Your branch circuit wiring typically makes up about 75%-80% of your home's wiring. The remaining wiring goes to dedicated appliances such as electric water heaters, dryers, and ranges. Aluminum wiring going to dedicated appliances is not usually a high safety concern. This wiring originates in your electrical panel at the circuit breaker, and usually terminates in only one location, either directly to the appliance it is serving or the electric outlet serving that appliance. The outlets which serve these dedicated appliances are typically rated for copper or aluminum wiring, and problems at these connections are fairly rare. Also, the aluminum wiring leading to these appliances is of a larger gauge than your lighting and outlet branch circuits, and is typically stranded wiring (multiple strands of wire wrapped together within the insulation) which makes it stronger and less prone to breakage while it is being bent and pushed into the electrical boxes.
The bigger danger of aluminum wiring is in your home's lighting and/or outlet circuits. There are several reasons why this kind of aluminum wiring can be more dangerous. Aluminum wiring is much softer than copper wire. If aluminum wiring is nicked during the insulation stripping process, this creates a weak spot in the wire that will easily break over time, especially if the wires are moved during the outlet installation. It also creates an area of greater resistance, which is prone to break or overheat if heavy loads are used on this circuit.
Another problem with aluminum wiring being so soft is it can only be bent a few times before it becomes brittle and breaks. Most of the homes that contain aluminum wiring are 30-50 years old and the switches and outlets in these homes have been updated many times. Every time you remove a switch or outlet and replace it with a new one, the bent aluminum conductors are pulled out of the electrical box, then pulled straight and bent back again when they are pushed back into the box. The more this happens to any wiring, the more the wiring breaks down, making it prone to breakage and overheating. Aluminum wiring is particularly susceptible to this danger due to its softness.
Aluminum wiring also needs to be terminated on devices specifically designated and rated for aluminum wiring. The typical switch and outlet you buy at the hardware stores are not rated for aluminum wiring. They are only rated for copper wiring. Aluminum outlets have terminals rated for the aluminum wiring, so you don't end up with corrosion caused by dissimilar metals. This is a very common problem with aluminum wiring. The terminal screws are also larger, allowing for the expansion of the aluminum, and the slightly larger aluminum conductor. Many inexperienced electricians, handymen and do-it-yourselfers install aluminum wiring on standard outlets that are rated for copper only, not realizing they are creating a fire hazard by mixing the dissimilar metals and using the smaller terminal screws. We've seen many burned up outlets as a result of this and these faulty outlets can cause much greater calamity in a person's home if gone unattended.
Sometimes, people will install a copper wire pigtail (a small piece of copper wire connected to the aluminum wires) and connect it with a wire nut to the aluminum wiring, then install the copper wire to a copper outlet. This is an approved method of installing standard copper outlets on aluminum wiring, but only if the proper aluminum to copper rated wire nuts are used. We've come across many homes that have standard wire nuts in place to connect copper to aluminum. These wire nuts have completely melted causing a serious fire hazard [insert picture of melted wire nut]. Standard wire nuts are not approved for connecting aluminum to copper and only specifically rated wire nuts approved for this installation should ever be used.
As you can see, there are many inherent dangers associated with aluminum wiring. If you're not sure if your home has aluminum wiring, or you know you have aluminum wiring and aren't sure if it has been installed or is working correctly, we highly recommend you schedule an Emerald State Electric electrical home safety audit [the blue text should link] to thoroughly examine your home's electrical system and reduce any potential fire dangers that may be lurking behind your walls.