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NZ Transformer Fault Current Calculator

Use this free tool to estimate prospective fault currents at the LV terminals of any distribution transformer — for switchboard fault rating, protection coordination, and AS/NZS 3000 compliance.

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.
Need a full fault study for your project?
Our engineers provide detailed prospective fault current studies, protection coordination, and AS/NZS-compliant switchboard schedules — from transformer to final circuit.
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