Blue Twist On Connector 150Jar Tight, square-wire spring threads directly onto conductors for fast connections with no slip. Tightens without ratcheting, easily removed for repeat usage on same size combination or larger. Fluted shell surface permits a firm grip for easy finger-tight c onnections. Thermoplastic shell shields against environmental extremes. Flame-retardant properties resist heat distortion up to 221 degrees Fahrenheit. Superior flash-over protection. High-dielectric shell features deep skirt design to prevent exposure of bare wire. Buttress threads screw into wire insulation. For copper to copper connections only.