Which connection method allows for voltage regulations through a standard transformer in a 120/240-volt system?

In a 120/240-volt system, an autotransformer connection is used to facilitate effective voltage regulation. An autotransformer operates by using a single winding to serve as both the primary and secondary windings, with part of the winding being shared between the two circuits. This design allows for efficient voltage adjustments, making it particularly advantageous for stepping up or down voltages while minimizing losses.

Autotransformers are especially valued in applications where only slight variations in voltage are needed, such as in certain types of lighting, HVAC systems, and motor applications. They are typically more compact and less costly than traditional transformers, which makes them suitable for various residential and commercial usage.

While the other connection methods have their applications, they do not provide the same level of efficiency in voltage regulation as an autotransformer. For instance, series and parallel connections primarily focus on how components are arranged in a circuit rather than regulating voltage through transformation. Grounded connections deal with safety and fault protection rather than voltage regulation inherently. Thus, the autotransformer connection is the most appropriate method for voltage regulation in the context of a 120/240-volt system.