Dry type Distribution transformers can be reverse connected without a loss of KVA rating, but there are certain limitations. Transformers rated 1 KVA and larger single phase, 15 KVA and larger three phase can be reverse connected without any adverse affects or loss in KVA capacity. The reason for this limitation in KVA size is, the turns ratio is the same as the voltage ratio.
Example: A transformer with a 480 volt input, 240 volt output - can have the output connected to a 240 volt source and thereby become the primary or input to the transformer, then the original 480 volt primary winding will become the output or 480 volt secondary. On transformers rated below 1 KVA single phase there is a turns ratio compensation on the low voltage winding. This means the low voltage winding has a greater voltage than the nameplate voltage indicates at no load.
For example, a small single phase transformer having a nameplate voltage of 480 volts primary and 240 volts secondary, would actually have a no load voltage of approximately 250 volts, and a full load voltage of 240 volts. If the 240 volt winding were connected to a 240 volt source, then the output voltage would consequently be approximately 460 volts at no load and approximately 442 volts at full load. As the KVA becomes smaller, the compensation is greater- resulting in lower output voltages. When one attempts to use these transformers in reverse the transformer will not be harmed; however, the output voltage will be lower than is indicated by the nameplate.
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