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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.

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.