Failure and Overheating of Aluminum-Wired Twist-On Connections

In this investigation, twist-on connectors of the type used for branch circuit wiring in buildings have been tested with solid aluminum and copper conductors. The connector and conductor combinations tested are approved in both the United States and Canada. All of the twist-on connections were made in accordance with the connector manufacturers' instructions. Testing consisted of on-off cycling at 90 percent of rated current and 13 percent duty cycle. Failures, involving increasing connection resistance and abnormal heating, have occurred in less than a year in a substantial number of the aluminum-wired connections. The failures involve both the electrical conductor (EC) and alloy aluminum conductors. In contrast the copper-wired connections have low resistance and are stable. The temperature of many of the overheating aluminum-wired connections has been high enough to cause hazardous deterioration of the conductor insulation. The difference in performance between the copper and aluminum conductors is determined to be due to failure to achieve and sustain low resistance conductor-to-conductor contact in the aluminum-wired connections. This results in the greater portion of the current flowing through the connector spring rather than directly from conductor to conductor. The connector spring is plated steel and is not suited to be the major part of the current path. Degradation of the contacts between the aluminum wires and connector spring follows by several well-known mechanisms, leading to increasing connection resistance and the resulting heat generation and high temperatures.