[TCML] primary tubing - now Aluminum wiring failures

[jimlux]

Quarkster wrote:
> Jim -
> 
> The terminations of the aluminum wire become oxidized, develop some 
> resistance across the termination, and eventually overheat.
> 
> There are several contributing factors:
> 1. Aluminum exposed to air immediately develops a high-resistance oxide 
> film;
> 2. Aluminum wire is quite soft and "cold-flows" under pressure;
> 3. Aluminum's high rate of thermal expansion can cause a 
> "self-degenerating" thermal-cycling problem where a small amount of 
> resistance creates localized heating, which causes the wire (and any 
> clamped connections) to expand. When the electrical load on that circuit 
> is removed, everything cools off, and if the aluminum has cold-flowed, 
> there is now less clamping pressure, which allows further oxidation in 
> the connection, which creates more resistance, which gets hotter the 
> next time the circuit is loaded, etc, etc.
> 
> I've personally seen wire-nuts on aluminum wiring that got so hot that 
> the plastic wire-nut body completely melted away, leaving only the 
> conical steel spring from inside the wire-nut holding the wires together.
> 
> Aluminum house wiring was a cost-saving measure that was implemented 
> without adequate testing, and turned out to be a complete disaster, a 
> great example of shoddy Engineering.
> 
> Regards,
> Herr Zapp



It's a bit more complex than that, even.  Aluminum comes in dozens of 
different alloys, some more suitable than others for wiring.  Another 
aspect implicated in the house wiring issues is that about the same time 
that they started using aluminum wire as a cost savings measure, they 
also changed from brass to steel screw terminals on receptacles and 
switches. That aggravated the differential thermal expansion problem.

There are modern aluminum wires that are perfectly safe, used with the 
correct terminations.

As others have pointed out, most distribution transformers these days 
use aluminum wire.  My old Piper Cherokee 140 used aluminum wire (for 
weight saving).  There are miles of coaxial cable out there with 
aluminum shield.



 From a TC standpoint, I don't think aluminum buys you a whole lot.  On 
the primary, the difficulty in making a good adjustable tap point is 
probably a deal breaker, even if copper tubing costs as much as it does.
On the secondary, it might be an OK trade, if you go up in size enough 
to account for the increased resistivity.  That will mean you get fewer 
turns per inch, so, for a given size coil you'll get fewer turns, so the 
inductance will be less, which will affect the tuning and the Lp/Ls ratio.

I know several folks have made experiments of one kind or another, but I 
don't think they've spanned the necessary range of conditions to fully 
account for all the various factors;e.g. how do you disentangle the 
effect of changing the inductance in the seconday.. you'd need to wind 
one with aluminum wire, then wind another with (smaller) copper wire so 
the AC resistance is the same, but making sure the turns per inch makes 
the inductance come out right.  And for close wound coils (like a 
secondary), the AC resistance calculation is non trivial, because the 
fields from turns i-1 and i+1 affect the current distribution in turn i. 
(aka proximity effect).  That would change for the spacewound copper 
wire coil.