knob and tube wiring

lisa98112July 22, 2005

Our 1915 house still has knob and tube wiring. The owners claimed they updated the electrical in 1984, but all they did was put in a new box and didn't completely rewire. The house has plaster and lathe walls, so rewiring is not an easy task. Has anyone else attempted to rewire themselves? Other than the extra difficulty of finding insurance, any reason to rewire?

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We had the knob and tube that was easy to get to, changed out. The stuff in the ceilings had to stay. I did have to remove the plaster and the lathe from the foyer, but it couldn't be changed out because it went to other rooms also. My electrician told me it wasn't a problem leaving what was left. This house was all still fuses when we bought it 7 months ago. The insurance asked if we had a circuit breaker. Nope, only fuses. She submitted our application and was told they would cover us as long as the fuse part was updated to a cb box. Never was asked about knob and tube...they probably figured? The PO of your house probably did update the wiring to the best of their ability. Meaning that unless you want all the plaster and lathe torn out, you pretty much have to leave it. The electrician we used was not going to try and fish all the needed wiring thru the walls unless I paid him out the ying yang. They had enough problems trying to get the main fuse box wiring down that was in the attic, down to the new CB in the basement. We knew our limitations, and electric isn't something we wanted to do. My sister lives up the road in a house almost as old as this one, and I know she still has knob and tube, some on cb, some still on fuses.

    Bookmark   July 22, 2005 at 2:36PM
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How do you know that you still have some knob and tube wiring? I always thought that the knob and tube wire wasn't insulated. Do you have non-insulated wiring going to breaker box?

    Bookmark   July 22, 2005 at 3:04PM
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Knob and tube wiring is insulated. Frequently, the insulation crumbles where it's been subject to heat, such as light fixtures, etc. In those instances, heat shrink can be used to reinsulate the wire. Otherwise, it's safe wiring as long as it remains undisturbed.

    Bookmark   July 22, 2005 at 3:17PM
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Ok. not to be stupid here (or take over the thread)
but if it is insulated how come the two wires are not together like in modern wiring? I thought that if the two wires touched it would start a fire and that is why K&T wiring was run about a foot apart and only brought together at the outlet or fixture (because it wasn't insulated)
Also how did you discover you had K&T wiring still?

    Bookmark   July 22, 2005 at 3:42PM
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Knob and tube is insulated, just with cloth rather than the plastic of modern wiring. Also it doesn't have a ground wire. I know we have it because when you open up a fixture or light switch box you can see the wires.

Everything is the breaker box is updated, but the wiring in the ceiling going to the fixtures and some of the outlets are not.

Here is a link that might be useful: photos of knob and tube

    Bookmark   July 22, 2005 at 4:17PM
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We have K&T, as well as other various types of "updated" wiring (up to and including Romex). Every single electrician we've spoken to said NOT TO WORRY about the K&T. Said replacing it would actually cause MORE of a problem, as the Romex could snag on stuff inside the walls/ceilings and potentially cause shorts/fires.

    Bookmark   July 22, 2005 at 9:18PM
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I know we had k&t because I saw it in the attic and the basement. I saw it when I removed the ceiling in the foyer and when I gutted the kitchen....I'm not an electrician, so I have no idea how they incorporated the new wiring with the old wiring. They didn't run the old wiring to the new breaker box, it's all new. I know they installed junction boxes all over, I'm guessing that has something to do with it.

    Bookmark   July 22, 2005 at 10:05PM
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It sounds like they used the old K&T to feed the new NM. Not the best idea since you still do not get a ground.
You do not have to destroy a house with plaster walls to replace K&T, you just need to fish the wires yourself or find an electrician who knows how to wire an older house without destroying it. It takes longer to fish wires to minimize wall damage than to just chop out the plaster, but you do not have to spend as much getting the walls repaired.
New wiring can pretty easily be installed from a basement up or an attic down. For a whole house with 2 stories I typically sacrifice one stud bay of wall to run a new sub-panel feeder from basement to the second floor. This is the time to install new water lines with the walls open. If you cannot find a place for a sub-panel on the second floor, run a bunch of 14/3 and 12/3 cables. Each cable will provide 2 circuits. A junction box has more latitude in placement than a panel.
Tricks of the trade include long drill bits (5 feet), or using extensions to reach fire stopping to drill through. A zip cutter works very well to cut plaster, but it can have problems with the wood lath. Cutting a square almost all the way through with a zip cutter, then finishing up carefully with a sawzall or keyhole saw leaves a clean hole. Any saw cutting of wood lath needs to be done at a shallow angle to the wall surface to avoid tearing the lath loose.
Shorter sections can be cut from a fish tape and used to pull wire. I actually have a bore scope for checking what is inside a wall before starting to work. Fixing a 1.4 inch hole is a lot easier than starting into a wall only to find some odd framing Âfeature that requires a larger than planned hole.
Plaster walls are actually easily repaired with setting joint compound (Easysand or Durabond) and will look as good as new. The biggest problem I encounter is textured plaster walls. Some textures are easily duplicated, while others are very hard to get a good match.
K&T used rubber covered insulated wires, soldered joints that were than tapped up for insulation, and Âloom to pair wires up to enter boxes. Bare wire means the insulation has fallen off. Some of the old wire was lead coated before the insulation was applied to try and protect the rubber from the copper wire. This insulation is usually in slightly better shape than rubber directly on copper. Any rubber insulation is going to be very dry and brittle and any movement will cause cracking of the insulation. Inside a box you can repair with heat shrink. Outside the box really calls for replacement, but Scotch33+ tape can be used for a temporary repair.

    Bookmark   July 23, 2005 at 9:27AM
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K&T wiring used two separate wires held several inches apart. It was nothing like today's cable wiring where everything is in the same sheath.

    Bookmark   July 23, 2005 at 9:47AM
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Basically, it's perfectly safe to leave it and just add new circuits where you need them.

The approved way around here is to run Romex between the old fuse box and a new breaker switch in the new breaker box. They over-amp the fuses in the old box, and the breaker switches are sized to match the old fuses. That way the only place you have to look for a popped circuit is in the new box.

My complaint about knob and tube is that it causes interference with RF signals ... the parallel wires are re-radiating crappy signals. If you hacve cable TV and internet it's OK, but broadcast TV can suffer.

    Bookmark   July 23, 2005 at 1:38PM
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Knob and tube was designed and installed when codes were more conservative. Generally the workmanship is superb - the electician knew how to splice and solder and tape properly. I've inspected the K&T in my attic and it all looks great. Insulation is not brittle or crumbling; no sign of overheating. Some of it has been replaced with entirely new circuits to the remodeled areas of the home. But what remains is in great shape.

A problem comes when homeowners or unlicensed electricians try to splice into existing K&T to add more circuits. Either the connections are bad or the original K&T is overloaded. The proper way to add new circuits is to put a new breaker in the panel and run Romex (or armored cable, or wire in conduit) to the new locations.

Another problem area is the dual wire pre-Romex that is actually wrapped with a rubber compound and fabric. The rubber tends to dry out and crumble with age. If disturbed, you can wind up with a neutral and a hot wire getting bared in very close proximity, perhaps contacting each other. This stuff should be replaced with Romex - providing the splicing into the old K&T that feeds it is done properly (as in a distribution box). My understanding is that the flying splices traditionally used in K&T are no longer allowed on new wiring.

    Bookmark   July 23, 2005 at 6:18PM
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We are in a house built sometime between 1930-1940. We have recently replaced the complete electrical boxes. However, in reading this thread, I remembered that one of the electricians who did some work on our house (actually maybe installing GFIC's that needed to be addressed after home inspection when we moved in)remarked about how old the wires were and they were black and kind of soot-y. I am wondering if this was actually knob and tube wiring? He was a young guy and there aren't many old houses like ours in our city. Were they just excessively dirty wires or was this K&T? They were black.

    Bookmark   November 23, 2008 at 9:51PM
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K&T was installed in my house in 1903. The insulation is far better than the metallic sheathed cable added in the 50's and neither has a ground. I would pay closer attention to the modifications and the newer wiring before worrying about the K&T.

    Bookmark   November 24, 2008 at 1:21PM
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Had an electrician here today who said that in our town it has been code for decades that all electrical had to be inside conduit. Thus, those fabric like wires I saw at one of the plug outlets were encased in conduit with the exception of that bit I saw. We went up to the attic and poked around for awhile and he said there was no knob and tube anywhere. I am relieved because hopefully this means its okay to insulate. Right now, it looks like someone threw one lousy cotton ball up there to insulate the whole attic. I can't believe the people who lived here before us never upgraded the insulation as you wouldn't believe how little it is. The insulation guy, the furnace guys, everyone who has been up there says there is absolutely no R value whatsoever up there. Guess that's why we had a $900 electric bill this summer.

    Bookmark   November 24, 2008 at 9:02PM
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is it ok to splice new romex wire into the old knob and tube wiring and is it safe to still use a 100 amp breaker box with todays electronics

    Bookmark   June 6, 2011 at 3:54PM
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"is it ok to splice new romex wire into the old knob and tube wiring"

Generally no, it is not allowed.

K&T remains in the NEC to cover grandfathering.
You are not allowed to extend K&T since there is no ground available.

    Bookmark   June 6, 2011 at 4:49PM
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I bought my house 2.5 years ago, and didn't know anything about k&t. I do now. Selling through relo, I can't sell until the k&t is replaced. So that means cutting into walls and ceilings and replastering. It's not cheap to have the work done, but at least once it is, wiring will be up to code. Old homes are great, but updating them is definitely not worth it, I'd never buy another house with just a standard full home inspection done.

    Bookmark   June 26, 2011 at 6:38AM
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"So that means cutting into walls and ceilings and replastering."

If you have basement and attic access it is not all that hard, and should only result in localized and minor damage to the plaster requiring patching.

You do not need to actually remove the old K&T, just make sure it is disconnected from its source.

Fishing wires into stud cavities does not go as fast as tearing out walls to gain access, but can end up saving money in the overall costs (especially if plasterers are hard to come by in your location).

Even without attic or basement access, you only need to make holes in the plaster to drill through studs for the new cable.
A low speed drill and 24 inch 'ship's auger' style bits get the job done while minimizing plaster damage.

    Bookmark   June 26, 2011 at 9:34AM
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