I have quite a lot of old scrap mains step-down transformers (220V-240V 50Hz), salvaged from old equipment during 30+ years.
Although I religiously marked the primary terminals of all of them as a "best practice", most of the times I lost track of (or I could never get) data about secondary maximum current and, especially, VA rating.
Most of them are classic E-type cored ones, but a few are toroidal.
Is there a way by simple measurements to discover a conservative estimate of their max VA rating? If I could (under)estimate that value within a 15-20% tolerance I would be happy.
IIRC I heard something about weighting the transformer and apply some proportionality factor (k VA/kg). Is that a reliable way? And which is this factor? Is it the same for any size/weight (I think the heavier I've got weighs about 10kg)? Does this method work also for toroids? Are there better methods that are more or less that easy?
(to clarify some points)
I asked for simple measurement techniques. Ideally they should be feasible with a True RMS multimeter and an oscilloscope, but this latter possibly only to monitor the secondary (I don't have an isolation transformer nor an high voltage probe). And I don't even have a variac, but I don't consider having a variac something in the realm of "simple" measurements.
I already know, as I've stated before, the primary voltage. Knowing the secondary voltage is just a matter of measurements. The only vital parameter to reuse those transformer is knowing an estimate of their VA rating.
Please, provide a clear procedure. Saying that by measuring the secondary wire gauge will tell me its ampacity is not enough. Which formula do I have to use? I never designed a transformer, so I don't know how secondary winding geometric parameters are chosen.