An off-line ambient temperature corrected transmission line ( l / L ) dynamic thermal rating system is being developed at PG&E to provide T/L ratings 1 to 24 hours ahead based on hourly ambient temperature measurements at a given location. A low minimum 2 ft/s constant wind speed is assumed at all times. Solar radiation is calculated by an analytical solar model assuming clear sky conditions. The IEEE/ANSI Ampacity Standard is used to calculate T/L ampacity.In the ambient corrected DTR system, a stochastic/deterministic model forecasts ambient temperatures. The forecasts and the standard deviations of forecast errors are updated every hour.T/L ratings were evaluated at two PG&E locations and gains up to 34% over the static line ratings were possible at both locations by the proposed DTR system. The benefits and the risk of the ambient corrected DTR system have been quantified.