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Transformer fault current.
Start with the nameplate.

Use the CWS calculator for a preliminary transformer-terminal estimate. Enter the transformer rating, secondary voltage and impedance, then review the assumptions alongside the result.

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CWS switchboard manufacturing in Invercargill
Before using the result

Keep the project conditions in view.

This tool supports an early specification discussion. It does not establish the rating or compliance of a completed switchboard. Confirm the source data, installation conditions and required verification with the responsible project designer.

Transformer Nameplate Data
Transformer rated output (kVA)
Enter a valid kVA > 0
LV line-to-line voltage (V)
Enter a valid voltage > 0
Per-unit impedance from nameplate (%)
Enter impedance between 0.1% and 20%
Calculation Method / 3-Phase (Dyn)
ParameterFormulaDescription
Full-load currentIfl = kVA × 1000 / (√3 × V)Transformer rated full-load current (A)
Sequence impedancesZ1 = Z2 = Z0 = Z% / 100For Dyn transformer: Z0 = Z1 (delta primary confines zero-sequence to LV winding)
3-phase faultI3φ = Ifl / Z1Symmetrical three-phase prospective fault current
Phase-to-phaseIpp = I3φ × √3 / 2Line-to-line fault ≈ 86.6% of 3-phase value
Phase-to-earthIpe = 3Vph / (Z1+Z2+Z0) = I3φWith Z0=Z1, symmetrical components give Ipe = I3φ at LV terminals
Phase-to-neutralIpN = IpeMEN system: neutral bonded to earth at transformer / fault path is identical
Sequence Impedance Assumptions (MEN / Dyn Transformer)
SequenceImpedanceBasis
Positive sequence (Z1)= Z% / 100Transformer nameplate impedance
Negative sequence (Z2)= Z1Assumed equal to positive sequence
Zero sequence (Z0)= Z1Dyn transformer / delta primary blocks zero-sequence from HV network; zero-sequence current circulates within the delta winding, so Z0 = transformer leakage impedance
Source impedance= 0Infinite bus (conservative worst-case / maximises fault current)
Cable impedance= 0Fault assumed at transformer LV terminals; add cable Z for downstream points
Neutral/earth path= 0MEN bond at transformer / neutral and earth at same potential at source
Engineering Disclaimer: This calculator provides simplified, conservative fault current estimates at transformer LV terminals. It assumes an infinite source, zero cable impedance, and simplified sequence modelling (Z0 = Z1 for a Dyn transformer on an MEN system). Earth-fault results are upper-bound approximations only and may exceed actual values at points further from the transformer due to earth-return-path impedance.

Results are for preliminary use only and must be verified by a qualified electrical engineer using a detailed fault study.
Move from estimate to specification

Bring the inputs to CWS.

Send the single-line diagram, source information, load schedule and site conditions with your enquiry. Simotrol is our primary platform for main switchboards and MCCs, with the assembly configured around the project requirements.

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Continue your project

MSB project requirementsDiscuss the supply and required assembly rating.Verification and standardsAgree the evidence for the proposed assembly.Electrical design scopeDefine the network data and study boundaries.
Contact CWS

Talk through your project.

Send your drawings, site requirements and project programme to our Invercargill team.

Printable quote checklist (PDF)Email your checklist, drawings and schedules to estimating@clivewilson.co.nz. Leave unknown items blank.

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