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.
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.
On smaller screens, scroll across the table to see every column.
| Parameter | Formula | Description |
|---|---|---|
| Full-load current | Ifl = kVA × 1000 / (√3 × V) | Transformer rated full-load current (A) |
| Sequence impedances | Z1 = Z2 = Z0 = Z% / 100 | Assumed Z0 = Z1 for this simplified model; confirm the actual transformer zero-sequence impedance |
| 3-phase fault | I3φ = Ifl / Z1 | Symmetrical three-phase prospective fault current |
| Phase-to-phase | Ipp = I3φ × √3 / 2 | Line-to-line fault ≈ 86.6% of 3-phase value |
| Phase-to-earth | Ipe = 3Vph / (Z1+Z2+Z0) = I3φ | With Z0=Z1, symmetrical components give Ipe = I3φ at LV terminals |
| Phase-to-neutral | IpN = Ipe | Model assumes zero additional neutral/earth return impedance at transformer terminals; verify actual paths |
On smaller screens, scroll across the table to see every column.
| Sequence | Impedance | Basis |
|---|---|---|
| Positive sequence (Z1) | = Z% / 100 | Transformer nameplate impedance |
| Negative sequence (Z2) | = Z1 | Assumed equal to positive sequence |
| Zero sequence (Z0) | = Z1 | Simplified assumption only; vector group alone does not establish the actual zero-sequence impedance. Confirm transformer data. |
| Source impedance | = 0 | Infinite upstream source for this single-transformer model; not a guaranteed installation maximum |
| 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 |
Confirm the transformer data and installation conditions with the responsible electrical engineer before selecting equipment or protection. See our switchboard fault-rating guide and Schneider Electric’s transformer-terminal calculation guide.
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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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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