Should old knob and tube wiring be replaced?

Question

I have been doing some electrical work in my house and I noticed it has old knob and tube wiring that is still hot, should this be replaced as found or is it fine to just leave it? It seems really old and brittle, so I'm a little concerned.

Answer 1

The advice my family once got from an electrician on this question was that if you have low amperage service and NEVER touch it, you're probably okay. If you have regular electrical service or touch the circuits at all, remove ALL of it. His basic theory was that if you keep the wires cool, and they haven't caused you trouble yet, it's unlikely to cause a problem. Granted that advice was about 15 years ago and none of that wiring is improving with age.

Once you touch the lines at all, rip it all out. It's very likely that you will introduce a problem between the coating and the wire. The house we were dealing with went another 10 years before we needed to rewire one of the old circuits. Once that happened we rewired the entire house all at once.

If you're concerned at all, turn off the effected circuits, and get rid of it. Better safe than sorry on this one.

Answer 2

Myths abound about K&T, and lots of it is ripped out unnecessarily.

The K&T wire is exactly as thick as modern wire, and was installed by skilled craftsmen using bulletproof soldered joints rather than plastic wire nuts. The ceramic tubes will outlast civilizations. With certain important exceptions your K&T will outlast the house itself, unlike modern wire.

K&T is more heat resistant than the equivalent modern wire, because the conductors are separated by an air gap. The ceramic tubes mean that even if the insulation deteriorates it creates no fire hazard. And in fact, if you check fire statistics you'll find K&T wiring is as, or more, safe than modern wiring. Hammered nails or screws can create subtle shorts and sparks in modern wire, but have little to no effect even if they pierce K&T wires. Poor installation is more common on modern wire, compared to K&T which was installed only by guild trained electricians.

From Home Energy Magazine 'Knob and Tube Not a Fire Hazard':

Legislation was enacted in Washington state to allow insulating over knob-and-tube wiring per Bonneville Power Administration (BPA) specifications. This resulted because there were no documented cases of a fire being caused by knob-and-tube wiring, whether insulation covered it or not.

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That said, much K&T wiring is either overloaded by modern uses, or has been hacked to death by bad renovation. Our jurisdiction (Berkeley, CA) requires an inspection by a licensed electrician prior to insulating over Knob & Tube (see here).

It is important to check:

• Fuses have been replaced with appropriate circuit breakers (15 amp for 14 gauge wire)
• Modern splices are proper, and made inside junction boxes.
• No branch circuit is overloaded (selectively add circuits to bring things in balance. Rewire your kitchen. Run separate new circuits for the dishwasher and laundry).
• Hot & neutral have not been mixed up between branch circuits.
• Insulating loom where K&T enters metal junction boxes is still in acceptable shape. This is weak point on K&T installs. Slip new insulation over the old.
• Consider an AFCI breaker for K&T circuits.

This bears repeating:

• Use great care when changing a light fixture or outlet on K&T. The loom where the wire enters metal boxes is a weak point, and is easily damaged.

Answer 3

A house in San Carlos CA, Having 50% knob-and-tube, 50% 1980 romex:

Insurance was no problem: No company cared (I ended up getting it from Travelers, via GEICO). So, I wouldn't let the insurance stories scare you until you pick up the phone and ask. What they do care about is the type of Circuit breakers you have. The modern ones that look like switches, or the old wire ones (that burn). That may be an issue if you have the old plugs.

knob-and-tube has 2 disadvantages:

1. Theoretical fire hazard: you see, they need to be able to "breathe". That is why they say that it is a problem to insulate over K&T. And why everyone will say, that if "you touch it" you may disturb it and hence you should remove it.
2. They have no grounding. So that "3rd" leg some appliances may have even if your socket accepts it, will do nothing. That is a higher risk of getting an electric shock.

Recommendation: It might be a problem with putting things "on paper" vs. verbally, but I interviewed many electricians and 3 permit employees (since I was remodeling the kitchen and "touched' some of the old K&T). My experience is that most of them, when not motivated to just make money out of you, say that K&T is just fine, there are plenty of old houses with it and you don't really see houses go up in flames around you do you? and when fire happens on the news, when did you hear that it was blamed on old K&T system (most likely you hear it was a heater or candles)

my personal comclusion, if it is not broken, don't fix it. do it only if you are doing some other thing which justifies the work.

• Expect $3,000 to $10,000 to re-wire the house or portion of it (I only needed 50%, and I had some walls and ceiling open anyway)
• notice that some electricians put in the quoted work the electric work, but you will have some cleanup work of patching walls and painting after they are done

Expect anything from $3000 to $10,000.

Answer 4

Generally, it is a good idea to replace it, especially if you can easily access it. One thing you don't want to do, is replace part of a circuit. Then the next guy might see the new romex and make some assumptions about the rest of the wiring. The other time you really really should replace it is if it runs through insulation, especially blown-in cellulose. That is a serious fire hazard. Knob and tube was designed to use open space as an insulator.

Answer 5

There may be issues with your insurance company. I recently bought a house and asked two insurance companies about the knob-and-tube wiring.

• Liberty Mutual said that they would insure the house, but would require that the knob-and-tube wiring be removed within 30 days by an electrician.

• Met Life said that they would not be able to insure the house until the knob-and-tube wiring had been removed.

Answer 6

Yes definitely. In some spots, knob and tube is a deal breaker for a house sale and must be replaced before the house is sold. It's fairly dangerous to have around, even more so if it's brittle. I'd hire an electrician if you're not comfortable replacing it all yourself.