How can I tell if an old Knob & Tube 3-way switch is still legal by the modern NEC? Specifically I am trying to tell if my switch is a "California 3-way" (or "Coast 3-way", "Hollywood 3-way", etc.) or if it is a Carter 3-way (see Wikipedia's descriptions here).

My house has a bit of Knob & Tube still in use (maybe 25%). As part of my bathroom project I have some "extra" access to walls I don't normally have open and easy access down to the basement. While this is open, should I go ahead and replace a 3-way switch.

One reason I don't really want to do this is I am already moving more and more outlets off of these circuits so I am less concerned about over loading. But mostly I don't want to because this 3-way controls 2 lights in my stairwell (one on the landing and one on the second floor) with a switch down on the 1st floor and the other on the 2nd floor. Because of this it would be pretty destructive to get to all of these locations. On the other hand, if it is really dangerous (i.e., the "Carter System") I might just go ahead and do it.

Update: My switch is not a "Carter 3-way". Which I guess is good, since these have been "prohibited by the National Electrical Code since 1923" (according to Wikipedia) and my house was built in 1926. My Knob & Tube is also copper so that is good since I am pretty sure it is connected to standard "copper compatible" receptacles and switches. For now this is staying in the house as one of the last 2 circuits that is still K&T.


Take a look at the diagrams in the Wikipedia article you linked to. These will tell you what you're dealing with. There are some big red flags that are easy to look for:

  1. First, kill the power to this circuit at the panel. This will allow you to poke around in the wiring safely. Use a non-contact voltage tester to make absolutely sure there's nothing live in the box before you start work.

  2. Now, examine the switch that is getting the "hot" lead from the panel. This may be hard to identify, especially with K&T wiring; I would recommend labeling where each wire to the first switch is connected, and then disconnecting and separating all wires from the switch. Then, turn the power back on and use a non-contact voltage tester to identify the "hot" wire. It will also be helpful to identify the "neutral"; it will be the wire between the panel and this box that completes the circuit but does NOT carry voltage when disconnected.

  3. With the power to the circuit back off, and the hot and neutral identified, look at where each wire connects. If the neutral wire coming from the panel is connected to any terminal on this switch, you have an illegal circuit, whether it's Carter (the hot would also be connected to the switch) or somebody just swapped hot and neutral while messing with it (the hot would then be "hard-wired" elsewhere in the box). This is unsafe, because of the "switched neutral"; if the switches are positioned such that the neutral is disconnected to break the circuit but the hot still has continuity, even though the light is off, the socket is still energized from the "hot" side and can give you a nasty or fatal shock of you touch it.

  4. Pretty much any other system that could have been used is "safe", as long as the neutral is never the side being disconnected (or the design of the switch guarantees that BOTH sides are disconnected simultaneously, such as DPDT switches; these are common in electronics but rare in home wiring).

  5. If the hot was connected to a terminal labelled on the switch as "Common", or somehow identifiable as such, and the neutral is not connected to the switch at all, you likely have Traveller-system wiring, which is perfectly safe and has been the best practice since "single-conduit" wire like the current Romex wire became available.

  6. If the hot side was connected to a terminal other than the common terminal, there is another wire coming from that same terminal going to the second switch, and the neutral is not connected to the switch in any way, that's California-system. As long as both switches are wired to properly follow the same system (understand this is an older system rarely used anymore, and may confuse even a competent electrical pro), it's fine to keep using.

Understand that this is all rather moot according to most modern building codes. It is illegal, under any circumstances, to:

  • Install new knob & tube wiring systems; K&T hasn't been code-compliant since the '40s.
  • Re-route existing knob & tube; even if you're using pre-existing wires, moving them to a new place counts as "new work", usually requiring a permit and inspections which will never pass if they involve K&T.
  • Add new wire runs to a knob & tube system; You can't add K&T because of the prior points, and you can't join the current Romex wiring to K&T because it's a fire hazard in itself and because it's misleading to anyone who's looking at the endpoint; they'll assume the whole circuit's Romex, join to it, overload the circuit and burn the house down.
  • Run more than 10A through K&T. K&T is quite safe from a fire hazard perspective if you keep it under 10A. However, you can't run much on 10A; you'll blow a fuse with the average modern vacuum cleaner. Fuses are interchangeable enough that it seems like a no-brainer to simply put in a larger fuse like 20A; this is how K&T wiring causes house fires (and the reason neither K&T nor fuses have been acceptable by code since better stuff became available).
  • Use copper-rated electrical nodes (switches and outlets) with aluminum wire of any type including K&T; quite a bit of K&T is aluminum, which has higher electrical resistance and thus gets hotter for the same amperage than copper or copper-clad wire, especially at junction points like screw terminals. Cu-only switches/outlets are not rated to withstand this heat; you have to pay extra for Al-rated stuff.

Given all the above, if you're doing anything more than an in-place replacement of a light switch, you could be in for an expensive renovation. If you are moving any part of the K&T circuit, it loses the grandfathered status, meaning the whole circuit has to be taken out back to the panel and replaced with Romex. That may also require the panel be brought up to code (I can't imagine you having K&T and a modern breaker box); if you need to do that, you'll likely invalidate ALL the K&T in the house and have to rewire the whole thing.

For peace of mind, if it were my house, I would contact either my home insurer, a trusted local licensed electrician, or the county clerk, and get the straight dope on where the upgrades would have to stop in certain situations. If messing with the wiring is going to result in you needing a full house rewire, DON'T TOUCH THE WIRING unless you have the scratch to take it all out, even if it's illegal; it's not your fault it's illegal until you change something.

  • 2
    Excellent answer, good detail, sound advise. I'd give you more than one up vote if I could. K&T although grandfathered in most areas is ancient, unsafe in my opinion and worth every penny to replace entirely. If anyone values living and keeping their home from turning into charcoal, replace k&t, and especially Carter 3-ways. When I inspect houses and see this situation, it heads up the report as an urgent safety hazard as 95% of the time, sections of wires are bare of insulation, dangerously close to plumbing, and accessible by unknowing hands. Spend the bucks and get rid of it. – shirlock homes Jul 21 '11 at 21:49
  • Thanks for the great answer. My city inspectors are fine with tying into existing K&T as long as it is a legal junction (inside a box) but I can certainly see that someone might see a romex wire coming out of a wall and assume that whole circuit is romex. I had not realized the wires coult actually be aluminum, is there any way to figure that out? – auujay Jul 21 '11 at 22:39
  • If the wire appears silver, it's aluminum. Pretty easy there. As for removing K&T, I wholeheartedly agree, but a full-house rewire back to (and including) the panel is an expensive job that will require cutting and drilling holes and fishing several hundred feet of new wire. $6000-8000 is a highly optimistic estimate. – KeithS Jul 22 '11 at 14:27
  • I must take exception with at least two points.As a registered electrician of 30+ years, I can't recall ever seeing aluminum K+T wiring. It is ALWAYS copper. The reason it is silver is because they used to anodize the conductors with a derivative of lead(pb)before covering with the cloth insulation. This served two purposes-first, protection from corrosion, and second, it allowed for easier removal of the jacket by the electrician. If you look closely at the cut end of any silver K+T conductor, you will see the color of the copper. This means that K+T in 14 gauge IS suitable for 15 amps not 10 – user52222 Apr 2 '16 at 16:15
  • Of course there are many reasons not to use K+T wiring-most notably the lack of an equipment grounding conductor and the overall age and condition of most K+T wiring which is ALL now approaching 100 years old! FYI --SAH Electric – user52222 Apr 2 '16 at 16:15

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