Safety issues with knob and tube wiring

I have an old house, built in 1920, which still has a lot of old “knob and tube” wiring left in certain parts of it. Ultimately, I would like to spend some money and have it all ripped out and replaced with modern wiring, but in the meantime, I am curious about just how sketchy the stuff is. In particular:

  1. I am thinking about installing a thermostat in the hall. This will require me to fish a wire through a wall which shouldn’t have any wiring in it, but I can’t be 100% sure without ripping it open. I have tried circuit finders and they don’t work very well through the lathe and plaster. Is it safe to run a metal fishtape through a wall that may have 80 year old K&T in it? Obviously I can cut the power while I’m doing the fishing, but I also wonder if the wiring is easily damaged.

  2. in the attic, there are places where the wiring is exposed and looks very brittle. Is this dangerous? If I were to go up there and put in fiberglas bats for insulation, do I need to take any extra precautions to keep from disturbing the wiring? Should I turn off the breaker if there is any chance of me touching it? Should I turn off the breaker and also make sure not to touch the wiring because it’s fragile? I am very cautious around it because it looks like it could break if you breathe on it wrong, and the insulation on it is all cracked and doesn’t look very … insulating. Am I being silly?

You’re not being silly at all.

First of all, we can definitively say that you can not put more attic insulation over the KT wiring. Since 1987, the National Electrical Code has forbidden this. The two main hazards are that the wiring can overheat to the point of starting a fire, and that insulation may get damp and short-circuit the wiring.

As for running wiring for a thermostat - yes, shutting off the power would be prudent before sticking a steel fish tape into the wall if you’re not sure that there’s no KT wiring in there. At the very least, you could hit a bare spot and get shocked. As you suspect, KT wiring is very easily damaged. One alternate to fishing wires and worrying about what’s inside is a wireless thermostat such as this one. Not cheap, but sometimes it may be a better option for retrofitting an old building instead of drilling, fishing and cussing.

Generally, if it’s left alone, KT wiring is safe, provided it was done right in the first place. If it’s lasted this long, it probably was done right. But now, it’s really fragile stuff and the sooner you can get rid of it all, the better.

I’m not considering putting insulation over the wiring, just mostly concerned about bumping the nearby wiring while I’m shoving the bats around. Sounds like a reasonable concern.

On the subject of a rewire… When doing wiring replacement, how exactly do you know when you’ve got it all? Much of the wiring goes through spaces that are easily accessible and/or mapped, but I have run across some spots where it makes some really unexpected turns, so I can’t be sure it doesn’t do something like that inside a wall. I’d hate to go to all the effort only to miss a chunk on a leg I didn’t even know existed. I would think the insurance company, who I’d definitely want to contact for my new K&T-free rates, would want some kind of assurance that it’s all gone, right?

Just remember the ‘insulation’ on KT is not very ‘insulating’ so it does not contain the heat generated by the copper the way moderns wires do and will not protect you from being shocked.

For rewiring the path the KT runs is largely irrelevant. The new wires can’t typically follow the same path(KT goes through joists, new wires are run under joists) When you rewire you provide new power to all the points previously powered by the KT. The insurance companies are kinda lax on checking out whether the KT is replaced. Most home owners just end up faxing a copy of the bill for the rewiring and they are good with that. The old KT and tube is cut out to the best of the electricians ability and abandoned in a manner that prevents it from being usable in the future.

Ten years ago I did a rewire from knob and tube in the house I owned back then. It was a fun project with the old man when he was still around.

We did what the previous poster suggested. It is impossible to track the path of the old circuits, so you just start with new. Run a fresh circuit from the box and start attaching the lights/outlets that are currently serviced by the knob and tube. Usually you luck out because the old knob and tube didn’t run to too many outlets. Back when it was popular you didn’t have microwaves, computers, TVs, etc. and it was mainly only for electric lighting. You didn’t have the modern setup with an outlet every four feet along the wall.

In my case, it fed the overhead lights and one outlet in each room. Modern wiring had supplied the other outlets. Best of luck

If it’s practical, I’d suggest that when you have the K&T disconnected you leave it all in place. It’ll make the house that much more interesting for future owners. :slight_smile:

yes the K&T can be damaged by fish tape. air as insulation is required to make it safe, you don’t know that you wouldn’t bend it or dislodge it from a knob, knock a knob loose or cause some building material to fall against it where it is bare or insulation damaged.

consider a wireless (radio wave connection) thermostat.

You should TURN OFF the breakers anytime you are doing ANYTHING with wiring. AND you need to BE SURE you are ACTUALLY turning off the power to the parts you are messing with. IF you CANT be sure, turn off the power to the whole house.

Mini safety freakout over.

the insulation is very brittle, if undisturbed and in place it can still remain safe barring rodents and building damage (rain leaks and so on). i would turn the breaker/fuse off, it is easy to turn around and bump into. just brushing it lightly, or vibrations on wood structure that normally doesn’t move much (stepping on the ceiling joists) can cause brittle cloth and rubber insulation to fall off. use good lighting and know well where it all is before hand.

you can abandon old K&T in place and route new wires through the walls when you want to replace it.

If I may expand the mini safety freakout just a hair: particularly on a house that old, you need to make sure “turning off the power to the whole house” actually turns off the power to the whole house. People have a nasty habit of neglecting to keep labels current, such as the genius at my father’s old place who jumped the innards of a switchbox and then painted it closed. The switch still moved, but it did nothing to interrupt the power.

The OP pretty much described my house - built in 1920, plaster and lathe, K&T wiring.

For a newcomer to rewiring, how difficult is this to do on my own? Say I take a weekend, cut the power in the morning, and start running new lines down to the breaker. What would I be in for?

Are there any books/websites you’d recommend to really read up on this before I begin?

You’re in for a **lot **of work. Also, if you don’t draw up plans and get a permit from your city or county, you’ll be in for a lot of grief. Different jurisdictions have different rules on whether or not this kind of work can be done by a layman or if a licensed electrician must do it.

I really wouldn’t recommend rewiring a house and upgrading the service entrance and main breaker panel as a first project. If youre new to wiring, this job is best fixed with the checkbook tool - hire an electrician who will know the various code requirements. You don’t want to get through it all then have the inspector come in and ding you for subtleties like not enough outlets in the kitchen, or not using arc-fault breakers on bedroom circuits, etc.

you are allowed as a home owner to repair old electrical with like. so you can replace receptacles, switches and light fixtures for a 2 wire system as long as the K&T wiring holds up. once you start to upgrade then all that circuit has to be brought up to electrical code.

for a big job like that i would recommend an electrician do it. you would want to plan a new circuit map for your house and bright it up to current electrical code. knowing the electrical code for residential and safe wiring practice (there are unique concerns to doing a rewiring) is no small deal, also doing so with minimal disruption to the structure and plaster/lathe walls takes experience (other wise you get to do lots of wet plaster repair). you might be able to work as the electricians helper, it will take two people to do quickly and that way you will never be without electricity overnight if the job is done right.

for someone who is good at DIY you could try it with lots of preparation. i would get every book on DIY wiring at your library and read them all until you understood them all. Consumer grade DIY wiring books do have errors in the code and sometimes don’t explain well to each person, multiple books help things come out in the end. Wiring Simplified by H. P. Richter, i think is the best book for a DIYer to explain doing things to the code, though other books may have good diagrams, drawings and photos to help you understand the techniques (you will do a lot of working blind in walls and some access hole making).

Check your local electrical code for rules related to “abandoned circuits” before acting on the above suggestion.

National Electrical Code covers it. You can not leave abandoned wiring in a manner that it could be reused in the future.

I would not recommend rewiring a house as a DIY project. Specifically one with horsehair plaster. While snaking wires is simple in concept in practice it takes a lot of skill to do it effectively and in a timely way. If you can avoid cutting holes in the plaster it is always best and an experienced electrician will know the best methods to avoid damage.

I’ll third or fourth the recommendation to hire a pro for this. With a lot of old homes the main breaker needs to be upsized to to meet all the requirements of the code so you might not even be able to strip it back to there and start anew. Add to that the difficulty of fishing wires through all the layers of old renovations that you run into with an older home and you’ll be glad to have someone experienced with you.

Around here the knob-and-tube is just disconnected and left where it is. I didn’t know there were areas where it all had to be removed. That would be a huge pain in the ass.

Do you have to remove it all or just remove the feed from the panel out to the branch circuits?

Thanks - that’s kinda what I was hoping.

Two additional questions: I plan on renovating my bathroom from a half bath to a full bath (second floor). There will be rewiring needed, in addition to plumbing and carpentry. (1) Do I contract each of those jobs out (plumber, electrician, gen. contractor), or will a general contractor be sufficient? (2) When I put in new wiring for that bathroom, does that then obligate me to upgrade everything else?

I don’t know what the code says, but I think it would be a good idea to remove it as much as possible. It’s no fun to run across wires that you must assume are live until proving otherwise.

Then again, it might be common for people to cut corners on that. I recently had a repipe done (the galvanized pipes were as old as my wiring), and at one point I was lucky to be standing there watching because the plumber held up a sawzall to recklessly hack through a pipe and what he assumed was dead wiring immediately next to it. I said, “don’t cut that wiring. It’s live.” He tried to argue that there was no way it was live because there was some romex nearby so obviously the house had been rewired. I pointed out how you could actually follow that wire he was about to cut back to a live junction box fed by new wiring. No dead plumbers in my basement, please.

A good general contractor will hire the right guys for each job. Ask to see the permits or get them yourself. If he’s getting the appropriate permits and inspections that’s a good sign.

Around here, if a licensed guy touches it he has to bring everything up to code. I’m not familiar with American codes.

Not a bad point but it sounds like that circuit shouldn’t have been connected to anything as the plumber said.