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The terminal's barrel is the section that actually meets the conductor material and is the area within the sleeves on insulated terminals. There are three basic barrel dimensions based on wire gauge, each with its own corresponding color. From left to right, they're 10- to 12-gauge (yellow); 14- to 16-gauge (blue); and 18- to 22-gauge (red). According to every technician we talked to, always choose a terminal barrel size range to match the wire gauge; an improperly sized barrel will prevent the essential wire seal and will likely fail.