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Thread: aluminum supply to sub panel - breaker tripping/wire heating

  1. #1

    Default aluminum supply to sub panel - breaker tripping/wire heating

    I have a 100 amp circuit going to a subpanel in the basement. It's worked fine until this week. the 100amp circuit has become very prone to tripping, especially when the dryer in the basement has been running for a bit. The house was built in 1994.

    I removed the panel cover for the main panel and noticed quite a bit of thermal build-up beginning @ the breaker and going for about 6" up the aluminum wire that feeds the basement subpanel. My first theory is, of course, oxidation. I have not pulled the aluminum wires from the breaker to see if there is oxidation, but I can see that an oxidation-inhibitor was applied to the other aluminum in the panel.

    I did not detect similar heat build up at the subpanel.

    So, here are the questions:

    If I do a visual inspection of the wires and find that there is oxidation, there is not enough slack in the wire to cut back more than 1" - I doubt that would be enough. Is there any way of installing a connecting block in the panel as a "splice" that would meet code? Maybe run copper from the block to the breaker? I'd really hate to replace the whole run (I'm guessing 70 feet) it would be quite expensive.

    should I replace the breaker itself? I would guess that it has oxidation on it as well.

    If a visual inspection does not reveal oxidation, what would you recommend as the next step in diagnosis?

    Thanks,

    Ian

  2. #2
    DIY Member bsperr's Avatar
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    Before you replace anything, I would make sure that the feeder wire to your subpanel is sized correctly based upon the load and the overcurrent protection you have. If it is, I would make sure that the connection between the feeder wire and the breaker is clean and tight. Loose connections can generate a lot of heat that would trip the breaker.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    WOrking with aluminum wire requires four things: proper size, proper torque, anticorrosion, and terminals designed to allow aluminum. Skip any one of them, and you will likely have problems.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    General Engineering Contractor ballvalve's Avatar
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    I would pull the wires out of the breakers, sand and clean them, recoat them with no corrode, and clean the breaker terminals. Retorque and retry.

    I would put a ammeter on the run FIRST and see what the amp pull is, especially with they dryer. Your dryer may be going out.

    Aluminum should also be RE torqued after a few days, makes a huge difference.

  5. #5
    Moderator & Master Plumber hj's Avatar
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    heating and melting of the wire insulation at a breaker usually means the connection was loose creating a high resistance connection. The heat will also cause the breaker to trip.

  6. #6
    Homeowner Thatguy's Avatar
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    The pass/fail spec for relay contacts is, at rated current, less than 30 millivolts is good and more than 100 mV is bad.
    These particular contacts should have more contact force than relay contacts so your numbers at the 100 A rating should be less. 0.03 V at 100 A is 300 micro-ohms.
    At least one panel manufacturer would/could not give me pass/fail resistance specs for these types of contacts. I would imagine they test with a four-terminal [Kelvin clips] micro-ohmmeter.

    For the next step in diagnosis, if you're very comfortable with electricity you could possibly bring out two thin, insulated test leads, one on each side of the contact interface, and confirm with a DVM how many mV you're dropping across this questionable interface. The test leads carry virtually zero current but they should be insulated to 600 v or so.

    Watch out for shock danger and arc-flash danger. These connections will be difficult to make.

    Another way to find a hot connection is to use one of these non-contact IR meters, but get one with adjustable emissivity. Smooth surfaces are generally low emissivity and so your temp readings may be off, or inconsistent.
    Last edited by Thatguy; 10-06-2010 at 08:16 PM.

  7. #7
    Electrical Contractor Jim Port's Avatar
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    Yes there are splices that can be made in the feeder conductor.

    A ampmeter would be the simplest way to start to narrow down the problem. I think your issue was a poor or loose connection, not too many amps.

  8. #8

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    Thanks to all you smart guys that suggested that it could be a loose connection @ the panel, because that's exactly what it was! I had checked for that half heartedly earlier by trying to move the wire, but not by actually twisting the terminal screw on the breaker. when i did that, i was shocked that it had nearly half a turn before it got tight again!

    all is well, thanks very much,

    Ian

  9. #9
    Moderator & Master Plumber hj's Avatar
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    You were lucky. Sometimes the clamping screw is welded to its socket by the heat from the loose connection. The recommendation is that the clamping screws on aluminum wire terminals be checked for tightness every five years or so, because they do expand and contract more than copper wires do.

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