The transformers for various supplies (120/240V 1-ph, 120/208, 277/480 etc.) are all connected to the same HV distribution network, so the frequency will always be the same.
60Hz is the standard frequency for public supplies in the U.S.A., Canada, much of Latin America, and parts of the Caribbean. Most of the rest of the world (e.g. Europe, Australia, most of Africa) uses 50Hz.
