A transformer's primary winding connects to the supply, while its secondary winding serves the load. The terms identify those roles, so the primary is not necessarily the higher-voltage side. Cooling media have changed over time, with the harmful effects of PCBs prompting the use of alternatives. In the household split-phase example, a neutral connection at the center of a 240 V secondary provides two 120 V legs. The frequency comparison is 60 cycles per second in the United States and much of North America, against 50 in Europe.
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Transformer windings, cooling fluids, and household split-phase power
Supply and load define the primary and secondary windings, while a center tap lets a household transformer provide two 120 V legs from a 240 V secondary.