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  1. #1
    Join Date
    Dec 2008
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    Erwin, TN
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    262

    Default What type wiring is this ?

    House was built in 1939, is this aluminum wiring, can you tell by cloth type sheathing ?

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  2. #2
    Join Date
    Aug 2008
    Location
    Maryland
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    Default Re: What type wiring is this ?

    Aluminum branch circuit wiring would havea a thermoplastic insulation. Tinned copper had the fabric braid over the rubber insulation.

    All answers based on unamended National Electrical codes.

  3. #3
    Join Date
    Mar 2007
    Location
    Ormond Beach, Florida
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    27,207

    Default Re: What type wiring is this ?

    Quote Originally Posted by Sam Morris View Post
    House was built in 1939, is this aluminum wiring, can you tell by cloth type sheathing ?
    Sam,

    Your question of "is this aluminum wiring" provides the answer.

    The "aluminum look" is from copper conductors where were tinned.

    The reason copper conductors were tinned is to keep the copper from reacting with the rubber insulation used at that time.

    Depending on what you see entering the panel enclosure (which is not shown in the photos): a) if each conductor enters separately in its own ceramic tube, you have cloth covered, rubber insulated, conductors on knob and tubes (Knob and tube); b) if the conductors enter in pairs contained in a single outer sheathing (also cloth), you are looking at cloth covered, rubber insulated, NM cable. Both would be tinned copper because of the rubber insulation.

    And that rubber insulation is going to be dried out and not very effective ... if effective at all ... meaning that those circuits should be re-wired with modern NM cable (which is in a PVC outer sheath and the conductors insulated in thermal plastic insulation).

    Jerry Peck
    Construction Litigation Consultant ( www.ConstructionLitigationConsultants.com )
    www.AskCodeMan.com

  4. #4
    Join Date
    Mar 2009
    Location
    Colorado Front Range
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    649

    Default Re: What type wiring is this ?

    Pay close attention to the area where the old wire enters the enclosure. If you can't tell what kind of sheath it is do some looking in a basement/attic if possible. There used to be a cable type (usually called BX) that was a tinned copper wire in a rubber insulation with a fabric outer cover that was inside an overall spiral wound metal sheath.

    This old stuff is particularly bad news as the rubber insulation can break down and allow the copper to contact the metal sheath. Bad on lots of levels as there are a number of different ways that the contact can happen. Sometimes you wind up with a live voltage sheath, bad news if someone tried to use the sheath for a ground as happens a lot, putting a charge on whatever exposed metal is trying to be grounded by that circuit.

    Other times both the hot and neutral can have insulation in different places and convert the sheath into an electric heater. I've raised a scuttle hole lid in more than one house and found the attic illuminated by a dull red light. As near as I can figure byproducts from coal heat found their way between the metal sheath and the framing and "just " managed to keep a fire from starting. Exactly what the distance between these faults is sometimes determines if you build a new house or not.

    If there is any of this type insulation feeding light fixtures the rubber almost inevitibly turns completely to dust when disturbed. If by some miracle it doesen't the stuff isn't rated at the 90 deg C required for most modern light fixtures and a work around for that would nee to be found.

    My contracts all require this stuff (rubber insulated tinned copper) be replaced when I'm working on a property because the very act of bumping it can cause major problems. And, 99.9 % of the time you can't see the damage.

    Occam's eraser: The philosophical principle that even the simplest solution is bound to have something wrong with it.

  5. #5
    Join Date
    Mar 2009
    Location
    Colorado Front Range
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    649

    Default Re: What type wiring is this ?

    One other thing with this old wiring. It was used when there was often little or no insulation used in attics and walls. The stuff is not supposed to be "in" insulation as that speeds up the deterioration of the rubber even more. Cellulose insulation around this old wire is just plain scary as there's almost no heat migration away from the wire.

    Occam's eraser: The philosophical principle that even the simplest solution is bound to have something wrong with it.

  6. #6
    Join Date
    Oct 2014
    Location
    Maryland, DC, and Northern Virginia, electrical only
    Posts
    155

    Default Re: What type wiring is this ?

    Quote Originally Posted by Jerry Peck View Post
    Sam,

    that rubber insulation is going to be dried out and not very effective ... if effective at all ... meaning that those circuits should be re-wired with modern NM cable (which is in a PVC outer sheath and the conductors insulated in thermal plastic insulation).
    I agree that the rubber is going to be dried out. Dave Dini's team did a study some years back, associated with UL, finding that old wiring tended to be good once you got a couple of inches away from terminations. I've found this to not always be true.

    All the same, it IS most commonly at the terminations that I found shorts, because of load and splice heating, and because the wires move. Inside the sheath, the crumbly rubber tends to serve as an effective spacer.

    Yes, it warrants replacement, or at least AF/GFCI protection until that time. Replacing it with NM is cheapest, and usually adequate. Replacing it with ACHH, modern BX, makes it immune from rodents and much more likely to trip any kind of breaker if penetrated.


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