knob and tube question

Discussion in 'Electrical and Wiring' started by mikejurasw, Jun 12, 2019.

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  1. Jun 12, 2019 #1

    mikejurasw

    mikejurasw

    mikejurasw

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    I have a bathroom with a wall switch that only governs an exhaust fan at the moment. I would like to add a modest over-vanity light fixture to the circuit so both fan and lighting come on when switch is thrown. The wiring is easily accessible, but is knob and tube. Can this be done?
    Thanks,
    MM
     
  2. Jun 12, 2019 #2

    Snoonyb

    Snoonyb

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    Sure.
     
  3. Jun 12, 2019 #3

    JoeD

    JoeD

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    No. You are not allowed to alter or add to an ungrounded circuit.
     
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  4. Jun 12, 2019 #4

    Michael Armstrong

    Michael Armstrong

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    The sections of the NEC dealing with extending an ungrounded branch circuit seem to assume you're extending the circuit to add additional receptacles (which must have an EGC (Equipment Grounding Conductor) terminal, and you must run an EGC in the cable extending the circuit. That EGC may be connected to several sources of ground:

    250.130(C) Nongrounding Receptacle Replacement or Branch Circuit Extensions. The equipment grounding conductor of a grounding-type receptacle or a branch-circuit extension shall be permitted to be connected to any of the following:

    (1) Any accessible point on the grounding electrode system as described in 250.50
    (2) Any accessible point on the grounding electrode conductor
    (3) The equipment grounding terminal bar within the enclosure where the branch circuit for the receptacle or branch circuit originates
    (4) An equipment grounding conductor that is part of another branch circuit that originates from the enclosure where the branch circuit for the receptacle or branch circuit originates
    (5) For grounded systems, the grounded service conductor within the service equipment enclosure
    (6) For ungrounded systems, the grounding terminal bar within the service equipment enclosure.

    So there may be some wiggle room here. First, there's the argument that you're not extending the circuit, but adding a new fixture (the lamp), in parallel with the existing fixture (the fan). There may be no grounding means in either the fan or the lamp, and therefore no EGC required, particularly if they're housed in nonmetallic enclosures. If that doesn't fly, there are lots of alternatives to actually bringing a compliant EGC to the switchbox or fan, and then to the lamp. I would probably just ignore all this and run a piece of 14-2 from the fan to the lamp. I'd love to know the correct interpretation of the NEC in this case, however, and especially if one of those options were available to me, I'd call or visit my electrical inspector, and see what he has to say. Whatever he says, of course, is "the correct interpretation of the NEC".

    In the very worst (and also very best, in terms of safety and compliance) case, you'd have to run a new, grounded, branch circuit from the service panel to the switch and both fixtures. It sounds like this might not be too difficult in your case.
     
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  5. Jun 12, 2019 #5

    nealtw

    nealtw

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    What does it say about adding to knob and tube.
     
  6. Jun 12, 2019 #6

    Michael Armstrong

    Michael Armstrong

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    Quite a bit, actually; start with Article 394 and go from there. There are many discussions in sparky forums about knob & tube, and it gets very deep very quickly. But K&T is currently recognized as a legitimate wiring method, it can be added to, and it will even support GFCI protection. But most experts will advise replacing a K&T circuit with modern wiring wherever possible, and don't mix the 2 in a single circuit. Easiest way to follow it would be to get a copy of the NEC (pricey), and curl up in your favorite chair and start reading.
     
  7. Jun 13, 2019 #7

    hornetd

    hornetd

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    "300.16 Raceway or Cable to Open or Concealed Wiring.

    (A) Box, Conduit Body, or Fitting. A box, conduit body, or terminal fitting having a separately bushed hole for each conductor shall be used wherever a change is made from conduit, electrical metallic tubing, electrical nonmetallic tubing, nonmetallic-sheathed cable, Type AC cable, Type MC cable, or mineral-insulated, metal-sheathed cable and surface raceway wiring to open wiring or to concealed knob-and-tube wiring. A fitting used for this purpose shall contain no taps or splices and shall not be used at luminaire outlets. A conduit body used for this purpose shall contain no taps or splices, unless it complies with 314.16(C)(2)."

    Install a box into which to terminate the new cable, leaving enough wire stripped to extend from the holes in the other side of the box to an insulator on each knob and tube conductor. Mount that box about 1 foot away from a set of knob insulators. Using a wood block or similar fulcrum and a pair of diagonal wire cutters
    [​IMG]
    pull the nail that secures the two halves of that insulator to the framing out enough that you can fit the wire from the new cable into the unused slot on the insulator. Place loom over the wire from were it starts in the box through the holes by which each conductor leaves the box all the way to the insulators that you will add the new wire to. Since woven fabric loom is no longer available You will need to use flexible plastic tubing for the loom. The split type of plastic loom is not acceptable because it is too easy to disturb and accidentally remove from the wire. The flexible plastic tubing provides physical protection to the wires. It is not insulation and does not need to be listed by a testing laboratory. Use a size of tubing that is just large enough to get the individual wires through and not something an inch or more in interior diameter. Secure both the original wire and the new wire in the insulator by gently tapping the nail through the middle of the insulator until it is snug. If you strike the nail too hard the insulator will shatter. Scrape a small length of insulation off of the knob and tube wires ~4 inches past the insulator. Cut the two wires from the box so that they are just long enough to go through the insulator slots and reach the bare spot on the original knob and tube wire. Strip the insulation off the 2 new wires about 1 inch from each end. Connect the two new wires to the bare spots on the knob and tube wiring with split bolts sized for the conductors. The split bolts should have a size range that has the size of the wires being used which is as close the the largest wire that the split bolt is marked for as practicable. Use a side cutting plier
    [​IMG]
    to hold the bolt head still and tighten the nut with a nut driver.
    [​IMG]
    The joint must be snug but not so tight as to cut into the wires. Tape the split bolt with 3 layers of plastic electrical tape. Find a way to run your new Equipment Grounding (Bonding) Conductor (EGC) to the nearest junction box on a circuit that has an EGC. Run that new EGC into the box where the added on length of nonmetallic sheathed cable comes off of the knob and tube wiring.
     
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  8. Jun 13, 2019 #8

    hornetd

    hornetd

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    If there are only two wires coming into the switch box, as seems likely to me, make no attempt to connect your new load to that box as it does not have a neutral conductor in it. Set the new box, the transition box to knob and tube wiring as described above, then run the new cable to the second new box that you will set for the light. If the existing fan has an unused knockout available for the Non Metallic Sheathed Cable (NMC) to connect to it will be far easier to begin the NMC run from there then to go through the steps of connecting to knob and tube wiring.

    --
    Tom Horne
     
  9. Jun 21, 2019 at 2:10 PM #9

    mikejurasw

    mikejurasw

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  10. Jun 22, 2019 at 5:16 PM #10

    raymond-

    raymond-

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    i'm more of a visual, car mechanic, computer geek but I have similar needs in my 1950 home. I will
    study the suggestions closely and hope it clarifies what I need to do and remain safe and to code.
    thanks to those more experienced for adding their comments above. r-
     
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