This bill, known as the High-Capacity Grid Act, amends the Federal Power Act to establish a best-available transmission conductor standard for certain transmission projects. This standard applies to both the construction of new transmission facilities and the modification or upgrade of existing lines under the Federal Energy Regulatory Commission's jurisdiction. A "best-available transmission conductor" is defined as one that provides the greatest feasible energy-carrying capacity and electrical efficiency, while also mitigating thermal sag. The legislation creates a presumption of prudence for public utilities that use these conductors, meaning their associated costs are presumed just and reasonable for recovery through rates. Conversely, there is a presumption against cost recovery for conductors that do not meet this best-available standard. The Federal Energy Regulatory Commission is mandated to issue regulations within 180 days to implement this standard. These regulations must establish a methodology for determining best-available conductors, consistent with the defined criteria, and provide for periodic review and updates to reflect technological advancements.