Transformer inrush and its breaker
Check a transformer’s switch-on inrush against its breaker.
Reads AS/NZS 3000:2018. Tables checked 2 Oct 2026.
Try a job 4
Each job fills in example values. Replace them with your values.
- Stop a transformer tripping its breakerWhy an isolation transformer trips a Type B breaker at switch-on, and what holds.
- Pick a breaker for a site transformerThe breaker and cable for a portable site transformer.
- Check a control transformer breakerWhether a small control transformer on an extra-low voltage secondary holds its breaker.
- Size the breaker for a step-down transformerThe smallest breaker of each type for a single-phase step-down transformer.
Inputs
Worked example with the default inputs. The live calculator replaces it when the page loads.
- Rating
- 15 kVA
- Phases
- Three phase
- Primary voltage
- 400 V
- Secondary voltage
- 400 V
- Circuits on the secondary
- One, with no protection of its own
- Inrush, × full load current
- 12
- Inrush lasts
- 0.1 s
- Breaker type
- Curve C
- Rating
- 32 A
The breaker at switch‑on
May trip
- Inrush current
- 259.8A
- Primary full load current
- 21.65A
- Smallest Type B that holds
- 100 A
- Smallest Type C that holds
- 63 A
- Smallest Type D that holds
- 32 A
- Secondary conductors, at least
- 21.7A
This result is a design aid. Verify it against the current standard.
Calculation steps
- Primary full load current21.65A
- Inrush, 12 × the full load current for 0.1 s259.8A
- Type C 32 A, typical instant band from160A
- Type C 32 A, typical instant band to320A
- The breaker at switch‑onResultMay trip
- Smallest Type B that holds100 A
- Smallest Type C that holds63 A
- Smallest Type D that holds32 A
- Secondary conductors, at least21.7A
AS/NZS 3000:2018 Cl 4.14.2.2
About this calculator
How this is calculated
When a transformer is switched on, its core can draw a large current for a few cycles while the magnetic flux settles. This inrush can be ten or more times the full load current. It lasts about a tenth of a second, and it is often enough to trip the breaker on the primary. This tool checks whether it does, and finds a breaker that rides through it.
The tool takes the primary full load current from the transformer tool, then multiplies it by the inrush multiple you enter, for the time you enter. No standard on hand prints these two figures, so they are your assumptions. Take them from the maker's data sheet. The multiple is a peak, and the tool compares it with the breaker's band, which is given in RMS multiples of the rating, as if it were RMS: that errs towards a trip.
It then places that current on the trip band of the breaker, the same band the MCB trip curve tool draws. The breaker holds when the band's earliest trip is no sooner than the inrush lasts. It trips when the latest trip is sooner, and it may trip in between. For each type, B, C and D, the tool finds the smallest rating that carries the full load current and holds.
With one circuit on the secondary and no protection there, AS/NZS 3000 lets the secondary cable go without its own breaker if it carries the primary current scaled by the voltage ratio. The tool gives that current.
| Step | Table or clause | What it decides |
|---|---|---|
| Full load current | The transformer tool | The current the inrush multiplies |
| Inrush | The multiple and time you enter | The current and time the breaker sees |
| The breaker's band | AS/NZS 3000 Cl 2.5.3.1 and Table 8.1, and a typical band | Holds, may trip or trips |
| The secondary cable | AS/NZS 3000 Cl 4.14.2.2 Exception | The least capacity of an unprotected secondary cable |
| An extra-low voltage transformer | AS/NZS 3000 Cl 4.14.1 Exception 2 | Whether Cl 4.14 applies at all |
Worked examples
- A 15 kVA three-phase isolation transformer on a C32 draws 259.8 A of inrush, inside the C32's typical instant band, so the breaker may trip at switch-on. A 32 A Type D holds.
- The same transformer on a D32 holds the 259.8 A and passes. The 32 A Type D is also the smallest D that carries the 21.65 A load.
- The same transformer on a B32 fails: the inrush is past the top of the typical instant band and the breaker trips. A 32 A Type D would hold.
- A 3 kVA 230/110 V site transformer on a C16 may trip on its 156.5 A inrush. A 16 A Type D holds, and the 110 V cable needs at least 27.3 A.
- A 10 kVA 400/230 V transformer, 10 × inrush, on a D32 holds and passes. A 32 A Type D is the smallest D that holds.
Limits
The inrush depends on where on the wave the transformer is switched and on the flux left in the core, so it differs from one switch-on to the next. The tool uses one figure for the peak and one for the time. A toroidal transformer can draw much more than a laminated one. A soft-start or inrush limiter changes the figures; enter what the maker gives.
The breaker's band is typical of its type, not a maker's curve. The tool does not check a fuse or a moulded-case breaker.
See also: MCB trip curve, Transformer and Cable size.
Questions
Each answer describes what this tool calculates. The result is a design aid. Verify against the current standard.
Why does a transformer trip its breaker when switched on?
The core draws a large magnetising current for a few cycles while its flux settles. The inrush can be ten or more times the full load current. If it reaches the instant band of the breaker, the breaker may trip.
How big is transformer inrush current?
It depends on the design and on where on the wave it is switched. 10 to 12 times the full load current for about 0.1 s is a usual first guess. A toroidal transformer can draw more. The maker's data sheet gives the real figure.
Which breaker curve suits a transformer?
Often a Type D, whose typical instant band starts at 10 times its rating. A Type C needs a larger rating for the same inrush, and a Type B larger again. The tool finds the smallest of each that holds.
Does the secondary cable need its own breaker?
AS/NZS 3000 Cl 4.14.2.2 lets one unprotected secondary circuit go without, if its cable carries the primary rated current times the primary to secondary voltage ratio. With more circuits, protect each one.
Is the breaker band the maker's curve?
No. It is a typical band for the type, the same one the MCB trip curve tool draws. Where the result is May trip, check the maker's curve or pick the next rating up.
Clauses and tables this tool reads
- AS/NZS 3000:2018 Cl 4.14.1Transformers outside Cl 4.14
- AS/NZS 3000:2018 Cl 4.14.2.2Transformer secondary circuits
- AS/NZS 3000:2018 Cl 2.5.3.1Operating points of circuit-breakers and fuses
- AS/NZS 3000:2018 Cl 4.14Transformer secondary circuits
- AS/NZS 3000:2018 Table 8.1Maximum earth fault loop impedance multipliers by MCB curve
Every clause and table, with how each was verified
Tables last checked on 2 Oct 2026.
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