The usual problem with aluminum wires is due to galvanic corrosion where the wire meets the brass screws in the outlet, especially if the wiring is daisy chained. The corrosion builds an oxide layer and the resistance slowly rises at that point, causing heat, and ultimately a fire. Pig-tailing is a common cheap solution, although it only delays reoccurance. It is done usually because there is not enough excess aluminum wire to trim back to a virgin section and re-attach to the outlet, which is just as good of a solution. I suppose creep could be a contributing factor, and pig-tailing with a spring-loaded connector would probably overcome that problem somewhat. I just ended up replacing all of my aluminum wiring bit by bit as I remodeled my house.
@Jon: Aluminum wire has a lower yield strength than copper wire, so you're right that aluminum can permanently deform under loads which copper would withstand. But this, in itself, would not be enough to cause loosening. (If it were, then torque-to-yield fasteners wouldn't work).
Resilience is actually a measure of the elastic energy which can be stored in a material. You can estimate it as the yield strength squared, divided by twice the elastic modulus. Even though copper has a higher yield strength, aluminum has a lower modulus, so the resilience is actually pretty close -- although you're right that the resillience of copper is somewhat higher.
But when you're talking about connections loosening over time, what you're talking about is creep.
When I first met my wife, her house (built in the early 70's) had aluminum wires. My first sign of trouble was a burning smell from a little used room. I found an outlet with a charred cover (nothing plugged into it to suggest an overloading problem, but hot enough to burn my fingers doing a touch test). The load was somewhere else daisy chained thru that box. It was "only" a loose connection, so tightening the screws on the outlet fixed it. I tightened ALL the outlets/switches in the house soon after.
The second sign of trouble was an outlet that was intermittent. Touching a cord plugged into it could make the connection come or go. The screws were all tight, and a new outlet didn't fix the problem, so what could be the cause? Turns out the wiring in the wall was intermittent! Aluminum is brittle, so a wire that was probably mishandled during installation eventually broke inside the insulation. So, not all the fire hazards are at the junctions. I had to replace that entire section of wiring with copper. If I remember correctly, they have special wire nuts to join copper to aluminum and special switches and outlets to use aluminum wires.
As for pigtails, from my experience any time you handle the wire (and I'm extra cautious, not bending it more than necessary) you run the risk of breaking it. Best to re-wire in sections as needed, or (smarter) just sell the house and leave the problems to someone else.
Jon: Regarding the approved pigtails to convert aluminum branch circuit conductors to copper terminations ...... while in theory that seems to be the ideal solution, it's been my experience that in practice this is not always workable. Many residential wiring plans resulted in using the smallest device boxes (least expensive!), and the MOST cables into & out of these boxes. The result? With so much room inside the box for grounding pigtails, circuit conductors, wire nuts, devices, etc., there isn't enough additional room to ADD even more wire PLUS "approved" crimp lugs.
And, rewiring an entire house to eliminate ALL the aluminum cable is for MOST installations an impractical solution since the original cables ("ROMEX", Type NM) is stapled at critical locations along the path.
About the best one can hope for is to periodically check heavy-use receptacles, especially those in the kitchen circuits, A/C circuits, etc. for secure connections.
How do I know this? Because I've been through it personally AND with relatives who have owned houses with aluminum wiring.
Hi, Dave. You are right about creep resistance as a factor, thanks for that information. I did mean resilience, though, as it pertains to copper's capability to "spring back" somewhat when deformed, so as you tighten a screw on copper wire, it offers resistance to its deformation that causes a tight bond. Aluminum doesn't, so creep resistance tends to loosen connections.
Manufacturers of copper wire use a "rebound tester" to measure wire resilience.
REplacing aluminum wire is a profitable project when it can be done. But in many installations it does not pull through that easily. Those are the ones where I save the boxes and simply install new copper wire. Aluminum wire is for clotheslines and such, not for inside the house wiring. REally. Aluminum wire has no place in inside wiring, since copper is so very much safer. Just burning one outlet box out of a wall undoes all of the aluminum wire savings. Als, buring up electrical wiring smells bad, and it is bad for your health.
@Jon: What you're referring to is not resillience, but creep resistance. Aluminum alloy 1350, which is what was used for wiring in the 1960s, has poor creep resistance. This means that, over time, it will permanently deform under load.
These days, 8000-series aluminum alloys (8030, 8176, etc.) are used for aluminum wiring. Their creep resistance is much better than 1350, and is comparable to that of copper.
As you aluded to, aluminum has a higher coefficient of thermal expansion than copper, and this is true no matter what aluminum alloy is used. This can cause thermal fatigue at connectors. Oxidation is also potentially a problem regardless of the type of aluminum. Galvanic corrosion between aluminum wiring and copper connectors is another potential issue. So even with the improved 8000-series alloys, aluminum wiring is not without its problems.
You're absolutely right about using "pigtails" with older aluminum wire.
A good article about aluminum wire (from an insurance company, no less) can be found here.
Yes, aluminum wiring is a problem. Copper has resiliance whereas aluminum doesn't. So aluminum simply compresses under a screw and offers little or no opposing force on the screw when you release torque. Thus thermal changes and even slight movement of wiring can cause an aluminum connection to fail. I know a family in Canada that has an electrician come in every few years to check its aluminum wiring for any problems. He usually findsseveral loose connections.
Instead of replacing aluminum wire you can use a special COPALUM crimp connector and an approved tool to splice a copper pigtail to the a aluminum wire and then connect the pigtail to your switch, outlet, light, fan, and so on. Many feedlines from a meter to a breaker box still use aluminum wire, but the connectors at each end handle the aluminum wire and some anti-oxidant "goo" helps protect the aluminum from oxidation.
First a comment about aluminum wiring: it is not a potential problem, it is a real problem. It fails. good jobs last longer than poor jobs before they fail, but they still fail if they are used. Unused aluminum wiring is less of a problem.
If the brass-type alloy in Jon's lamp had been the slightly springy mix, and iff it had been correctly swedged, that would not have been the part that failed. Of course the problem with the 3-way sockets is that only the center contact has any spring, and so the other contact is challenged to stay connected. It was a poor design before it ever left the drawing board. Of course the Edison screw base is a challenge, but when they are made to the original standard they work quite well, and very few people cross-thread them. The main advantage, aside from the huge installed base, is that they don't depend on material springiness to hold the bulb in place. Most alternative bases demand much closer tolerances or much better materials in order to keep the contact presure high enough. And the wonderful sockets in your car would probably not pass the shock hazard and stupid user tests, or else they would be too expensive to compete.
I looked for some brass sockets with brass rivets with the idea that a bit of solder would provide a safer circuit. No such sockets exist, at least on the sites I reviewed. I also thought about running a 2-56 bolt through the brass tubing, tightening it and locking it in place. I might do that if I can't find an Eagle or Leviton replacement with a solid connection. We'll see...
Focus on Fundamentals consists of 45-minute on-line classes that cover a host of technologies. You learn without leaving the comfort of your desk. All classes are taught by subject-matter experts and all are archived. So if you can't attend live, attend at your convenience.