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Dynamic Thermal Line Ratings is a new field of endeavor that holds much promise for electric utilities seeking to maximize existing line capacity, increase transfer capability between utilities and regions while providing adequate system security. Various approaches to increasing the thermal capacity of transmission lines without reconductoring or extensive restructuring are considered. The dynamic thermal rating (DTR) methods considered involve monitoring weather conditions so that during periods of favorable wind and ambient temperature the maximum allowable line current can be increased. The relative importance of the various weather parameters and transmission line thermal limits are discussed. Certain problems in the application of these methods and some cautions concerning the usefulness of such time variable thermal ratings to system operators are reviewed. Momentary increases of up to 300 percent and average increases in the range of 50 percent above traditional static thermal ratings (STR) are theoretically possible. Actual increases may be considerably less. In particular, the ability to place monitors in those spans which limit the whole line rating due to clearance or annealing limits on the line has not been demonstrated. Also, concerns about operation of conductors at higher average temperatures and the system operator's need for a reliable estimate of thermal capacity some hours into the future are noted.