Fault Current Calculator | Kewo Engineering

System

Source

Primary voltagekV Primary system

System voltage Voltage, line to lineV Configuration

Infinite source

Three-phase faultkA X/R Phase-to-ground faultkA X/R

Phase-to-phase faultkA X/R Phase-to-neutral faultkA X/R

Transformer

Rating Secondary voltage Rating, otherkVA Secondary, line to lineV Configuration Impedance%Z X/R Primary winding Primary connection Impedance tolerance Reduce impedance by%

More transformer settings

Zero-sequence ratio Z0/Z1 Half-winding R multiplier Half-winding X multiplier

Secondary cable

Conductor Size Installation Cables per phase Lengthft Insulation Conductor spacing (GMD)in

Drag to turn. Double-click to reset.

More cable settings

Neutral size Conductor temp., faults°C Conductor temp., voltage drop°C Enter the cable impedance instead R per conductor at 75 °CΩ/1000 ft X per conductorΩ/1000 ft

Equipment rating at the end of the cable

Short-circuit ratingkA

Finds the shortest cable that keeps every fault at the end within the rating.

Load

for voltage drop

Load Power factor, lagging Load connected

Report details

Location Prepared by Reference

Available fault current

Fault current

Symmetrical rms, bolted faults

Voltage drop

Fault current along the cable

Inputs

Impedances and asymmetry ohms at the secondary voltage

Basis

Bolted, symmetrical rms faults by the ohmic method with complex impedances; prefault voltage at nominal; no motor or inverter contribution.

Source impedance from the three-phase and phase-to-ground fault currents and X/R. Transformer impedance from %Z and X/R; single-phase 120 V faults use 1.5R + j1.2X of the full winding (half-winding approximation).

Cable R and X from NEC Chapter 9 Table 9 (conduit); for direct buried and overhead cable, R from Table 9 and X from the geometry of the cable drawn. R corrected for temperature; phase-to-ground and phase-to-neutral faults return on the neutral.

Voltage drop by the IEEE Std 141 formula; transformer regulation at nameplate impedance with the primary at nominal voltage.

Method

Bolted, symmetrical rms fault currents by the ohmic method with complex impedances. Prefault voltage equals nominal. Motor and inverter contributions, arc impedance and transformer taps are not included.

Source

Three-phase fault: Z₁ = V_LN / I_3φ at angle tan⁻¹(X/R).
Phase-to-ground fault: 2Z₁ + Z₀ = 3V_LN / I_LG, which gives Z₀. Left blank, the ground-fault current is taken equal to the three-phase value (Z₀ = Z₁).
Infinite source: Source impedance zero; only the transformer and cable limit the current.
Single-phase transformer: Connected phase to phase, the transformer sees Z₁ + Z₂ = 2Z₁. Connected phase to neutral, it sees (2Z₁ + Z₀)/3. Source impedance is referred to the secondary by the square of the turns ratio.
Cable only: The fault current entered is the value available at the start of the cable. For a single-phase 3-wire source left without a phase-to-neutral value, 1.5 × the phase-to-phase value is used.

Transformer

Impedance: Z_T = (%Z/100) V²/S, with R = Z/√(1 + (X/R)²). Typical %Z and X/R fill in when the rating changes; replace them with nameplate or test values.
Tolerance: “Minimum, IEEE C57.12.00” reduces the impedance by 10 % when it is 2.5 % or less and by 7.5 % above (C57.12.00 §9.2), for the highest fault current.
Three-phase connections: Delta to grounded wye: secondary zero sequence is the transformer’s own, Z₀/Z₁ × Z_T; the delta blocks the source. Grounded wye to grounded wye: source Z₀, referred, adds in series. Delta secondary: no phase-to-ground fault current.
Single-phase, 3-wire: Faults on the 120 V half winding use 1.5R + j1.2X of the full-winding per-unit impedance on the half-voltage base (Eaton Bussmann, Selecting Protective Devices). At the terminals this gives 1.33 to 1.67 times the 240 V fault.
Primary line current: Secondary fault current times V_s/V_p, the highest line shown. A delta–wye bank carries a secondary phase-to-phase fault as 2/√3 and a phase-to-ground fault as 1/√3 of the ratio-adjusted current on the worst primary line.
Withstand basis: The terminal fault is compared with the IEEE C57.12.00 Table 12 multiple for Category I units (40, 35 or 25 times rated current by size).

Cable

Conduit: R and X from NEC Chapter 9 Table 9 (600 V cables, 60 Hz, 75 °C, three single conductors in conduit) for PVC, aluminum or steel conduit.
Direct buried and overhead: R from the PVC column. X = 0.02298 ln(D/GMR) Ω per 1000 ft at 60 Hz, from the construction drawn (below). Triplex cores touch in a triangle; quadruplex cores close round a ring as tightly as they fit, a square when equal. Three-phase and phase-to-phase faults use the GMD of the phases; the phase-to-neutral loop uses the spacing to the nearest phase, X = 0.02298 [ln(d/GMR_phase) + ln(d/GMR_neutral)]. GMR is that of Class B stranding on the conductor diameter. For open wire, the GMD entered.
ACSR messenger: R of aluminum of the same size from Table 9. GMR of the aluminum strands; the steel strand carries no current and its magnetization is ignored, which gives slightly more phase-to-neutral fault current than published GMRs of 6/1 ACSR.
Temperature: R_T = R₇₅ (T + k)/(75 + k), k = 234.5 for copper and 228.1 for aluminum. Faults default to 25 °C (lowest resistance, highest current); voltage drop to 75 °C.
Return path: Cables in parallel divide the impedance. Phase-to-ground and phase-to-neutral faults return on the neutral, one per parallel set: for the cable, 2Z₁ + Z₀ = 3 × the phase-to-neutral loop, which is Z₀ = Z₁ + 3Z_N when the phase reactance is the same in both. Parallel earth and raceway paths are ignored.

Cable drawing

In conduit: One parallel set: Type THHN/THWN-2 (PVC and nylon, UL 83) or XHHW-2 (XLPE, UL 44). Copper is Class B (NEC Table 8), aluminum compact (Table 5A); diameters over insulation from Table 5 or 5A. The conduit is the smallest trade size of PVC Schedule 40 or rigid metal conduit (Table 4) at the Table 1 fill, without a grounding conductor. Conductors 1 AWG and larger are drawn black with phase tape, as commonly supplied; smaller ones in colored insulation.
Direct buried: 600 V URD, Type USE-2, compressed conductors, XLPE 60 mil (8–2 AWG), 80 mil (1–4/0), 95 mil (250–500 kcmil), 110 mil (600–1000 kcmil) (ICEA S-105-692, UL 854). The neutral carries three yellow stripes, the utility marking; NEC work needs white or gray marking (200.6).
Overhead triplex and quadruplex: Service drop to ICEA S-76-474: compressed 1350 aluminum phases with XLPE 30 mil (6–4 AWG), 45 mil (2 AWG) or 60 mil (1/0 AWG and up) on a bare messenger, ACSR 6/1 of the same aluminum area up to 4/0 AWG and 18/1 above, or bare copper with copper phases.
Open wire: Bare Class B conductors on porcelain spools of a secondary rack, the neutral on top. For three phases, the rack spacing is GMD/1.26.
Colors: Customary, not required by the NEC: black, red, blue with a white neutral up to 250 V; brown, orange, yellow with a gray neutral from 277 to 600 V.

Fault currents

Three-phase: I = V_LN / |Z₁|
Phase to phase: I = V_LL / |2Z₁| on three-phase systems; on single-phase, the 240 V loop through both line conductors.
Phase to ground or neutral: I = 3V_LN / |2Z₁ + Z₀|; on split-phase, the 120 V loop through one line conductor and the neutral.
Asymmetry: X/R of the fault loop; peak i_p = κ√2 I with κ = 1.02 + 0.98e^−3R/X (IEC 60909-0); asymmetrical rms at ½ cycle I√(1 + 2e^−2πR/X).
Shortest cable for a rating: Each fault is a loop I(L) = V / |A + BL|, A at the start of the cable and B per foot. The length that brings every fault type down to the rating is solved directly.

Voltage drop

Formula: IEEE Std 141: VD = E_s + IR cosθ + IX sinθ − √(E_s² − (IX cosθ − IR sinθ)²).
Circuits: Three-phase per phase, times √3. Single-phase over the loop: both line conductors for a 240 V load, line and neutral for a 120 V load.
Transformer: Regulation at the load uses the nameplate impedance with the primary held at nominal voltage.
Guides: 3 % for a feeder or branch circuit and 5 % overall, from the informational notes to NEC 210.19 and 215.2.

Checks

Each case is solved live by this calculator.

Case	Reference	Expected	Calculated

The engine also agrees with a separate per-unit implementation within 1 × 10⁻⁵ over 400 random cases covering both phase counts, every transformer connection and every installation type. The constructions drawn match published diameters over insulation within 0.006 in, and assembly diameters within 5 % (Southwire SPEC 83230 and 83248, Prysmian PowrServ, 600 V URD Type USE-2).

Limitations

No motor, generator or inverter contribution; source X/R held constant.
Bolted faults only. Arcing faults draw less current.
Cable data are for 600 V cables. For medium-voltage cable, enter R and X under More cable settings.
The drawing shows typical constructions; strand counts and diameters of compressed and compact conductors vary by maker.
Transformer at nominal tap, prefault voltage 1.0 per unit, 60 Hz.