Around the beginning of the 20th century, there were over 4,000 individual electric utilities, each operating in isolation. Almost all of those used low-voltage connections from nearby generating power plants to the distribution lines serving their local customers.
As the demand for electricity grew, particularly in the post-World War II era, electric utilities found it more efficient to interconnect their transmission systems. In this way, they could share the benefits of building larger and, often, jointly-owned generators to serve their combined electricity demand at the lowest possible cost, and to avoid building duplicative power plants. Interconnection also reduced the amount of extra capacity that each utility had to hold to assure reliable service. With growing demand and the accompanying need for new power plants came an ever-increasing need for higher voltage interconnections to transport the additional power longer distances. Over time, three large interconnected systems evolved in the United States.