The voltages for an auto-wound transformer are normally specified as a string of numbers starting with a zero and ending with the highest voltage. There is no primary or secondary as such, any two points on the winding can be connected to the supply and likewise a load may be connected across any two points. A minimum of two voltage taps are required for an auto-transformer to perform a useful task. An auto-transformer does not provide electrical isolation between the input and output so must not be used in safety critical applications such as portable tool transformers, arc welders or car battery chargers. Suitable applications are in supply voltage matching where only a small difference exists between input and output voltages. see also Winding Types for further information.

E.g. a 0-220-230 V winding will allow an auto-transformer to be connected to a 220 V supply and deliver 230 V to the load. Alternatively it may be connected to a 230 V supply and output 220 V.

Multiple tappings are possible, e.g. 0-380-400-415-440-480 V.

It is also permissible to have one or more taps before the zero to permit fine adjustment of the output voltage.

E.g. 10-5-0-380-400-415 V, which will allow selection of the following voltages: 380, 385, 390, 400, 405, 410, 415, 420 and 425 V. 5 and 10 V are also available but would not generally be used as outputs (or inputs) in their own right.