[TowerTalk] XHHW wire

[jimlux]

On 2/24/20 5:46 AM, Rob Atkinson wrote:
> I have a few problems with this wire.  My experience with aluminum is
> that it is not worth the trouble integrating it into the rest of the
> antenna system when it comes to bonding it to other metals.  I
> appreciate the light weight and lower cost but aluminum has another
> undesirable property:  It cold flows so clamps on it eventually come
> loose and then you have resistance and heat.  This is why aluminum
> house wire which was tried in the 1960s came to a quick end--too many
> cold flows, heat and fires.   I hope it isn't coming back for that
> purpose.


The lugs in your panel box are aluminum as well as the wire, so there's 
no CTE mismatch, so no loosening problem. For that matter, the circuit 
breakers engage with the aluminum bus bar.  Utilities have been using 
aluminum wire for a very long time with no problems. It was when they 
started to go to smaller sizes AND the use of the "wrap around the 
screw" type termination.

The problem with aluminum house wire isn't cold flow (mostly) - it's 
that about that same time they changed the material of the screw 
terminals on receptacles and switches from brass to steel.  Steel and 
Aluminum have significantly different CTE, so with temperature cycling, 
the connection would loosen, and then you'd have the heating/fire 
issues.  This is aggravated by aluminum rapidly forming an oxide layer

  Today, with push in spring loaded connections, that isn't an issue - 
the spring makes a gas-tight seal with the aluminum. Modern aluminum 
wire is a different alloy than that used pre-1970s and has a different 
CTE (and slightly different creep, but apparently that's not 
significant)  If you do need to connect to aluminum wire, they have 
wire-nuts specifically designed to join copper and aluminum.


Airplanes have used aluminum wiring for large conductors for a long 
time, without many problems. They also use crimp connectors designed for 
aluminum wire.  I understand that the A380 uses a lot of aluminum for 
smaller wires, but it's some exotic plated stuff with multiple layers. 
You're not going to be getting it for a ham antenna in a surplus sale 
any time soon.





> 
> I have my 200 amp service drop to my home using aluminum.  I don't
> like it but I can't make the power company use copper.  Every few
> years I check the lugs on the main breaker to make sure they are still
> tight.   This is one area where an IR camera would come in handy.   It
> WILL cold flow eventually and you'll know it when your lights flicker
> or dim briefly when a big load like A/C comes on.

Well, you probably don't check the lugs under the meter, and those are 
aluminum too. You can get an IR camera and check, but I suspect you'll 
not find any problems. Don't forget to put a piece of tape on the lug so 
the emissivity is what the camera is expecting, otherwise the temp reads 
partly what's reflected.

Light flicker is more likely due to insufficient distribution 
transformer capacity, or a failure of the grounded (neutral) conductor. 
We used to have the former because when the tract was originally laid 
out it had half the number of houses it was finally built with, so all 
the pad mount transformers wound up with twice as many houses as 
expected.  A co-worker had the neutral failure (on infrastructure 
installed in the 1920s and 1930s in South Pasadena) - changing the loads 
would change the voltages on the two "phases".  They replaced it with, 
of course, aluminum.



> 
> I was dismayed to see that QST ran an article by some sort of RF
> expert on how to construct open wire feedline using aluminum AWG 8 or
> maybe it was 6, stranded wire.  Ridiculous overkill for ham power, and
> he spliced line sections together using what appeared to be steel
> clamps.  Those clamps will lose their grip in a few years and he'll
> have problems, besides the use of steel for RF.   None of this came up
> in the article.    Then there was the line Z due to the spacing and
> diameter of AWG 8 cable but that's outside the focus here.

yes, proper crimps with a proper tool would be a better way to connect 
it. Picking the right alloy is important.

As far as loss goes, I suspect that for open wire line (already low 
loss) the larger aluminum compared to a smaller copper might actually 
come out ahead. Lots of people homebrew open wire line with AWG 14, and 
AWG 8 is twice the diameter.  If we assume that skin depth is small 
compared to diameter (which is worst case here), then the fact that 
aluminum is half the conductivity of hard drawn copper would be made up 
for by having twice the diameter.  If the skin depth is bigger, then the 
fact that the AWG 8 has 4 times the cross sectional area makes it even 
better.

Stranded wire has it's share of RF problems, but sometimes, you use what 
you have.