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rewiring knob and tube

| Posted in General Discussion on February 4, 2004 07:36am

Hi All,

I’m trying to install the new whole house fan before the attic gets into the 90’s or above.  It’s a small house, so the ideal location is the only location – above the hallway.

In the attic above the hallway are two K&T circuits, the hot and the common/neutral.  I need to move these and two of the branch circuits to make room for the fan.  I pulled a permit, so everything will be inspected.  The fan itself will be wired to a new NM circuit from the basement subpanel, so it will be grounded.

What are NEC approved methods for tying into K&T?  Taunton’s Code Check says only that the box should be no less than 12″ from a knob.  I’ve heard that staples are unacceptable.  Code Check diagram shows a metal box with cable clamps in the knockouts.  Are metal boxes and clamps required?  Can I reuse some of the old knobs or are new ones available?

NEC at the city office says connections may be soldered (less perferable for me) or made with “approve wired connection devices.”   Does that mean wire nut?

If this is too much, what section of the NEC should I look in?

Thanks

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Replies

  1. Scooter1 | Feb 04, 2004 10:40pm | #1

    Good luck Ray.

    In our city, once you break Knob and Tube, the wire has to be pulled directly off the service panel. No splices in K&T.

    I have the stuff in my house, and I just made the cut, wrapped the wire in some romex scraps, and clamped it in a box with a romex clamp and then made the splice inside the box to romex. The box can be buried in the attic but must be accessible. This is the way I did it; I didn't dare get it inspected for fear I would have to rip it all out all the way to the main service panel. Ugh.

    Maybe some of the sparkies here will provide you with better advice that I did. Good luck, pal.

    Regards,

    Boris

    "Sir, I may be drunk, but you're crazy, and I'll be sober tomorrow" -- WC Fields, "Its a Gift" 1934

  2. davidmeiland | Feb 05, 2004 12:03am | #2

    My last house had knob and tube. My electrician tied one circuit into a metal box by running the wires through short pieces of a rubber-like bushing material he had that then went through a romex clamp. One wire, one jacket, one clamp. Two knob and tube conductors in, one 12/2 romex out, AND one green #12 ground out and back to the panel. The inspector looked at it and signed off.

    I have seen it done with a tube run through a romex clamp and into the box to sleeve the wire. Not sure if that's code or not, but it seems a little iffy with the brittle porcelain in the clamp.

    Easiest thing is always to call your inspector and ask what s/he'll accept. They're all different, except for one thing--they all take calls first thing in the morning and then leave for the field.

  3. 4Lorn2 | Feb 06, 2004 04:25am | #3

    The best, most sure and long lasting, solution is to rewire the whole house. Failing that replacing the most used circuits, usually kitchen, heating and baths, makes good sense. As mentioned by other posters a reasonable solution for K&T wiring is to replace the circuit.

    All of these are best done by an electrical contractor experienced in old work and rewiring. Old work is more an art. A different game from wiring a house during construction, new work. Different techniques, psychology and tools.

    You can easily spot old work electricians. Usually we are a bit taller, smarter, cleaner and better looking than the others. We are the ones with the big 'S' on our chests that the other electricians call 'sir'. We leave a trail of broken hearts and hallowed ground where our shadows fall. That's why we make the big money. Kidding of course.  

    We don't do anything an average person couldn't. Assuming the person is smart, handy and very careful. Most of all you need to do your homework. Posting on this forum is a good sign. Go to the local library and check out a few books on home wiring. Even if what you are trying to do has little to do with what the book is talking about you can pick up a feeling for the mindset and meticulous nature of electrical work.

    Always check your tools. A circuit checker that fails to tell you about a live circuit can get you killed. Check against a known live circuit. Be careful. Know if your grounded. Avoid being grounded if at all possible. Work circuits dead even if it takes longer. I frequently work circuits live but I have years of experience, training and no small amount of luck. Knowing the risks and stacking the odds in my favor doesn't hurt either.

    Remember you can't see, outrun or fight electricity. It can kill or maim in a fraction of a second. Let your fear feed your care. Do not become complacent. If any of this does NOT give you pause. Do NOT do your own electrical work. You are too confidant.

    If none of the above suggestions are going to happen your planning to cobble together a solution. It can be done.

    Be dead sure you have both sides of the same circuit, hot and neutral. Get this wrong and you can blow stuff up or overload the neutral. (You sure it wouldn't be better to get an electrician?)

    A hot and neutral set are usually run parallel about 16" apart. Sometimes more or less. If there is any remaining insulation you may be able to tell hot from neutral. Hot usually black cotton with brown woven in. Neutral off white with brown woven in. As the off white darkens with time and the brown lightens the insulation can be difficult to tell apart even under a strong light. By flashlight in an overheated attic with both covered in dust the difference is best detected through prayer and a Quiji board.

    Assuming the neutral connections are sound a 'Tick tracer' will tell hot from neutral. A ground lead, a long length of #12 copper run back to the panel, can give you a valuable reference. (If any of this sounds complicated refer back to the second paragraph.)

    Once you know what is going on hot and neutral try to find a set of knobs relatively close together. One on the hot run and another on the neutral run. Try to select ones in good condition that seem to be gripping the wires firmly. You may need to adjust the fastener holding the knob to get them to grip the wire. Tightening the screw or gently tapping in the nail. Knobs are typically porcelain so gentle but firm is the way.

    If any insulation remains on the wires you have to make a judgement call. After cutting the wires about 2' from each knob. (Did you remember to turn off the circuit and double check the wires against the reference before you touched them?)  You need enough slack for the wires to pass each other by about 10". Look carefully at the insulation. Is it dry, cracked, has holes. Bend the wire. Does the insulation bend smoothly? Or does it crack, maybe fall off? If it is anything less than pristine you need to reinsulate.  

    Grab your handy heat shrink tubing, the stuff that comes in length of about 3' and is about 3/16" diameter. Slip the tubing over the wires. If any remaining insulation is not too furry just go over it. If the insulation is furry it can give you problems getting the tubing on so just carefully strip the remaining insulation. You want the wires to be insulated all the way from the knob to well inside the box your going to mount.

    I use a propane torch to shrink the tubing, bring a spray bottle of water to put out the fire you start. Finding that you have forgotten the bottle after you have a fire burning and having to go back to the truck to get it is considered bad form. Good way to meet firemen and see the shiny red truck though. Be careful.

    I use a non-metallic box. My favorite is a Bakelite box with small plate type clamps that slip into place to keep the wire from backing out. Don't use a metal box as it can lead to shorts in this situation situated between two uninsulated live lines. The fact the box has to be grounded only makes things worse. Technically no clamp is needed if the knobs grip the wires well and are within 8" of the box measured along the wires. 

    Two inspectors, Florida, told me that there is no hard rule for how far to the knob. 8" is illustrative. Smoothly done 12" might not be too much. Idea being to keep it as small as practical. Avoid installing any staples or contacting any surface with the wires, even though they are reinsulated.

    K&T wires must always be assumed to be uninsulated and installation made in a manner that they do not depend on any insulation other than the porcelain knobs or tubes used to go through wood. Loom being a more modern addition to the system.

    This usually means you need to mount the box between the two knobs. Ideally you want to mount the box so the wires can enter opposite sides of the box without touching anything. A smooth 90 or 180 degree bend works well. I usually mount the box with a couple of screws, Piffins, drywall screws, work well. Pounding in nails above a ceiling old enough to have K&T wiring is just asking for trouble.

    Once in the box give yourself six inches of slack. Cut to length. Strip the ends and make up to the romex, or other wires if you went BX or conduit. You do remember which wire was neutal? A quick wrap with a piece of white tape is a good idea. The Romex needs to come in a hole not used by one of the wires from the K&T.  Use good quality wire nuts.

    Be sure to staple the Romex or clamp the BX or conduit so they don't get yanked out of the box. Install a cover on the box. Use a Sharpie to write the circuit number or identifier.

    Be sure to ground the romex. Don't use plumbing or any mechanical system as these can electrify these systems. A line back to the panel, made so much simpler if you replace the entire K&T circuit, (You sure you don't want to replace the whole circuit?) is the best remaining option. There is a way of using a GFI but you will have to search under 'GFI or GFCI and grounding' or post a second thread.

    Never, ever allow an attic with any K&T wiring to have insulation installed. Remember K&T wiring is assumed to not have any insulation, the system was designed before durable plastics were available. The cotton and rubber based insulation was expected to disintegrate in an attic. Crawling through an insulated attic with live conductors hidden in the insulation makes the snake-in-the-grass analogy come to life. If you don't electrocute a tradesman or two their boots are likely to snag wires and destroy the system.

    (Are you really sure hiring an electrician wouldn't be better?)

    1. alwaysoverbudget | Feb 06, 2004 08:07am | #4

      i wasn't the orig. poster, but was reading your desription on how to do this and i read  with intrest your comment on heat shrink tubing. i am always running across a cieling light that has had  3-150 watt bulbs screwed into it! first hint is the label that says max 60 watt is burned off :) . i usally remove the fixture to find the insulation is crisspy critters [this is "modren" romex,25- 40yrs old]. the way i repair the wires is to put 2 layers of heat shrink over them and install a new light. is this repair acceptable, or should i be cutting off a foot,put a box in,and pigtail  60c rated wire into the orig box?let me know what you think as this is one of my most common problems with my rentals [except for the one with alum. thats another story]thanks larry

      1. 4Lorn2 | Feb 07, 2004 08:20am | #7

        I run across these situations once in a while. If the insulation is still good at least an inch or so after the wires emerge from the the cable's jacket, the covering over all three conductors that provided mechanical protection, then heat shrink will serve.

        Sometimes the cable can be pulled in to allow you to strip back enough jacket to make this possible. The important point is that the jacket has to extend into the box. I usually use a single layer of a good quality heat shrink and overlap the factory insulation by a good inch.

        If the jacket won't extent into the box with some good factory insulation remaining then things get more complicated. If the run leading to the ceiling box can't be reasonably replaced a junction box will need to be added to accept the connections for a new piece of Romex.

        Whenever possible this box should be mounted. In a ceiling under another floor where there is only access to the space is by dropping the ceiling box and working through a 4" diameter hole or half a 4" hole if the ceiling box is a pan mounted on the joist, it is often a greater help to any electrician following to adding a 'throw away box'.

        This is a standard box made up, including cable clamps and cover, that is allowed to go unmounted. This allows the ceiling box to be dropped and hang down on slack cable so the added box can be worked. 

    2. User avater
      NickNukeEm | Feb 06, 2004 01:30pm | #5

      Excellent stuff.  You need to write an article for FHB on the subject, I'm sure there are plenty of remodeling contractors as well as DIYs who would appreciate it.

      Thanks.

      I never met a tool I didn't like!

    3. raynb3 | Feb 06, 2004 08:53pm | #6

      Thank you 4Lorn1 for your in depth knowledge and advice.  I intend to rewire the house when I have the $.  We do electrical on most of our landscape construction jobs, usually outlets, switches and timers for LV lighting, pumps, etc.  So I'm experienced with 120V, NM cable, and conduit.   I want to do this right the first time, so the inspector won't make me redo anything.

      A couple of quick questions:  the heat shrink tubing - will a heat gun work on it?  Hold on tight, the attic insulation is shredded wood fiber, probably redwood by the looks of it!    And if your pants are still clean -  all the K&T is buried!  So I'm really hesitant to use a torch..., but rewiring is definitely in the future. 

      Do I really need to ground a 5' section of NM when the K&T on both sides of this NM is ungrounded?

      Do know of a source for knobs?  This way I don't have to splice in 10' away at a knob and then deal with the branch circuits in between that knob and where I want the splicing to actually occur.  Are there alternatives to knobs as staples are out?

      And lastly, do you happen the NEC section code for K&T splicing?  If the inspector is unsure about the installation, I want to be able to tell him, "Look it up here."

      Thanks again.

      1. Matthew | Feb 07, 2004 03:20pm | #9

        Hello Raynb3,

        I have many extra knobs and tubes that I saved when I rewired my house. They looked too good to just throw out. If you need a few, let me know and I can send them your way.

      2. User avater
        BillHartmann | Feb 07, 2004 07:20pm | #10

        Yes a heat gun will work fine for shrinking the tube.

        But is works better if you have the attachment. It shaped like an semi open hand. Where the wrist is is open and slips onver the heat gun. And the "curled finger" direct the heat around all side of the tubing.

        They make make ones for propane torches also.

        The problem with a stright heat gun is that it does not get the back side unless you move it around a lot.

    4. User avater
      JeffBuck | Feb 07, 2004 11:41am | #8

      I skimmed thru ...

      all sounded good ...

      'cept I think ya forgot the most important thing ...

      K and T is perfectly safe ... as long as it's "fused" right ...

      as in ... make sure it's on a 15 amp breaker.

      20 ... and you have a very good chance of it over heating and burning the house down.

      Me ... I do play with electricity a bit ... Dad had me rewiring stuff under his guidance at about age 10 ... but I call my electrician when it comes to K and T.

      Too much to go wrong.

      Aside from putting the illegal splices I've found thru out my own home into boxes ... I don't touch the stuff.

      The only good advice for the original poster .. who has too many questions to do it safely .... is.....call in a pro.

      JeffBuck Construction   Pittsburgh,PA

           Artistry in Carpentry                

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