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An over-moded evacuated waveguide line was chosen for use in the transmission system for the proposed JET enhanced performance project (JET-EP) electron cyclotron resonance heating (ECRH) system. A comparison between the quasi-optical, atmospheric waveguide and evacuated waveguide systems was performed for the project with a strong emphasis placed on the technical and financial aspects. The evacuated waveguide line was chosen as the optimal system in light of the above criteria. The system includes six lines of 63.5 mm waveguide for transmitting 6.0 MW(10 s) at 113.3 GHz from the gyrotrons to the launching antenna. The designed lines are on average 72 m in length and consist of nine mitre bends, for an estimated transmission efficiency of similar to90%. Each line is designed to include an evacuated switch leading to a calorimetric load, two do breaks, two gate valves, one pumpout tee, a power monitor mitre bend and a double-disc CVD window near the torus. The location of waveguide support is positioned to minimize the power converted to higher-order modes from waveguide sagging and misalignment. The two gate valves and CVD window are designed to be used as tritium barriers at the tot-us and between the J1T and J1D buildings. The last leg of the waveguide leading to the torus has to be designed to accommodate the torus movement during disruptions and thermal cycles. All lines are also designed to be compatible with the ITER ECRH system operating at 170 GHz.