Why Do Aluminum to Copper Connections Fail?

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Jimmy H. Beard, M.S.E.E., P.E., CFEI

Electrical connections between aluminum and copper wires present special hazards for property owners. Aluminum to copper connections may deteriorate over time if not installed in a proper manner. A deteriorated connection can lead to overheating, which can result in a fire.

Why do aluminum to copper connections fail?

One reason aluminum and copper connections fail is because the properties of the two metals are different. For example, the thermal expansion of aluminum is about 25% more than copper. As temperature increases, the aluminum expands more than the copper, which forces some distortion of the conductors. Then, when the temperature falls again, the aluminum contracts more than the copper, leaving a small amount of space between the aluminum and the copper conductors. The space between the conductors reduces the physical contact between them and results in an increase in resistance at the connection.
Aluminum also combines with oxygen in the air to form aluminum oxide. While aluminum is a good conductor, aluminum oxide is a good insulator. Space between conductors in a splice can result in air moving in and filling the space. The air provides oxygen for aluminum oxide to form. The aluminum oxide that forms on the conductor increases the resistance of the connection, resulting in the formation of heat. Too much heat can melt insulating materials around the splice and can lead to a fire.

Proper connectors are essential

Good quality connections between aluminum and copper wires start with the right connectors. When installing aluminum to copper connections, using connectors made for only copper conductors can have disastrous consequences. Figure 1 shows a failed aluminum to copper connection that was made using a twist on connector (wire nut). The insulation of the connector melted, exposing the energized, bare conductors. Figure 2 shows a failed aluminum to copper connection that was made using a split bolt connector. In both cases, the connectors were not listed for use with aluminum conductors and fires resulted when the connections later failed.

JHB Fig 1
Figure 1. Failed aluminum to copper connection made with a twist-on connector


JHB Fig 2
Figure 2. Failed aluminum to copper connection made with a split bolt connector


The National Electrical Code (NEC) requires that conductors of dissimilar metals not be mixed in a connector unless the connector is identified for that purpose. The NEC also requires that devices such as switches and receptacles that are connected to aluminum conductors be listed and marked for such use. Switches and receptacles listed for aluminum wire should be marked CO/ALR. Failure to use proper splicing connectors and devices can present fire hazards for the property owner and result in liability for the installer.

If you have a property loss that you suspect may have been the result of faulty aluminum to copper connections, EDT can help. We have engineers who are knowledgeable about proper splicing methods and can assist in determining the cause of your loss.


About the Author

Jimmy H. Beard, M.S.E.E., P.E., CFEI is a consulting engineer with our Birmingham Office. Mr. Beard provides consultation in the areas of cause and origin analysis of electrical system failures; scope of damage and analysis of costs to repair or replace electrical equipment; and engineering consultation related to fire investigation. You may contact Jimmy for your forensic engineering needs at jbeard@edtengineers.com or (205)838-1040.

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