Aluminum wires were used for branch circuits in some older houses between the 60s and the late 70s. They are no longer used for residential branch-circuit wiring due to fire risk (old AA-1350) and economic (AA-8000) considerations, but existing wires can be maintained in a way that makes them relatively safe. AA-8000 wire and cable will be marked as such -- cable not marked as such can be assumed to be AA-1350.
Aluminum wiring was used in the late 60s until the early 70s in home branch-circuit wiring, as a replacement for copper. Several downsides were discovered soon after:
- Aluminum is softer than copper, so it is easier to damage
- Aluminum corrosion is not capable of carrying electrical loads like copper, so it poses a fire risk when that happens
- Aluminum cannot be used in conjunction with copper wires (without a splice connector) or copper fixtures, as this contributes to the corrosion
- Early aluminum wiring was made of utility-grade AA-1350 aluminum, which has expansion and creep properties which are not compatible with the steel screws used on branch circuit devices at the time, which created a fire risk due to connections loosening over time as a result
As long as aluminum is connected only to fixtures that are ALR rated or is properly pigtailed to copper for connection to branch-circuit devices, and treated regularly with anti-corrosion chemicals, existing aluminum wiring is considered safe. Aluminum wires are no longer used for home branch circuit wiring for economic reasons; however, the current AA-8000 family alloy wires are legal for branch circuit wiring as per 2014 NEC 310.106(B) and can be found in larger gauges, like for an electrical service to the main panel, for feeders to subpanels, or for high-current branch circuits such as those used to feed electric ranges and furnaces.
2014 NEC 310.106(B):
(B) Conductor Material. Conductors in this article shall be of aluminum, copper-clad aluminum, or copper unless otherwise specified.
Solid aluminum conductors 8, 10, and 12 AWG shall be made of an AA-8000 series electrical grade aluminum alloy conductor material. Stranded aluminum conductors 8 AWG through 1000 kcmil marked as Type RHH, RHW, XHHW, THW, THHW, THWN, THHN, service-entrance Type SE Style U and SE Style R shall be made of an AA-8000 series electrical grade aluminum alloy conductor material.