The origins of the twist-on wire connector are obscure, but references to their use can be found since the early 1920’s. Originally called “porcelain connectors,” twist-on connectors are often referred to by the industry name “wire-nut®,” which has been the trademark of Ideal Industries since 1939. The early twist-on connectors were made of porcelain and similar inorganic materials, and did not contain an inner spring (see Fig. 12). By 1930 plastic Bakelite connectors were beginning to appear, and by the 1940’s inner springs were added to better twist and more securely capture the conductors. By the late1960’s thermoplastic (nylon, polypropylene, etc.) twist-on connectors were becoming common, and aluminum wire combinations were added to accommodate the increased use of aluminum wire. In the mid-1980’s, enhanced requirements were added to the