I myself think that, it's not always 5A because exactly, it's the specification which is deduced from theory, but infact, there's many factors that effect it. So, it results in the fact that, you can not get 5A as indicated by manufacturer.
In addition, due to loss via heat or your rectifier can not accomodate the current of up to 5A, so you can not get a current of 5A go through your load!
Current ratings on transformers are typically RMS values. (Yours can handle 5 Amps AC current through the secondary windings). Keep in mind, that you have a center tapped transformer and 5 Amps is using the secondary in parallel.
Actually, the current is the limiting factor (because of heating in the windings). If you operate the transformer at a slightly higher voltage, you can push more watts through it. If you operate at a slightly lower voltage, you limit your watts. Despite the fact that current is the real rating, all larger power transformers are rated in VA, kVA, and MVA.
If you can REMOVE heat from a transformer you can INCREASE current through a transformer. That's why many power supplies have fans.
VA is the product of RMS Voltage times RMS current. Because of phase differences in current and voltage (power factor) VA is not necessarily Watts, but will always be greater than or equal to Watts.
The center tap splits the secondary into two halves.
When you ground the center tap and use each half of the secondary to supply current. I call that using the secondary in parallel. In this case your transformer can supply 12V at 5 Amps (but really 2.5 Amps from each leg).
If you don't use the center tap and you ground the secondary at one end, the other end supplies 24V. I call that using the secondary in series. In this case your transformer can supply 24V at 2.5 Amps.
In short, you have a 60 VA transformer.
The terminology better applies to transformers with two secondaries. If the two secondaries are identical 12V, you can wire it to be 12V without a center tap (truely parallel), 24V without a center tap (truely series), or as a center tapped transformer like you have. Most ratings for center tapped transformer give the amp rating for a parallel configuration. I hope this clears up my terminology.
hi!
there are many factors involved in this which are regarding to the operation and behaviour of transformer which are includes in its working for example eddy current loss, windings behaviour,magnetic field loss and othre parameters so.