connecting different metals?

Discussion in 'General Electronics Chat' started by Mathematics!, Dec 30, 2010.

  1. Mathematics!

    Thread Starter Senior Member

    Jul 21, 2008
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    This is for Electrical Engineering / Electrician ?

    I have helped an electrician with installing copper wire circuits over existing aluminum ones. We used purple wire nuts with this non-corrosive gel in them.

    What I am curious is this just for copper to aluminum or must you use this gel on any 2 metals that are connected together that are different.

    For example what if I had copper and silver would I still have to use a gel?
    Would this gel be the same as the one used for aluminum.

    Also what is the big hazard about not using these special wire nuts?
    I would think the worst that could happen is a open circuit or a voltage drop at the connection of the 2 distinct metals caused by corrosion. Can't see this being dangerous. But maybe I am over looking something.

    Thanks
     
  2. Markd77

    Senior Member

    Sep 7, 2009
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    The problem is that corrosion takes a while so at some point there is a very small amount of contact carrying the current. That gets very hot and burns the house down.
     
  3. #12

    Expert

    Nov 30, 2010
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    The problem is the aluminum. It forms an oxide layer and it expands because of heat. A tiny defect makes itself worse and worse over the years and causes FIRE!
     
  4. Mathematics!

    Thread Starter Senior Member

    Jul 21, 2008
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    OK, so what if you put a heat sink at the contact areas wouldn't this dissipate the heat so it doesn't get hot enough to burn down the house?

    what about for different metals other then aluminum does this still apply?

    How easy would it be to have a house start on fire with not using non-corrosive stuff? Seems to me the breakers would trip before the wire got anywhere near as hot as it needed to to start a fire. Correct me if I am wrong though.
     
  5. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    If you have a house wired with silver wires, I want to buy it. Then again, what about gold and silver? How about depleted uranium and copper?

    John
     
  6. #12

    Expert

    Nov 30, 2010
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    Be practical. You can't put a heat sink on every wire nut in the building, and if you use the proper connectors, there is no reason to. Fix the problem, not the symptoms.

    Yes, it's just for aluminum.

    A 15 amp breaker will pass 1800 watts of power without thinking about tripping. 1800 watts under a wire nut will cause enough heat to run a glowing, red hot, room heater, all in a tenth of a cubic inch.
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    The whole point is that you desire maximum power transmission to the load, with minimal losses along the way.

    If you wind up with a high-resistance joint in your wiring, power will be dissipated in that joint instead of the load.

    As the price of copper went up, there was a lot of pressure to keep construction costs down to maintain profit margins while keeping homes affordable. Unfortunately, there wasn't a lot of thought given to corrosion in earlier years.

    Aluminum works OK as long as it's protected from corrosion. However, it still has a much higher resistance than copper of the same gauge, and is a much worse conductor of heat.

    Pick up a copy of Forrest Mims III "Engineer's Mini-Notebook Volume IV", available at Radio Shack again after a long absence:
    http://www.radioshack.com/product/index.jsp?productId=10852492
     
  8. marshallf3

    Well-Known Member

    Jul 26, 2010
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    What I am curious is this just for copper to aluminum or must you use this gel on any 2 metals that are connected together that are different.

    Not on Cu to Cu but it's a must with anything Al and I hope the wire nuts themselves were rated to handle this type of connection - most are not. The safest way to connect Al to Cu is with small buss bars that have the screws in them then cover them in silicone grease followed by a lot of tape.

    If you've got Al wiring in your house it's not uncommon to find a lot of incompatibilities all the way from the breakers to the duplex outlets, switches and light fixtures. http://www.faqs.org/faqs/electrical-wiring/part2/section-16.html
     
  9. debe

    Well-Known Member

    Sep 21, 2010
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    Back in the early 70s when I worked as a power linesman ETSA the company was replacing copper cables with aluminum. To join the copper to aluminum we first wire brushed it, then wire brushed it with a grease to exclude air. Then an aluminiumn sleeve was used to join the wires & a hydraulic crimper was used to crimp the sleeve. This joint was then covered with a sealing material for weather proofing. This was done on street mains & must have been fairly good. As i dont recall having any trouble.
     
  10. #12

    Expert

    Nov 30, 2010
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    www.copper-aluminum-wire.com/copalum.htm paints a pretty bad picture of aluminum wire and then offers a fix with Tyco brand connectors saying: "This method is the only other method recommended by the U. S. Consumer Product Safety Commission." (besides rewiring the whole house with copper)

    I'm not sure this page is still the truth, but I am sure aluminum wiring has burned a lot of houses.
     
  11. marshallf3

    Well-Known Member

    Jul 26, 2010
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    Al is still used for high voltage transmission service in some places (mainly because it's lighter) but is being replaced over time with Cu because the line losses are so much less that they can recoup their costs in a short order of time.

    http://www.tpub.com/neets/book4/11e.htm
     
  12. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    And even more House Trailers/ Manufactured Homes.
     
  13. floomdoggle

    Senior Member

    Sep 1, 2008
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    The "grease" is sold as Noalox.
     
  14. #12

    Expert

    Nov 30, 2010
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    STOP!

    I used Noalox until I found that it was banned for being flamable.
     
  15. marshallf3

    Well-Known Member

    Jul 26, 2010
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    Plain old silicone electrical grease, available from any auto parts supplier in tiny squeeze packs all the way up to large tubes or a lot of other places as well. I suggest just getting a 4 or larger ounce tube, good stuff to have around.
     
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