What is the key technology for connecting aluminum?

There has been a number of recent posts concerning aluminum conductors. IIRC, the problem occurs because of poor connection between Al
conductors and devices such as switches and sockets. Presumably, with proper devices and other techniques developed after gthe initial wide use of Al, reliable connections can be made.
What is the key to make good connections? Doesn't oxide form in a way to introduce resistance and consequent heating? Is there actual metal to metal contact without an intervening layer of aluminum oxide?
Bill
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You wire brush the aluminum conductors to remove the aluminum oxide and then apply a paste (nolox) to prevent formation of new oxide. So, I would guess, you're correct, no significant intervening aluminum oxide. Also, the CuAl connectors prevent the unequal expansion and contraction problem.
j
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To avoid problems with aluminum oxide, you use a special paste on the aluminum. The paste contains some kind of air-excluding grease plus fine metal particals. You apply it to the wire, then work it in with a wire brush. The wire brush breaks up the existing surface oxide, and the grease keeps air away so new oxide doesn't form. The metal particles help penetrate the surface of the aluminum too.
Also, aluminum has a problem flowing away from areas of high applied pressure. The old screw wire clamps with a point that digs into the copper wire are a problem with aluminum. Devices designed for aluminum use have a large contact area with less pressure per unit area.
Apparently, much of the problem with aluminum with with the "backstab" connection points on cheap devices. They typically have a single sharp piece of metal that bites into the side of the wire - somewhat iffy with copper but a failure waiting to happen with aluminum.
    Dave
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Those ought to be outlawed for any sort of wire. I've seen receptacles on multiple occasions that were melted and charred, one was red hot inside due to backwire connectors which had developed a poor contact and a large load was further down the chain. In my own house, the downstairs had been wired with the 49 cent variety of those, plugs wouldn't stay in them so I went to replace them and with almost every one, wires popped right out as I pulled the receptacle out of the box.
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How reliable is solder joint between two aluminum wires?
Or between one single strand aluminum conductor and one multi strand tinned copper wire?
Which solder is better for this kind of connection? Solder with lead or without lead? I know worldwide people are changing to lead free solder but I want know which type of solder will make a reliable joint in aluminum conductors.
Thanks
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You can't solder aluminum. Unless someone has come up with something new since I was doing a lot of soldering 30 years ago.
Bill Ranck Blacksburg, Va.
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snipped-for-privacy@vt.edu wrote:

Alusol http://www.cupalloys.com/products.php?productIdS
I've been using it for years..
Very, very effective and a lot easier than welding aluminium (although welding is obviously more fun an more effective..) -- Sue
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J wrote:

http://www.aws.org/wj/2004/02/046 /
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Where would you use a soldered joint? I was under the impression that there are essentially no soldered joints in modern line voltage wiring (I'm not counting knob and tube).
    Dave
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My application involves using polyester coated winding wire. Becuase of copper wire price going up peole have started using pure aluminum wire and copper clad alumium wires
http://en.wikipedia.org/wiki/Copper-clad_aluminum_wire
but at some stage these wire needs to be connected to a copper wire like mains lead or any input or output lead.
regards
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James Sweet wrote:

I don't think any devices were ever approved for use with aluminum in "backstabs" - copper only. (But that doesn't prevent improper use.)
Tests show "properly" made connections could also fail.

One of the great mysteries of life - why does UL continue to allow "backstabs". At least the holes are too small for #12 now.
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| James Sweet wrote: |>> Apparently, much of the problem with aluminum with with the "backstab" |>> connection points on cheap devices. They typically have a single sharp |>> piece of metal that bites into the side of the wire - somewhat iffy with |>> copper but a failure waiting to happen with aluminum. |>> | | I don't think any devices were ever approved for use with aluminum in | "backstabs" - copper only. (But that doesn't prevent improper use.) | | Tests show "properly" made connections could also fail. | |> |> Those ought to be outlawed for any sort of wire. | | One of the great mysteries of life - why does UL continue to allow | "backstabs". At least the holes are too small for #12 now.
Oh great! Now they will be encouraging idiots to change the wire size to number #14. Let's see, what is the ampacity of #14 AL and how safe will it be on a 20 amp circuit? OTOH, I suppose for the most part this is the Darwin Effect in action (as applied to that error in the gene pool).
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snipped-for-privacy@ipal.net wrote:

#14 Al wire has never been allowed for US branch circuits.
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| snipped-for-privacy@ipal.net wrote:
|> | James Sweet wrote: |> |>> Apparently, much of the problem with aluminum with with the "backstab" |> |>> connection points on cheap devices. They typically have a single sharp |> |>> piece of metal that bites into the side of the wire - somewhat iffy with |> |>> copper but a failure waiting to happen with aluminum. |> |>> |> | |> | I don't think any devices were ever approved for use with aluminum in |> | "backstabs" - copper only. (But that doesn't prevent improper use.) |> | |> | Tests show "properly" made connections could also fail. |> | |> |> |> |> Those ought to be outlawed for any sort of wire. |> | |> | One of the great mysteries of life - why does UL continue to allow |> | "backstabs". At least the holes are too small for #12 now. |> |> Oh great! Now they will be encouraging idiots to change the wire size to |> number #14. Let's see, what is the ampacity of #14 AL and how safe will |> it be on a 20 amp circuit? OTOH, I suppose for the most part this is the |> Darwin Effect in action (as applied to that error in the gene pool). |> | | #14 Al wire has never been allowed for US branch circuits.
But I was referring to an idiot's choice of wire. Likely he'll find that #14 Al is nearly impossible to find. And that would be sad because we'll lose the chance to remove that idiot gene from the pool.
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On Sun, 13 Apr 2008 14:54:40 -0400 Michael A. Terrell
| | snipped-for-privacy@ipal.net wrote: |>
|> | |> | #14 Al wire has never been allowed for US branch circuits. |> |> But I was referring to an idiot's choice of wire. Likely he'll find that #14 |> Al is nearly impossible to find. And that would be sad because we'll lose the |> chance to remove that idiot gene from the pool. | | | Not if it's someone elses's home, or it lasts long enough to sell it.
If you hire an idiot as an electrician, you get what you deserve. This is what inspectors are for (among other things).
If you buy a home from an idiot (and don't inspect it to discover this error), you get what you deserve.
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On Sun, 13 Apr 2008 16:58:44 -0400 Michael A. Terrell
| | snipped-for-privacy@ipal.net wrote: |> |> On Sun, 13 Apr 2008 14:54:40 -0400 Michael A. Terrell
|> |
|> |>
|> |> | |> |> | #14 Al wire has never been allowed for US branch circuits. |> |> |> |> But I was referring to an idiot's choice of wire. Likely he'll find that #14 |> |> Al is nearly impossible to find. And that would be sad because we'll lose the |> |> chance to remove that idiot gene from the pool. |> | |> | |> | Not if it's someone elses's home, or it lasts long enough to sell it. |> |> If you hire an idiot as an electrician, you get what you deserve. This is |> what inspectors are for (among other things). |> |> If you buy a home from an idiot (and don't inspect it to discover this error), |> you get what you deserve. | | | You are a self righteous idiot. I hope that is a close family member | of yours who dies at the hand of one of the free range goof balls.
Gee, and I thought you had come out of your four year long attitude of making personal attacks on people. But I guess I was wrong.
[note: fours years is a long as I have observed this; it might be longer]
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snipped-for-privacy@ipal.net wrote:

A bit unrealistic.
Most people who hire an electrician don't know enough to determine that he/she is incompetant.
A normal pre-sale inspection will not include disassembly of the electrical system to determine wire size.
Ed
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When I first posted this question. I was really trying to find out what happened in Al wiring that caused.
I know that pure copper (Cu) will cold weld when it flows with sufficient pressure applied. Is there welding taking place with Cu under binding posts? Certainly Cu oxidizes. Are Cu oxides conductive?
Does Al cold weld? How important is that in making connections? How do the anti-oxidizing pastes work?
Bill
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Salmon Egg wrote:

I think it was covered in the replies, but I'll do it here. Two factors cause the problem: 1) al and cu have different coefficients of expansion. 2) Aluminium oxide is quick forming and is an insulator. The combination is (slowly) deadly - with different expansion, when the splice heats due to current flow, some of the al that was in contact with the cu gets exposed to air and oxidizes where exposed. The joint cools when current stops and contracts, but with a bit less of the al in contact with the cu due to the oxide. The next application of current "sees" a higher resistance (because the contact area is smaller) so creates more heat. More heat means greater expansion, so more al gets exposed and thus more oxide is formed. Repeat the process enough times and you can get into real trouble.
The paste works to prevent oxidation. No oxidation = no problem. Of course, that assumes the joint was properly made in the first place. Paste won't "rescue" a poorly made splice.
As far as I know, cold welding does not occur in residential wiring with either material.
Ed

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ehsjr wrote:

When aluminum heats, it expands faster than the screw it is constrained by. The aluminum may extrude and become smaller, which makes a looser connection, more heat, and progressive deterioration. I think that is a little different from what Ed said.
"New technology" wire and CO/ALR devices fix the expansion problem. But "old technology" wire (from before the change in UL standards) could still have problems.

I would add the obvious, that paste keeps oxygen away from the aluminum to prevent oxide. In general, for good connections you would like a 'gas tight' joint. Some pastes allegedly have metal particles to bite through the oxide.
For 15/20A circuits, the recommendation based on testing done for the CPSC is to apply antioxide paste, then abrade the wire, and use paste in the connection.
As an example of oxide problems, wires connected with a wirenut may have little metal-to-metal contact because of oxide. Large lugs deform the wire which breaks through any oxide (which is very thin). Wire nuts don't force the wires together hard enough. But the spring in the wire nut can cut through the oxide and make contact. The spring may be relatively high resistance and is not intended to be the current carrying element. If a couple turns make contact they can form a heater and can get red hot.

I don't think it occurs either.
For larger wire, Cu and Al, one splice method is to put the wires in the ends of a tube and compress the tube with high pressure. This forms a cold weld from wire to sleeve. I believe the Copalum splices, which gfretwell referred to, use high pressure compression and form a cold weld. But as he said they are quite expensive.

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