Question on transformers in parallel

[Idea04]

My question is if you connect multiple transformers together with the secondaries in parallel, how do you calculate the sum current from the transformers. Is the current calculated by dividing the voltage on the secondary winding by the impedance of the secondary windings.
Or is it based on the turn ratio.

 

[otto9K9otto]

Running two IDENTICAL secondaries in parallel will double you current. The voltage will stay the same. But:

Don't do it! Any slight difference in winding impedance will create a circular current from one transformer to the next that will do you no good but will heat up the transformers. Running secondaries in series is OK, provided insulation limits are not breached.

 

[m.s.j]

Running secondaries in series is not applicable and it is not advisable to parallel operate power transformers with widely different power ratings as the natural impedance for optimal design varies with the rating of the transformer.
The power divides between parallel-connected transformers in a relationship which is inversely proportional to their impedances; a low-impedance transformer operated in parallel with a higher-impedance unit will pass the greater part of the power and may be overloaded. A mismatch in loading of up to 10 per cent is normally acceptable.
For more information you can refer to Transformer Riddle No.41 from http://electrical-riddles.com