Major transmission lines in the Western Interconnection are grouped into numbered paths for planning and operational purposes.
Two measures of path congestion are the U75 and U90 metrics. These measure the percentage of time in which flow was above 75 or 90 percent of the path's operating limit.
Many factors determine operating limits. A low U75 or U90 does not necessarily indicate a path is underused, nor do high values necessarily indicate congestion. For example, some paths were built to carry electricity from large plants—high flow is not unusual for these paths.
