size Sets / phase (parallel)

Cable impedance reduces downstream Isc — every run added lengthens the total impedance path.

TransformerSize, secondary voltage, %Z / X-R

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Rating Construction

Secondary voltage (V, L-L) Vendor family (reference only)

208 V

240 V

480 V

600 V

4160 V

Use actual nameplate %Z / X-R instead of typical table

Nameplate %Z Nameplate X/R

Secondary Cable (optional)Between transformer secondary and equipment

Length Unit

Conductor Raceway

Conductor size Sets / phase (parallel)

This is the cable feeding the "secondary equipment" — the final result below is the Isc it must be rated to interrupt.

Secondary Grounding (optional)Solid, low-resistance, or high-resistance neutral grounding

Grounding type

Specify NGR by

Neutral grounding resistor, R_NGR (Ω) System charging current, I_C (A, optional) Resistor duty rating

Single-Line Diagram

updates live as components are added

ANSI/IEEE one-line symbols · schematic, not to scale

Result — Secondary Equipment

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kA symmetrical RMS

Max Isc, secondary equipment

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Isc at transformer secondary bus

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Total Z to fault point

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Resulting X/R

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Peak asym. (crest) current

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κ (asymmetry factor)

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Include Calculation Ledger in report Include OhmZone brand mark in report

Report template (.docx, optional)

Calculation Ledger

ohmic (Ω) method, all impedances referred to secondary voltage base