An autotransformer is a type of transformer that has the primary A certain amount of the autotransformer's energy emanates from the connection of the primary and secondary while the rest of the energy comes directly from the supply. Consult a local inspector and utility companies before installing any autotransformer to see whether they comply with building codes. Using an autotransformer can save considerable amounts of money compared to regular separate winding transformer. The secondary current rating of an autotransformer must be at least equal to the primary current rating of the subject transformer. In simple terms, a subject transformer with a primary current draw of 5 amps should be used with an autotransformer with a secondary current rating of 5 amps or more. To increase voltage over a subject transformers design rating when using an autotransformer, limit the total VA of the subject transformer to its specified rating. That is, if you boost the voltage 10%, lower the subject transformers secondary current rating by 10%. Transformer VA is equal to (voltage x current) of all of the secondary windings.
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