Replacing knob and tube wiring

[katwalatapan]

Hello,

While conducting minor renovations to a vacant group home, knob and tube wiring was discovered in the ceiling. It is definitely a possibility that other circuits in the group home would also be knob and tube type.

As per Canadian or US Electrical Codes, would it be required to remove only the discovered part of the knob and tube and replace with standard wiring method or would the code mandate replacement of all of the knob and tube type wiring.

On a safety perspective, all of the knob and tube wiring (discovered and undiscovered) should be replaced. I suppose the AHJ would also have a similar requirement. But is there an acceptable and safe scenario, where only the discovered part of knob and tube wiring and circuit is replaced and the remaining part of knob and tube wiring is inspected for insulation deterioration and left in place to be replaced during a major renovation project. Is wiring permit usually approved on partial replacement of knob and tube wiring? Also is there a possibility of safety hazard when the two different type of wiring are present at the same time i.e. knob and tube and standard conduit?

Thank you.

 

[waross]

Knob and tube wiring was abandoned more for economic reasons rather than safety reasons. With the approval and availability of NMD type cable (Non-Metalic, Dry) trade named Loomex or Romex) the high labour cost of installing knob and tube wiring caused it to be phased out.
The existing wiring has probably been in service for 60 years or more. Not a bad safety record.
One issue with knob and tube is the lack of a grounding conductor.
However GFI devices depend on sensing the imbalance between the line and the neutral conductors and provide excellent ground fault protection without a grounding conductor.
But, the bottom line:
Discuss this with the AHJ. He has the last say and if he requires changes before he signs off on the structure, there may be insurance coverage implications if the work is not completed to the satisfaction of the AHJ.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[racookpe1978]

Usually, a remodel or renovation that affects only a part of the building is not looked at (very deliberately!) by the inspector checking off the new parts and wiring.

But, this old wiring is so far out-of-spec with today's requirements and safety that (indeed) the only reason it could be considered "usable" or "adequate" now is that "Well, it hasn't failed. Yet." (In Vallejo, CA, I did see bare knob and tube wiring in one house they wanted to sell to us in the mid-80's. Just looking at that was enough for me to immediately walk out of that house and away from that real estate agent, so, you may want to consider replacing it on that re-sale and price reason alone.)

Seriously, unless you absolutely need to maintain the house for historic purposes with that wiring, you need to replace the wiring functionally. And even historic wiring displays could be left as-was, with the new working (energized) wires run out of sight separately. Don't pull the old wires unless you want to salvage the copper. You will find the lights are too few, the number of wall sockets too few, the rating too low, the grounding unsat, the sockets all two-prong, the basic functions for stoves, microwaves, freezers and refrigerators, TV's, computers, etc too low-rated for today's appliances.

 

[bacon4life]

A big part of the danger comes when insulation is added. Knob and tube was designed for free air movement and it can easily overhead when buried in insulation. It is unsafe to ignore "undiscovered" knob and tube wiring if the building has had insulation retrofits. It would also be prudent to verify that the fuses or breakers are properly sized and have not been changed to a higher size by a resident frustrated over a lack of capacity from the knob and tube system.

The NEC does not require replacement of knob and tube. Article 394 limits the use of knob and tube to extensions of existing installations in unfinished attics, roof spaces and ceilings, and does not allow Knob and tube where insulation could envelop the conductors.

 

[waross]

Good point on the insulation, bacon4life.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

When the rubber hits the road, the real issue will be that the AHJ cannot force you to replace systems in use that are untouched, but they may disallow any connection to a system that does not meet current code. So if your K&T wiring is in something that is not going to be disturbed, he will have no jurisdiction over it, it is what is commonly called "Grandfathered". But if, for example, you are adding an outlet to a room in which all of the rest of the wiring is K&T, the AHJ may force you to re-wire that ENTIRE CIRCUIT to the current code, even if part of it gets into another room. Then because a lot of old original K&T wired houses used a scheme in which several "hot" lines may have shared a common neutral return path, you end up having to rip and replace almost everything anyway. A lot of it has to do with the local AHJ and whether or not they allow any connection / extension of old existing wiring. A LOT of that will have to do with the grounding issue and the insulation issue as well. I remodeled a house in Seattle that was built in 1906, WITHOUT electric lighting, then they replaced the gas lamps with electric in the '20s by using the gas pipes as electrical chases for K&T wiring, it was a real mess. But I called for an inspection prior to starting the work, which the inspector initially refused to do because he said he was not an advice source, but I explained what I INTENDED to do and he came out to check the feasibility. So basically he allowed me to leave the existing K&T wiring dropping down from the ceiling into older wall sconce light fixtures, because they did not "need" a ground wire. But I had to cut it off long, extend it above the blown-in insulation to a junction box on a stanchion, then re-connect to anything else using modern systems. Everything else was rip and replace, mostly because all outlets and wall switches needed to be grounded, which they were not.

This might give you some insight.

http://inspectapedia.com/electric/Knob_and_Tube_Wiring.htm

"Will work for (the memory of) salami"

 

[Mbrooke]

A GFCI does not need an equipment ground to function. The imbalance does not have to go through an EGC, it can be anything else grounded. Granted a 3 prong tester will not trip it (your supposed to use the button anyway)but it will still protect people. Code allows for a GFCI to take the place of a non grounded outlet provided it be labeled "no equipment ground".

 

[stevenal]

Article 394 of the NEC deals with the subject, and it doesn't say to rip it all out. In fact it provides great installation detail, so I wouldn't say it isn't code compliant. It does speak of the insulating material issue.

I had this stuff in my old house, and was impressed by the workmanship involved. Every connection was carefully twisted, soldered and wrapped. Definitely installed to last.

 

[Mbrooke]

What gauge wire was used in knobe and tube? If it was 14 guage wire fused at 15 amps like modern NM-B, I would think insulation wouldn't matter in terms of over heating? Even in insulation modern NM that is 2 conductors under a jacket gets only warm under full current.

 

[SceneryDriver]

I believe that many homeowners' insurance policies will not cover you if there's K&T wiring present and in use. There may be a provision for continued use during a remodel, but only until it can be replaced. I can imagine a similar policy existing for insurance on a group home. K&T is usually fine until it is disturbed; insulation of that age and makeup is very brittle and any bump (or passing rodent) risks damaging it.

SceneryDriver

 

[ameyerrenke]

Mbrooke, the main concern with overheating on K&T is with the neutral wire getting too hot because an entire house would be run back to 1 neutral wire.

In my area I've not had any problem getting homes with K&T insured, may be different for a group home, idk.