Why aluminium is not a primary choice in cables? Does it have something to do with corrosion? Does it solder differently?
It is the primary choice in HV power transmission cables- aluminum with a steel core for strength.
As well as the well-known issue of cheaply and easily making reliable field connections with small gauge simple aluminum wires (eg. house wiring), the higher resistivity means things like transformers and motors would need more iron. They may still be somewhat more economical, all things considered. From here is a comparison of 2.5MVA electrical transformers made with copper vs. hybrid vs. aluminum:
The surface of an aluminum object will oxidize when exposed to air, producing a thin layer of aluminum oxide.
This is great for manufacturing, because it protects the material from further degradation. It's not so great for conductors, because the oxide layer is an electrical insulator. (You may have seen small white plates of aluminum oxide form between components and heatsinks.)
Terminating aluminum cables requires specialty components and anti-oxidizing paste. Installation therefore requires more effort and is thus more expensive and fault-prone. So aluminum is only used for long stretches of cable where it will eventually be cheaper than copper.
For example, many underground cables are aluminum.
You may have seen copper-clad aluminum cables. You can connect these using standard terminals, but the aluminum is only present for mechanical strength, not for conductivity.
In addition to the other answers, Aluminum has a 30% greater co-efficient of thermal expansion than copper. This is not a problem per se. However, in some cases where aluminum wire has been used, proper account for the greater thermal expansion has not been taken into account. As a result, with repeated thermal cycling, termination connections have been loosened to the point where the connection resistance has become excessive and resulting in charring and fires.
(image from Grace Electrical Services
As mentioned, the thermal coefficient of expansion does not, in itself, make aluminum inappropriate for uses, such as building wiring. However, it does require that appropriate termination techniques and materials be applied. Aluminum wiring is still permitted by the US National Electrical Code for building wiring. (citation here )
Aluminum is marvelous, underrated stuff
And North American AC electrical wiring has achieved mastery of using it, including the unfortunate exception which proves the rule.
Never mind copper being twice the weight for the same ampacity. It's 10 times the cost for the same ampacity!
Copper had to come first, because it exists primordially - you can find copper nuggets in nature, so it was one of the first worked metals in human history. The affordable process for refining aluminum requires staggering amounts of electricity, so by definition, aluminum enters a world that is already fully built-out in copper.
What are the tricks to working with it?
In practical application in house wiring, there are two "golden rules" called out on the first page of the Code book.
Use terminals designed and rated for aluminum wire (NEC 110.3). Generally, the simplest way to do this is to use a lug terminal which is itself made of aluminum. Panel neutral bars, large lugs, Alumiconn and Polaris style connectors all use this method.
This works because aluminum has a different coefficient of expansion than copper. When the aluminum lug envelops a copper wire, this thermal expansion works favorably.
The exception that proves the rule is the 1970s experiment with small aluminum branch circuit wires in North America. This was done in response to a copper shortage, and the government ordered the certification agency (UL) to fast-track approval of switches, receptacles and splices. UL hastily certified ordinary devices made of brass without adequate testing. This caused frequent failure, and the postmortem revealed the rule I state above. Design standards were significantly revised (to CO-ALR), and those devices have terminals made of indium, a soft, conformant metal that mates well with aluminum.
Torque terminals to spec with a torque wrench (NEC 110.14). This was a universal "best practice" that electricians did on large high-current terminals, and those have proven reliable in both metals. However, for "the small stuff" (15-50A wires), no one bothered - they just cranked them down "gud-n-tite" with no particular standard for what that means.
And totally unrelated to aluminum, this was found to be a major cause of failure for copper connections, and of course there's no reason to think mistorqued aluminum connections were performing any better. This was coupled with some trade-show testing that proved pro electricians couldn't set torques "by feel" any better than their spouses. It turns out the entire industry had been in the bottom quartile of the Dunning-Kruger curve, and many are still in denial.
Use the proper alloy. The 1970s effort involved taking AA-1350 alloy "outdoor transmission line" wire and shrinking it down. This did not perform well. A new AA-8000 alloy was specially developed for interior electrical wire.
So with those application techniques mastered, aluminum is proving to be a winner for architectural electrical distribution. It has always dominated the skies of utility distribution.
That said, in residential the stigma of the 1970s mistake lingers, and don't wire with aluminum smaller than 50A if you want to get good money when you sell the house.
How do you design for aluminum?
A worthy question. And please - if you are designing an EVSE, accommodate aluminum wire. Almost none do, and it greatly increases installation cost (6 AWG copper NM vs 6 AWG aluminum SER/SEU).
The gold standard in the electrical industry is an aluminum-bodied lug connector. So have a copper lead or bar come out of your machine and land on an aluminum bodied lug with two screw terminals - one for your copper lead/bar, and one for the customer supplied wire connection. Torque your side to spec and have a label reminding the installer to do the same. It also helps to use 75C thermal connections, which allows a smaller wire size to be used.
If you are attaching these lugs to a PCB or plastic molding, they do not need to be insulated and can be quite inexpensive.
Reason for no: Tokamaks
If you're trying to run a cyclotron or magnetic isotopic separator, you need the maximum possible magnetic flux in the least possible space. This can be important for motors, too, if you need to cram an extremely high power motor in the limited space between railroad rails, for instance.
For that, copper can't be beat.
Reason for no: you want to sell your house.
NEC has no quarrel with use of aluminum wire for small branch circuits, subject to the new rules.
However, if you do, the buyer's inspector will find it, mistake it for the problematic old stuff, and tell the buyer not to buy your house. They will demand a discount, remediation, or walk away.
Use for >50A circuits continues to be perfectly fine. A few superstitious jurisdictions raise that standard to 90A.
Reason for no: flexible cordage
For fine-threaded flexible cordage such as found between an appliance and its wall plug, aluminum is really too brittle. It does not like the constant flexure.
Reason for no: physical working
Aluminum has no fatigue limit so if it's in an application where it'll be carrying a lot of physical weight and moving a lot, such as an overhead line, it's going to accumulate metal fatigue and fail. Copper has the same problem, though.
The cure for this is ACSR, which is aluminum wire around a steel core. Now the steel core takes care of the physical strain of supporting the cable, and the aluminum is just along for the ride. This is not used in residential "service drops"; that's just regular duplex/triplex/quadplex.
- Aluminium is soft. Copper isn't the toughest of metals, but it is when compared with aluminium.
- Aluminium oxidises on contact with air. The oxide layer is thin, but it's an insulator. You have to break through it to make contact.
- If you mix dissimilar metals, and damp gets in, then one of those metals will suffer from electrolytic corrosion.
Screw terminals for wires are often made of brass. It's tough, and it's mostly made of copper. So if you put a copper wire into a brass terminal, and do the screw up reasonably tight, it will make good contact for many years.
Put an aluminium wire in the same contact, and you'll have to do the screw up tight to break through the oxide. That crushes the soft wire, making it liable to snap. Then damp gets in, and the aluminium starts to corrode. It now gives a bad contact. So you do the screw up tighter, and the already corroded wire snaps.
You can't make the screw terminals out of aluminium instead; it's too soft.
Where aluminium was used for house wiring and telephone cables in the 1970's to 80's it's now failing. It's good for large cables when used with specialised terminals, particularly crimp terminals. But for smaller wire gauges, it's terrible stuff. UK wiring standards now ban it for the sizes of cables you would find in houses.
Some of the problem is history... bad history...
Aluminum wires are larger, softer, and more brittle than copper, leading to poorer connections and connection failures. Aluminum’s oxidation process also creates a surface coating with high electrical resistance. That resistance is one factor in creating excess heat buildup in the connection. It is this breakdown of the connections at outlets, light switches, and other junctions that can create enough heat to start a fire.
A study done by the U.S. Consumer Product Safety Commission (CPSC) revealed that “homes wired with aluminum wire manufactured before 1972 (‘old technology’ aluminum wire) are 55 times more likely to have one or more connections reach ‘Fire Hazard Conditions’ than a home wired with copper.” A typical home can have 200 or more connections, exponentially increasing the risk of fire. (Source)
Did you notice the phrase "'old technology' aluminum wire"? That's part of the problem. The industry has overcome most of the issues involving aluminum in electrical conductors, perhaps all of them... except one.
People have really long memories when it comes to bad experiences.
Why does the U.S. avoid nuclear energy? Because we all remember Three Mile Island and refuse to believe things have improved. Why isn't aluminum used in, e.g., house wiring? Because we all remember the bad days when houses either burned down or the wire became brittle and broke apart inside the insulation forcing expensive wire-pulling-and-replacing contractor visits.
Never underestimate a human being's ability to refuse to let go of the past.