a single 3-way switch controlling a light fixture on a knob & tube circuit - why?

Discussion in 'Electrical Forum discussion & Blog' started by J Mike, Jun 12, 2014.

  1. J Mike

    J Mike New Member

    Messages:
    3
    Location:
    NY
    I am in the process of replacing all the K&T wiring in my house. There is one light fixture that is controlled with a 3-way switch. There is no second switch. All of the terminal screws on the switch have wires to them (except ground screw). I have heard that neutral switching was common with K&T. Is that what is going on here, and why a 3-way switch? Thanks.
  2. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,825
    Location:
    Cave Creek, Arizona
    They could NOT have hot and neutral wires on the same switch. Either there is another 3 way somewhere, or the extra wire is "dead", or you have a light, somewhere, that turns on when you turn the 3 way "off".
  3. J Mike

    J Mike New Member

    Messages:
    3
    Location:
    NY

    I did not say that there were hot and neutral on the same switch. And I don't think the extra wire is dead - I did a quick check with a pen volt tester. I am 100% sure that there is no other switch or light, having lived in the house for 13 years. This is why I am wondering how this thing works. Thanks for any advice.
  4. LLigetfa

    LLigetfa DIYer, not in the trades

    Messages:
    4,055
    Location:
    NW Ontario, Canada
    It does not make any sense. Have you actually confirmed that it is a 3-way and not a double-pole?
  5. J Mike

    J Mike New Member

    Messages:
    3
    Location:
    NY
    Yes, it is a 3-way. There are 3 wires in the switch box, all of which are connected to the switch. two come from the floor below the switch. both of these register voltage with the volt pen. the third wire continues up the wall, into the attic, then over the the light fixture. the other wire coming off the light fixture goes to another single k&t wire running through the attic.
    Last edited: Jun 12, 2014
  6. JustAHomeOwner

    JustAHomeOwner Member

    Messages:
    41
    Location:
    NY
    A 3-way switch has one screw that's either a hot leg or a switch leg depending on where the feed is. The other 2 screws are the travelers that would normally connect to the travelers on the second 3-way. If you have one 3-way and the normal screw to the feed is always hot: with each alternating throw of the switch, one of the travelers becomes hot and the other not. When the light is off, something else is becoming hot. Where's the wire go that doesn't control the light? If it's dead ended at the light fixture then a single pole switch would work instead. Just wire nut the unused wire on both ends.
  7. ActionDave

    ActionDave Electrician

    Messages:
    351
    Location:
    Colorado
    Your problem is a "volt pen" is going to give you some false indications of juice on a particular wire when you are dealing with three way switches.
  8. JustAHomeOwner

    JustAHomeOwner Member

    Messages:
    41
    Location:
    NY
    I've done lots of 3 and 4 way switch installs, never owned or used a volt pen but I'm curious why you said that. Don't they work as intended?
  9. ActionDave

    ActionDave Electrician

    Messages:
    351
    Location:
    Colorado
    They do work as intended. They use capacitance as a way to check for voltage so they can give false positives in some cases.

    What is capacitance you ask? Well that is when something does something to the wire, electricity, and insulation in a circuit. That is all I know about that. I do know that volt pens are a quick and dirty way to check for power but they have their limitations.
  10. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,825
    Location:
    Cave Creek, Arizona
    From your description, the two wires from the floor are both hot, (one a feed and the other going downstream to another device), and the switched wire goes to the light. The two terminals are probably "bridged" with a jumper.
  11. PatrickH

    PatrickH New Member

    Messages:
    17
    Location:
    Virginia
    Wow! Knob and Tube! Brings back horrible memories! You mentioned that the two wires coming from the bottom of the box are hot. Did you disconnect them from the switch to read them. If it's a three way, the hot wire will feed one leg of the switch in one position, and the other leg of the switch in the other. The hot is typically attached to the odd colored terminal of the switch. Given your house is really old! (knob and tube), you could have a switch covered over somewhere that feeds the light in question. You should try to determine where this is, and eliminate it, by tying the light back to the controlling switch.

    If the switch were switching a neutral then it would have to be double pole, double throw, not a three way.

    Good luck with your old house, and be careful for asbestos while changing things or working in crawl spaces. Breathing is important!
Similar Threads: single 3-way
Forum Title Date
Electrical Forum discussion & Blog Can I use a (mistakenly purchased) 3-way switch as a single pole switch? May 21, 2012
Electrical Forum discussion & Blog single 'master' 3-way? switch at top of the stairs controlling 2+ zones downstairs? Dec 17, 2010
Electrical Forum discussion & Blog Converting Two Recessed Lights on a Single Switched Leg Jun 16, 2014
Electrical Forum discussion & Blog Can I use 2 single pole 30 amp pushmatic breakers as a double pole set up May 28, 2014
Electrical Forum discussion & Blog Multiple lights/switches on single circuit. Aug 30, 2013

Share This Page