433, 415 or 400 volts?
The Australian LV secondary.
Three numbers name the same low-voltage winding. Which one a transformer carries, and why they differ, is the difference between nominal, no-load and on-load voltage.
Specify a distribution transformer in Australia and its secondary is almost always written as 433 V — yet the nominal supply is 400 V, older drawings say 415 V, and New Zealand still works in 415 V. They are not contradictions. They describe the same winding at three different moments in its working life.
The three voltages
| Voltage | What it is |
|---|---|
| 400 V | Nominal. The harmonised three-phase value the network is named for — 230 V phase-to-neutral, 400 V phase-to-phase — under AS 60038, with a supply tolerance of +10% / −6% at the point of connection. |
| 433 V | No-load (open-circuit). The voltage the secondary produces on its nominal tap with nothing drawn. Australian distribution transformers are wound high on purpose, so that once load pulls the terminal voltage down, what reaches the customer is still nominal. |
| 415 V | The old nominal. Australia used 240 / 415 V until the 2000 harmonisation to 230 / 400 V; the numbers on the network moved, the copper mostly did not. New Zealand retains 415 V as its working nominal, so the calculator keeps it a one-click preset. |
Why wind for 433 V?
A loaded transformer delivers less than its no-load voltage. The gap is voltage regulation — the internal impedance dropping volts as current flows. For a typical 1000 kVA, 5%-impedance unit at 0.8 lagging power factor, that drop is about 3.7%. Start at 433 V no-load and full load leaves roughly 417 V at the transformer terminals — before a single metre of distributor's cable takes its own share on the way to the customer.
Winding for 433 V is headroom, not error. It budgets for the transformer's own regulation and the volt-drop across the LV network downstream, so the point-of-supply voltage lands inside the statutory band under load rather than only at no-load.
This is why picking the secondary voltage in a sizing calculation is a real decision, not a default. Compute at 400 V and the full-load current, fault level and cable sizes read one way; compute at 433 V and every one of them shifts. The Transformer calculator surfaces the choice explicitly — 433 V as the Australian no-load convention, 415 V as a New Zealand preset, and a free target so you can pin the exact on-load secondary you're designing to and let the tap recommendation follow.
The tap connection
Off-circuit taps exist precisely to move the no-load secondary in steps — typically ±2.5% and ±5% — so an installation can be trimmed to its actual upstream voltage. On its nominal tap a 433 V-rated unit sits at 433 V no-load; drop a tap and it falls, raise one and it climbs. The calculator recommends the tap whose no-load secondary lands closest to your target, and shows the full-load current shift that comes with it.
Standards: AS 60038 (standard voltages) · AS 60076.1 (power transformers, tolerances & reference temperatures) · regulation figure computed by the Ampacities Transformer engine for the unit stated. Nominal voltages and tolerances are quoted from the current AS/NZS editions; this note is guidance, not a substitute for the standard.