Re-wiring knob and tube

[JHZR2]

Hi,

My home has modern 200A service, all circuit breakers, and most all modern wiring. However, there are three circuits that are still knob and tube.

I am getting some work done, and there is a state cost-share because Ill do over 25% energy savings. However, the requirements are no oil tank (thus my other thread), and no knob and tube visible.

I know there is knob and tube in the walls for light fixtures, etc., and that stuff is not practical to me to replace. It is in good condition, including at the light fixtures and plugs that Ive looked at (and Ive taped/shrinkwrapped all of them for extra measure).

Any plugs that I can fish wires for, Ill rewire with romex. The boxes for plugs have a compression, screw down connector for the leads, which is very much like the romex strain relief things on modern boxes.

I guess what my question is, is what is the best way to deal with K&T in the basement. There are lots of the nice soldered connections, all in good shape. But this means that there are LOTS of junctions. How do I deal with those? Is it best to just do one "wye" per covered junction box, or can I do more if the wiring alignments allow?

Also, I plan to verify what goes where by running a known load, either a 300W halogen or a 400W resistive heater, through the plugs and then using a clamp-on ammeter to verify the correct amount of current flowing. Is this sound? Can I get a reading on both the hot and neutral side? If not, how do I verify which neutral is correct if two go up the same wall cavity?

Thanks in advance for any insight.

 

[duaneb9729]

They make an inductive "pen" looking thing, it lights up on the power side of your knob and tube wiring, and not on the neutral.

If I remember right, it set me back about $20 at home depot

I like your idea of using the clamp on amp meter to wire.

Sorry I dont have any advice for you on figuring out which two wires in a set of four go where.

 

[river_rat]

I'm not 100% sure I understand your question, but using a known load will help verify which wires feed a given outlet, and yes it will read amps on both hot and grounded circuit conductor (colloquially = neutral), but will not tell you which is which.

Like posted above, you can try a glow pen to determine which is grounded/neutral, but is some cases these fail to do so.
In that case you will need to take a voltage reading to a known ground.

Hope that helps.

 

[JHZR2]

OK thanks!

So really the other question is what is best practice for a junction box where wiring branches out? In the knob and tube system, there are very nice solder joints at each location where some "user" branches off. Since Ill be replacing with Romex, I would imagine it may be easier to do multiple junctions in one box, e.g. three "users" coming off of the feed line out of the breaker box in one location).

But is there a best practice? Obviously it is not practical to put plugs in series vice parallel off of one feeder that I can easily set up in the basement.

Thanks!

 

[tomcat27]

It's a judgement call whether you simply replace each existing junction with a box, or use one JB to feed a few runs. OK - your "series" idea is really best practice - but if its not practical to run your wiring in that manner... do whatever you want. the most that an inspector is likely to do is shake his head and make a few snide comments.

 

[JHZR2]

The K&T wiring was in parallel and has worked for the last 80 years or so anyway...

 

[Stu_Rock]

Originally Posted By: JHZR2

Any plugs that I can fish wires for, Ill rewire with romex. The boxes for plugs have a compression, screw down connector for the leads, which is very much like the romex strain relief things on modern boxes.

This is probably the correct thing to do when possible. Usually those clamps are labeled "N T" which indicates certification for "nonmetallic cable" (that is, Romex) and "tube." I'm not 100% sure the "T" stands for "tube," but it stands to reason, since it's already in service with K&T.

Originally Posted By: JZHR2
I guess what my question is, is what is the best way to deal with K&T in the basement. There are lots of the nice soldered connections, all in good shape. But this means that there are LOTS of junctions. How do I deal with those? Is it best to just do one "wye" per covered junction box, or can I do more if the wiring alignments allow?

You can supply as many branches as you want, provided that you follow the wire fill guidelines for that size of box. If you really want to have a lot of branches going off of one junction box, get a deep 4 11/16 box. I find it hard to come anywhere close to 100% fill on those.

Originally Posted By: JZHR2
Can I get a reading on both the hot and neutral side? If not, how do I verify which neutral is correct if two go up the same wall cavity?

This can be a rather subtle diagnosis process. I prefer direct measurements with multimeters (either in ohmmeter or voltmeter mode, depending on the situation).

Since you're going to be cutting exposed K&T anyway, I say cut one neutral and see what doesn't work. Do this with only one K&T circuit energized at a time in case there's a shared neutral between two of them. Use an incandescent lamp for testing and ensure that nothing else that could possibly be plugged into a K&T circuit is turned on (as a redundant check against false test results from a shared neutral situation).
"First, shut off all the K&T circuits and then cut one neutral. Then re-energize only one circuit at a time in case there's a shared neutral..."

 

[JHZR2]

Originally Posted By: Stu_Rock

Since you're going to be cutting exposed K&T anyway, I say cut one neutral and see what doesn't work. Do this with only one K&T circuit energized at a time in case there's a shared neutral between two of them. Use an incandescent lamp for testing and ensure that nothing else that could possibly be plugged into a K&T circuit is turned on (as a redundant check against false test results from a shared neutral situation).


In a paralleled circuit, even if all other circuits in my box are off, using a multimeter I could still hit a viable neutral. In fact, won't I always hit a viable neutral, even if from another circuit because the breaker is on the hot? I could hit another wire and still show 120V between the one hot and the other circuit's neutral.

 

[Stu_Rock]

Sorry--I wasn't clear before (just got back from a long conference/road trip).

For the cut method: Turn off all three K&T circuits to make the cut, then pick one to turn back on.

The clamp ammeter will nondestructively let you know which wire is carrying current. To see which wire goes to a receptacle, plug in a sufficient load there and find the neutral that has a nonzero reading. All the others will be zero. Fortunately, it sounds like you already know which is hot and which is neutral (I presume they used different color wires).

 

[Audi Junkie]

My plan is mostly complete. It is to rewire as many old outlets as possible w/Romex (all get new receptacles) and to add a few new boxes w/outlets with new direct wiring to the panel box and breaker, basically a new circuit. I really only want the existing K&T to serve ONLY the overhead lights and the wall switches.

Just stating what I'm doing and maybe that's applicable for you. I don't mind a new outlet box at the foot of the wall, some people might. That gives one heavy-duty outlet for the main equipment in the room. It's just important to find a good location.

 

[Stu_Rock]

Also, regarding the above: do not cut a neutral while its circuit is energized!

To restate one more time: I mean shut all K&T off, cut one K&T neutral, then re-energize only one K&T circuit at a time until you find your target receptacle not working.

 

[ddrumman2004]

My oldest son who is a licensed electrician helped me out in my wife's 140 year old house with knob & tube wiring.
Where a circuit branched off, we installed a metal junction box and branched off from there.

All of this wiring is run in the attic and since the house has no insulation in the walls of any kind, it was easy to replace the wiring in the outside, as well as the inside walls.

We still have quite a bit to replace when funds allow.

 

[JHZR2]

Well, I guess the branching best practice is the major question. What I was thinking is aligning the old K&T circuits on 15A breakers, since some of it may feed lights where I cannot replace the wiring. If possible, I might actually leave the remaining K&T on its own breaker feeding lights, etc., and put the other "user" plugs, etc on their own circuits. But here is my thought:

I would run a 12/3 line off of the breaker, following the trace of the main wire from the K&T. I would then run 14/3 wires from this via wire nuts and junction boxes to the users. Where I broke off, I would cut the 12/3 wire, and then connect two x 12/3 wire ends and as many 14/3 wires as made sense to feed the users, combined with a wire nut. So the main run that everything feeds from is a 12 ga and the users are all on 14ga, protected by a 15A breaker.

In locations where I needed to combine with old K&T, I would keep the 12Ga ground wire intact, and just combine the old K&T like with the cut 12Ga hot and neutral via wire nuts in its own junction box (plastic) with the K&T well-covered with a combination of liquid and physical electrical tape.

Does this make sense?

 

[Stu_Rock]

The plan sounds good. I have just a couple comments:

I want to ask if you're talking about "12/2 with ground" and "14/2 with ground" NM cables. The /3 have four wires--black, red, white, bare. If you do actually need both hots, it sounds like a shared neutral or split-feed setup, which requires a little more planning.

If funds allow, use all new 12 gauge wire and 20 amp breakers for the circuits with all new wire. You may have to use AFCIs on those circuits if the job is subject to inspection.

For the junction boxes where you go from NM to K&T, you should have wire loom covering the wires from 2 inches inside the box all the way to the first supporting knob or ceramic tube (just like they did in the old days). You can salvage old loom from your tear-out, but if it's too short or disintegrating, get new loom. Finding new loom is very difficult. Fortunately McMaster now has a modern substitute--it's SKU is 7408K49.

 

[JHZR2]

you think that 0ga stuff is the right choice to go over the fabric insulated wire? I guess you cant really be too big...

 

[Stu_Rock]

It's for a 0 gauge bare wire. Accounting for the insulation on 12 or 14 gauge, it's the right size. I have used it--it's good stuff.

 

[JHZR2]

ok, thats what I thought.

 

[JHZR2]

Well the wife and I got part-way through one of the three original K&T circuits in the house working for a few hours on sat and sunday. Amazing how much stuff was on this one circuit. A number of illegal splices into BX and romex too. Mapping the circuits is the toughest thing, and then re-mapping the next hardest. We used a resistive heater and a current clamp in a bunch of places to verify what was what. Worked great. Found a few bad cuts in wires where there was a splice removed. Found one wire showing 120V that we would not have expected... dangerous.

We are converting the one circuit into three circuits, one for the front walls of the house, one for the rear walls of the house, and then one that will just rewire the light circuits that go up in the walls as best as I can (leaving k&t in the ceiling), and will use an arc flash breaker in the box vice a regular breaker.

Fortunately romex fits nicely through the old tube holes and rewiring first-story plugs is fairly easy...