How can I know the current of a secondary trafo winding?

[avitsur]

Hi, is there a table or other source of information that can tell me how much current I can withdraw from a transformer's secondary winding? I have one winding that provides 28V. I've measured the wire's diameter - it's 1mm. the second winding provides 29V (all AC, of course) wire diameter 1.2mm. (measured with a caliper).

[Benta]

If you know nothing about the transformer, the only way forward is to look at the physical size to estimate its VA capabilities. Compare it with known transformers.

[Ian.M]

To explain further:
For electrical grade enamelled copper wire, the CSA (cross section area  D²𝝅/4) only determines the resistance per unit length.  The type of coating on the wire determines its max. service temperature.  For wires in free space or in small bundles the temperature rise can be calculated from the RMS current through the wire(s), so, given the ambient temperature, it is possible to determine the max. continuous RMS current a wire may safely carry.

However in transformers the temperature rise is largely determined by the total losses in the transformer, its shape and surface area, as the thermal resistance transformer surface to ambient typically dominates (except for actively cooled transformers) though the RMS current through the wire does contribute to localised heating due to the thermal resistance winding to core or other windings.  Transformer manufacturers generally try to balance the I²R losses in the primary and secondaries to avoid hotspots, but are limited by the minimum wire gauge their winding equipment can handle which may result in the primary or high voltage secondaries being heavier gauge than ideal, and by the wire gauges they actually stock. Also the length of wire for N turns is determined by the distance from the core, so a winding at the surface will have higher losses.  Therefore a particular wire gauge secondary can have different ratings in different transformer and can even have different current ratings in the same multi-secondary transformer depending on where it is in the winding stackup.
 

[Andy Chee]

Do you have access to a set of kitchen scales?  I make an informed guess of power rating, by comparing the weight with several other transformers with similar weight and voltage rating.

[Vovk_Z]

3.5-5 А/mm2.

[Kleinstein]

There is no hard current limit. The more current the hotter the transformer get and the shorter it will last. The current density is first approximation only.
It depends on how compftable you are with the temperature and how heat resistant the windings are.

[BILLPOD]

Good Morning Avitsur,  The way I approach Xfmrs with unknown specs.
is to apply a load and monitor the temperature of the unit.   Starting well below what I 'Guestimate' the maximum amperage to be.  I then slowly increase the load until the xfmr gets very warm to the touch, but not burning hot.   If you have a DMM with temperature measurement capabilities you could measure the temp.   Compare the results with the
draw of the circuit you want to power, or to just mark the xfmr for future
applications.

[Phil1977]

Another quick way to get a power limit is to measure the output/input efficiancy. As long as you are in the plateau between idling and copper losses, you should be fine.

By measuring the idle power over input voltage you also easily get a value where saturation starts.