The combination of extreme weather and the growth of renewables have focused attention on the nation's aging and inadequate transmission infrastructure.

On Dec. 16, 2021, the Federal Energy Regulatory Commission (FERC) issued Order 881 concerning transmission line ratings. The rule changes the definition of ratings to allow utilities to include assumptions about ambient weather conditions. Ambient weather, including temperature and other atmospheric conditions, can affect the transmission capabilities of power lines. As the commission notes, warmer temperatures tend to lower a transmission line’s rating while colder temperatures tend to increase it.

According to FERC, many transmission line ratings are currently calculated based on assumptions about ambient conditions that are "not regularly adjusted.” These ratings do not accurately reflect the near-term transfer capability of the transmission system.

These assumptions can lead to power lines that are either more or less restrictive than they need to be when the assumptions do not match the real-time ambient conditions. The results can be either not enough or too much power travelling over lines at a particular time.

To address this issue, FERC adopted the rule that includes a definition of “ambient-adjusted ratings.” According to that definition, the rating applies to a time of not greater than one hour and reflects an up-to-date forecast of ambient air temperature across that time.

FERC believes that the new definition will reduce safety problems, avoid equipment damage, result in “increased system transfer capability and, in turn, lower costs for consumers.”