The biggest blackout in U.S. history that occurred less than two months ago helped to renew a long-standing debate over the reliability of the nation’s electrical grid.

“The age of the electrical grid may not be as pressing an issue as the amount of energy transmitted across the network,” says Bruce Wollenberg, professor of electrical and computer engineering at the University of Minnesota. “The grid isn’t overloaded, but they need more capacity so there’s more spare room to withstand outages under heavy loading conditions.”

Michehl Gent, president of the North American Electric Reliability Council (NERC), says that far less money has been spent to build new transmission lines than to add plants that generate power, and more power is racing across the same system. U.S. investment on transmission lines fell from $1.3 billion in 1990 to $270 million in 1998, according to figures in the president’s National Energy Policy.

However, the number of transmission lines may not be the only issue. According to Wollenberg, because companies transmitting electricity have modest profits, there are concerns that they aren’t investing in regular maintenance of transmission lines to keep them in working order.

The growth of energy trading also has exploited the lack of capacity on the electrical grid because some parts of the network aren’t equipped to handle the frequent transmissions of large amounts of power between energy traders.