For a dual-voltage control transformer with a line voltage of 480 VAC, how should the primaries be connected to achieve 120 VAC?

To achieve 120 VAC from a dual-voltage control transformer with a line voltage of 480 VAC, the primaries should be connected in series. This configuration effectively allows the transformer to utilize the full voltage across its windings. When the primaries are combined in this manner, the output voltage can be stepped down appropriately to deliver the desired 120 VAC.

By connecting the primary windings in series, you effectively double the voltage that each winding sees in the system, enabling the transformer to output the necessary voltage efficiently. This method takes advantage of the transformer's design to reduce high line voltage to lower, usable levels for control circuits, ensuring safe and effective operation in electrical installations.

In contrast, connecting the primaries in parallel would maintain the voltage at 480 VAC rather than stepping it down, which is not suitable for achieving a reduced 120 VAC output. Using only one primary would not yield the desired voltage level either, and suggesting that it cannot reach 120 VAC would misunderstand the basic operation principles of transformers and their ability to adjust voltage levels through proper winding connections.