Hello,
Does anyone out there know why aluminum is not used for house wiring, but is used as service feeds? Is it not a good conductor at lower voltages? Does it over heat? I am looking at a remodel project of a home that was wired with aluminum and the owner is concerned that the home is at risk.
Replies
corrosion at the joints
alu expands and contracts more than copper when heated/cooled by use, and this abrasion can allow corrosion to build up in a loose joint, eventually leading to a resistance that can lead to heat. In the worst case, fire.
There are special al/cu jumpers that can be added (only by a licensed electrician) with an air tight seal which will isolate all of the joints in the aluminum from exposure to air (oxygen) and eliminate the risk. After installation, all the devices and joints that are in the air will be copper x copper.
Aluminum itself conducts power just fine (the size of the wire is increased to compensate for the slightly lower conductivity than copper), it is only the joints that are exposed to the air that are a concern.
Talk to an electrician about quoting this (install the al/cu jumpers) for the whole house.
If that is too much, the next best thing is to check every connection in the house (every one!) is fully tight and cover each one with a special protectant (that helps to keep oxygen away from the joint, reducing the chance for corrosion to occur).
On the other hand, main feed wiring is done with aluminum because it is cheap, lightweight and has only one joint inside the house (at the panel) and is quite large and is not as much as a risk as all of the little connections you've got all over a house for every switch and receptacle. These joints at the panel (and meter) are covered in the same oxygen barrier goop to protect them from corrosion and were torqued down with a torque wrench when they were installed (pretty well controlled conditions = safe).
Hope this helps,
Norm
Thank you for the info. You have armed me with the info I need.
Pat
Aluminum has several problems.
One is that aluminum (Al) is more plastic. That is that it does not return to its original position when exposed to pressure as well as copper (Cu) does. All wiring heats up under load. This is a simple function of resistance. Most materials expand when heated. So when under electrical load the Al expands and presses against any restraint. As it cools it tends to pull away because the material has oozed out away from the increased pressure. This exposes the Al previously shielded by direct contact with the lug to air.
This brings up the second, possibly more important, point. Copper oxide is a pretty good conductor but aluminum oxide is an insulator. An insulator that forms a film second in hardness only to diamond. It also forms incredibly fast. Slice into a piece of copper and the oxide will have formed before the knife leaves the cut. The presence of aluminum oxide in a connection increases the resistance in the joint. Increased resistance increases heating under load.
Load to heat
Heat to expansion
Expansion to loosening
Loosening to oxidation
Oxidation to increased heating
Repeat this cycle every time the circuit is put under increased load and then allowed to cool.
Larger cables are less prone to this effect and specialized lugs, marked "Al/Cu", are made to increase reliability. Larger cable connections are generally better shielded from combustibles inside panels and other enclosures. Properly made and maintained aluminum feeders, not branch circuit wiring, is considered reliable and safe. IMHO a close second to copper at a substantial saving in cost and weight. Improperly installed or not maintained it can be quite miserable and dangerous.
Tips:
Wherever possible remove, abandon and disconnect any branch circuit conductors made of aluminum. Rewire with copper.
When making a connection with aluminum wiring clean the aluminum well, coat it with an approved anti-oxidant and reclean through the compound. The grease keeps the oxygen away from the aluminum so won't instantaneously oxidize. After this recoat the wire before inserting it into a connector. No need to have massive gobs of compound squeeze out but all exposed aluminum need a thin coating to be protected.
A small of compound dab on the threads of any set screws, back them most of the way out before applying, helps greatly it making sure the set screw does not bind or corrode in place. A seized lug can make tightening a loose connection impossible without replacing it. A costly job in many cases. A pennies worth of compound can save hundreds of dollars.
You can use emery cloth, be sure to remove the grit after cleaning, or a knife to clean the conductors but I have found that I get better result with a small stainless steel wire brush. These units look like a heavy duty toothbrush and are available where welding supplies are sold.
When making up aluminum joints make sure the connectors or lugs are rated for aluminum. Tighten to the listed torque range and come back in a few minutes and recheck the torque. Often a good connection will relax after being initially tightened. Then it is a good idea to recheck these connections every year or so. Caught early a bad connection is a simple repair. Allowed to cook for a few years the same problem can lead to more expensive repairs, destroy a panel or, more rarely, cause a fire.