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Because of its high-capacity, low losses and no electromagnetic interference, the gas-insulated transmission line (GIL), which is a replacement of an overhead line in special environments, has been used for over 40 years, mainly as bus ducts of gas-insulated substations or underground connections in power plants. The insulated and cooling gas in GIL is mainly SF6 or its mixtures, which have high global warming potential (GWP) and are listed as restricted gases in the 3rd Conference of the Parties to the United Nations Framework Convention on Climate Change in 1997. Therefore, a lot of researches have been made to find a substitute gas for SF6. In this paper, the cooling capability of gas mixtures with fluorocarbon, which has a lower GWP than SF6 and outstanding insulating performance, is investigated. Heat transfer coefficients of gas mixtures are calculated by measuring the surface temperature of a metal pipe under a certain current. The heat transfer coefficients of gas mixtures with fluorocarbon are analyzed and compared with that of SF6 mixtures. Based on the heat transfer coefficients, the maximum permissible currents of our GIL model with fluorocarbon mixtures or SF6 mixtures are also calculated and compared.