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Treat the secondary of a "buck boost" no different than the secondary of a standard transformer. Make your intial calculations ignoring the fact you will use it as a buck boost. The amount the transformer can carry has to do with the way its low voltage windings are connected.

So a 500VA transformer with a 12/24V secondary wired for 24V has conductors rated to carry 20.83A.

BigJohn;

Glad that we all were able to assist with the Buck/Boost Transformer sizing criteria.

As JBD described above, to figure the capacity of the Secondary Windings - in Amperes, divide the Transformer's rated Volt-Amps by the Secondary Voltage.

Using JBD's example Transformer above, these are the results:

500 VA Transformer - Secondary = 12/24V

* Secondary setup in Parallel for _12V_:
500 / 12 = 41.67 Amps maximum load Current through Transformer.

* Secondary setup in Series for _24V_:
500 / 24 = 20.84 Amps maximum load Current through Transformer.

Determine the "Per-Line" Amperes, which the Load Device will require, by dividing the Apparent Power (Volt-Amps) by the Voltage; or, if available, just use the nameplate Amperes of the Device.

If there is any doubt per these figures, feel free to reply as necessary! We are all here to assist.

Scott

BJ...

Wire size, insulation rating, temperature rise...

Coil saturation at 60Hz normally never a concern.

Normally the designer spec'd small gauge high-temp conductors with insulation rated to 150 Centigrade. Even so, there's only so many amperes you can shove through the transformer.

This limit is found when the B-B is trying to bump/buck a lot of current a minimal voltage.

Normally this is no sweat since the typical B-B transform is 277 down to 240 ... or 208 up to 240. You do note that the RATIOS are the SAME. Stacking two such common B-B permits 277 to buck down to 208; which was a proposition on another thread.

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Thus you have multiple limits: how much the magnetic circuit can handle and how much the conductors can handle. There are frequency limits, also, but at 60 Hz you not be troubled.