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

kVA Rating

Transformer rated output (kVA)

Enter a valid kVA > 0

Secondary Voltage

LV line-to-line voltage (V)

Enter a valid voltage > 0

Impedance %

Per-unit impedance from nameplate (%)

Enter impedance between 0.1% and 20%

Calculation Method — 3-Phase (Dyn)

Parameter	Formula	Description
Full-load current	`I_fl = kVA × 1000 / (√3 × V)`	Transformer rated full-load current (A)
Sequence impedances	`Z₁ = Z₂ = Z₀ = Z% / 100`	For Dyn transformer: Z₀ = Z₁ (delta primary confines zero-sequence to LV winding)
3-phase fault	`I_3φ = I_fl / Z₁`	Symmetrical three-phase prospective fault current
Phase-to-phase	`I_pp = I_3φ × √3 / 2`	Line-to-line fault ≈ 86.6% of 3-phase value
Phase-to-earth	`I_pe = 3V_ph / (Z₁+Z₂+Z₀) = I_3φ`	With Z₀=Z₁, symmetrical components give I_pe = I_3φ at LV terminals
Phase-to-neutral	`I_pN = I_pe`	MEN system: neutral bonded to earth at transformer — fault path is identical

Sequence Impedance Assumptions (MEN / Dyn Transformer)

Sequence	Impedance	Basis
Positive sequence (Z₁)	= Z% / 100	Transformer nameplate impedance
Negative sequence (Z₂)	= Z₁	Assumed equal to positive sequence
Zero sequence (Z₀)	= Z₁	Dyn transformer — delta primary blocks zero-sequence from HV network; zero-sequence current circulates within the delta winding, so Z₀ = transformer leakage impedance
Source impedance	= 0	Infinite bus (conservative worst-case — maximises fault current)
Cable impedance	= 0	Fault assumed at transformer LV terminals; add cable Z for downstream points
Neutral/earth path	= 0	MEN 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.

Get in Touch

Common NZ Transformer Fault Current Reference Table

Pre-calculated prospective fault currents for standard New Zealand distribution transformer sizes at 400V secondary. Values assume infinite source impedance (worst-case / conservative). Impedance values are typical for NZ distribution transformers per AS 60076.

NZ Transformer Prospective Fault Currents at 400V Secondary
Transformer Rating	Typical Impedance (Z%)	Full Load Current (A)	Prospective Fault Current (kA)	Common Switchboard Rating
200 kVA	4%	289 A	7.2 kA	10 kA
500 kVA	4%	722 A	18.0 kA	20 kA
750 kVA	5%	1,083 A	21.7 kA	25 kA
1,000 kVA	5%	1,443 A	28.9 kA	35 kA
1,500 kVA	6%	2,165 A	36.1 kA	40 kA
2,000 kVA (2 MVA)	6%	2,887 A	48.1 kA	50 kA
2,500 kVA (2.5 MVA)	6%	3,608 A	60.1 kA	65 kA

Notes: Full load current calculated as I = kVA x 1000 / (1.732 x 400). Prospective fault current = Full load current x 100 / Z%. These are transformer-only fault levels. Actual fault current at the switchboard will be lower due to cable impedance and source impedance. Always verify with a site-specific fault level study. Clive Wilson Switchboards manufactures boards rated from 10kA to 65kA+ to suit all common NZ transformer sizes. Contact us on +64 3 214 4264 for fault level advice.

Parameter	Formula	Description
Full-load current	`I_fl = kVA × 1000 / V`	Rated secondary full-load current (A)
Series impedance	`Z₁ = Z% / 100`	Transformer leakage impedance (per-unit)
Bolted fault current	`I_sc = I_fl / Z₁`	Prospective short-circuit current at LV terminals
Phase-to-earth	`I_pe = I_sc`	MEN system: earth fault path through transformer winding impedance only at LV terminals
Phase-to-neutral	`I_pN = I_sc`	MEN system: neutral bonded to earth at transformer — fault path is identical