The following information relates specifically to single phase line voltages common in residential, commercial, and light industrial applications.
It does not attempt to address three phase voltages, or line voltages likely to be encountered in higher powered industrial installations. These will be treated separately.
A detailed handling of line voltage goes beyond the scope of this discussion; however, in practical terms we can consider two ranges, 100-127 V and 220-250 V. The map below provides a graphical representation.
The primary voltage ratings of transformers designed for the North American market and tested to American National Standards Institute (ANSI), Underwriters Laboratories (UL), or Canadian Standards Association (CSA) standards are usually referenced to 120 V or a multiple of 120 V. The primary voltage ratings of transformers designed for the European market and tested to EN (European) standards are most often referenced to 230 V as the European Union has codified 230 V -10% +6% as their standard line voltage.
For dual voltage transformers, ratings of 115/230 V or 120/240 V are often used interchangeably and the difference is of little practical importance. Special consideration is required for countries where the nominal line voltage is 127 V as this is near the high end of the safe working voltage for transformers with rated primary voltages of 115 V or 120 V. These countries include Mexico, Monaco, Morocco, the Antilles, Saudi Arabia, Senegal, Spain, Suriname, Tahiti, Togo, Tunisia, and parts of Vietnam. The same caution applies to countries where the nominal line voltage is 250 V.
These include parts of Australia, India, and South Africa. Japan also often requires a unique transformer as the line voltage is 100 V and low output voltage may be an issue if using a transformer designed for 115 V or 120 V. When designing equipment for sale in emerging markets including parts of China and India voltage stability is a known issue and the use of a ferroresonant or constant voltage transformer may be required to insure proper equipment operation. If you have specific questions not covered in this discussion, please contact the Foster Transformer Company.