In the late 1960's and early 1970's copper prices rose and contractors/electricians switched from copper to lower costing aluminum wiring. Although no longer common for distribution circuits, aluminum wiring is still used today in certain applications. For example, 240 volt circuits for stoves and dryers. It sometimes is used on the main service entrance wire from the road to the house.

Concerns with this type of wiring have arisen, for example, when aluminum wire is connected to devices (eg. receptacles, light fixtures) which were not designed for aluminum, or, when aluminum and copper wires are attached. In these cases a reaction can occur causing the connections to fail, perhaps become disconnected, and/or, potentially overheat, spark and catch fire. Symptoms of this can sometimes be seen in the discolouration of receptacles, flickering lights, or the smell of hot plastic insulation.
The conductivity of aluminum is not as good as copper so a different, thicker, gauge wire must used. For example, today the most common copper wire size is 14 gauge. The comparable aluminum wire size used was 12 gauge.

In their most recent 1997 Safety Notice, Ontario Hydro states:
"Aluminum wiring in residential installations will operate as safely as any other type of wiring if the proper materials are used and it is installed as per the manufacturer's instructions and the Ontario Hydro Electrical Safety Code."
Special care must be taken to ensure, for example, that connections are made to receptacles that are suitable for aluminum wiring. Further, where aluminum and copper wires are connected that proper paste/flux, and/or, the appropriate wire connectors, are used.
Regardless of the wiring type used, no circuits should overloaded or over fused.

What do you do if you suspect a problem?
Have a qualified electrician check:
1) Terminations at devices without removing or disturbing them.
2) Cut back any damaged aluminum conductors and join these to a copper tail using a connector approved for use with aluminum. These connectors are coloured either brown or purple, depending on the manufacturer. The copper tail is then terminated at the terminal screws of an ordinary device (which includes approved receptacles, etc.). Or, cut back any damaged aluminum conductors and re-terminate at a new device bearing the appropriate marking. Only devices bearing the mark CO/ALR are currently approved for use with aluminum wiring.
3) Panel board terminations for signs of overheating.
4) Fuses present for heavy loads are temperature sensitive type (D or P).

If a home has aluminum wiring and you suspect problems may exist further professional advice from a qualified electrician, experienced in repairing aluminum wiring concerns, is recommended.