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Inspected an old house today that had OLD KNOB & TUBE wiring present at least on the first floor (observed from the basement) How do my fellow home inspectors rate this type of wiring Defective old wiring present, recommend a licensed electrician to evaluate for safety or List the wiring as Marginal old wiring present or say nothing at all. Thanks SM

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Tell client what you see, Determine if any is energized if possible, even if not tell them there may be energized in home, Recommend replacement (updating) by contractor familiar with it, or at minimum it be evaluated by qualified contractor, (repair as necessary) and given written cert it is safe.... Also tell them some Ins companies will not insure home with active knob and tube (or edison base fuses) at least here in WA...

I find it where previous inspectors did not tell their client it was in home, I come in and mention it to client (verbal and in report) and to get it checked out or replaced .... really pisses off owners when selling and the inspector had not said anything to them 3-4 years before when they bought the home.

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Inspected an old house today that had OLD KNOB & TUBE wiring present at least on the first floor (observed from the basement) How do my fellow home inspectors rate this type of wiring Defective old wiring present, recommend a licensed electrician to evaluate for safety or List the wiring as Marginal old wiring present or say nothing at all. Thanks SM

I don't call it defective unless it is. (It often is.) However, even if it's in excellent shape, I call it obsolete. I write a short, well-reasoned paragraph about the problems associated with it in that particular house and I recommend that the customer replace it.

- Jim Katen, Oregon

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Pretty much what Jim does. Short paragraph explaining it and why it's less than desirable.

Since I've never seen one that wasn't all mangled somewhere, I always tell folks to replace it.

If someone wants to argue that it's just fine, that's their prerogative, although they're prerogative is usually ill formed and illogical.

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I normally tell folks that there's K & T present and, like Jim, call it obsolete.

I usually don't go into an explanation of why it can be an issue because I normally have several other comments in the report pointing out various issues with the electrical system that are related to the K & T.

I do like Jerry, point out that some insurance companies will refuse to insure the house while others will likely charge higher premiums until/unless it's replaced and that some don't seem to care.

I tell 'em that if it were my house, I'd be figuring out how to replace it as soon as possible and tell them to figure on a bill in the thousands to find, disconnect and bypass all of this stuff and install new modern grounded receptacles.

In Washington State, K & T can still be added onto and can be covered with insulation if it's first inspected by a licensed electrician and he puts his blessing on it.

ONE TEAM - ONE FIGHT!!!

Mike

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Article 394 in the 2008 National electrical code allows the installation of Knob and Tube wiring for the extension of existing installations or by special permission.

Quoted from the 2008 NEC:

394.10 Uses Permitted.

Concealed knob-and-tube wiring shall be permitted to be installed in the hollow spaces of walls and ceilings, or in unfinished attics and roof spaces as provided by 394.23, only as follows:

(1) For extensions of existing installations

(2) Elsewhere by special permission

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Article 394 in the 2008 National electrical code allows the installation of Knob and Tube wiring for the extension of existing installations or by special permission.

Quoted from the 2008 NEC:

394.10 Uses Permitted.

Concealed knob-and-tube wiring shall be permitted to be installed in the hollow spaces of walls and ceilings, or in unfinished attics and roof spaces as provided by 394.23, only as follows:

(1) For extensions of existing installations

(2) Elsewhere by special permission

That's all true, of course. But my recommendation to replace old K&T isn't based on an NEC rule, it's based on the fact that the wiring is, well, obsolete.

The K&T that I see is uniformly between 90 and 110 years old and the insulation is made from rubber and cloth. Are there any objects made from rubber & cloth are still in useful condition after 100 years of service?

- Jim Katen, Oregon

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There's always the idea that one can't even see all of it; approving it without being able to see it always struck me as a little odd. Without dismantling every junction and splice, how could anyone know if it was in satisfactory condition. The simple act of opening the splice to check it would probably screw it up.

Then there's the NFPA piece about anything >40 years old having problems.

Why would anyone want to stretch the envelope on something like old wiring?

I might stretch and patch a roof on my own house, but wiring.....(?)....

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Trust me I was in no way advocating the use of Knob & Tube !

I ripped tons of that crap out. I would never suggest adding on to it.

I view K & T the same way I view an old fuse panel -

Was a very good thing when first installed as it was the latest and greatest items on the market, but if still in use ,it's time to replace it.

I was pointing out that it is still in the National Electrical Code as being allowed to be extended.

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The K&T that I see is uniformly between 90 and 110 years old and the insulation is made from rubber and cloth. Are there any objects made from rubber & cloth are still in useful condition after 100 years of service?

- Jim Katen, Oregon

Not to argue that K&T is good, but a couple decades ago, when my father expressed concern that the K&T wiring would start a fire in his house, I went up and removed from circuit a short sample of a K&T conductor. I brought it to him and bent it in the middle, folding it back 180 degrees and then straightened it back out. The insulation didn't split or damage in any way. It looked like brand new. I suggested to him that the issue with the wiring had more to do with devices (outlets and switches) that were so old and the lack of grounding than from the quality of the insulation on the conductors themselves. This was long before becoming an inspector but after my education in electrical engineering.

Marc

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The idea behind the wiring is very much like the idea behind the building codes.

Just because something isn't built "to code", that doesn't mean the house blows up. It means there is some small incremental percentage of houses that will have a problem.

Same thing with K&T. I grew up with it. Worked on it all the time. Some of it was just fine. Some of it would literally turn to dust when I'd open up the friction tape splices.

So, gauging the condition of all the wiring based on a few observations, or a single observation, is dangerous imho.

I still see K&T wiring that's just "fine", or at least it looks that way. But, I still say you can't tell by looking.

And, this isn't a shot at EE's or any other engineer, but the goofiest electrical systems I see are invariably in the homes of electrical engineers. I am not aware of any EE education path that teaches about residential wiring peccadilloes.

Just like the craziest heating systems always seem to be in the homes of mechanical engineers.

And never, ever buy a home owned by a contractor, and especially not one they built "especially" for themselves.

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The K&T that I see is uniformly between 90 and 110 years old and the insulation is made from rubber and cloth. Are there any objects made from rubber & cloth are still in useful condition after 100 years of service?

- Jim Katen, Oregon

Not to argue that K&T is good, but a couple decades ago, when my father expressed concern that the K&T wiring would start a fire in his house, I went up and removed from circuit a short sample of a K&T conductor. I brought it to him and bent it in the middle, folding it back 180 degrees and then straightened it back out. The insulation didn't split or damage in any way. It looked like brand new. I suggested to him that the issue with the wiring had more to do with devices (outlets and switches) that were so old and the lack of grounding than from the quality of the insulation on the conductors themselves. This was long before becoming an inspector but after my education in electrical engineering.

Oh, heck, you'll still find plenty of K&T that's nice & flexible. But remember that electrical wiring is a weak-link-technology. It doesn't matter if it's in pristine condition in the middle of a wiring run if the wires just above the light fixture have the consistency of Cheetos. That's where the trouble will start.

Just pull a ceiling fixture in an old house and look at the two crispy critters that are feeding it.

- Jim Katen, Oregon

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The K&T that I see is uniformly between 90 and 110 years old and the insulation is made from rubber and cloth. Are there any objects made from rubber & cloth are still in useful condition after 100 years of service?

- Jim Katen, Oregon

Not to argue that K&T is good, but a couple decades ago, when my father expressed concern that the K&T wiring would start a fire in his house, I went up and removed from circuit a short sample of a K&T conductor. I brought it to him and bent it in the middle, folding it back 180 degrees and then straightened it back out. The insulation didn't split or damage in any way. It looked like brand new. I suggested to him that the issue with the wiring had more to do with devices (outlets and switches) that were so old and the lack of grounding than from the quality of the insulation on the conductors themselves. This was long before becoming an inspector but after my education in electrical engineering.

Marc

Exactly,as long as knob and tube wiring hasnt been hacked/butchered by somebody reading a book at the hardware store thinking their an electrician I generally have no problem with it myself as a contractor.

We still have many houses in the older parts of omaha with knob and tube

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I don't remember where I learned this but it stuck.

One of the problems with K & T is that, even though we see new panels all the time, it used to be protected by fuses and there's a strong likelihood that it's going to have been pennied more than once.

There's a weird characteristic of copper wiring - if you run it above it's rated ampacity and overhead it for too long by pennying it, you'll lower the wire's ampacity; in other words, 12ga. wiring will overheat with a 12-amp fuse and 10ga wiring is going to overhead with a 30 amp fuse. So, I was taught that electricians, to be safe, will go down one size for all of the branch wiring when replacing the fuse boxes in houses with K & T.

Has anyone heard of this phenomenon?

ONE TEAM - ONE FIGHT!!!

Mike

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I don't remember where I learned this but it stuck.

One of the problems with K & T is that, even though we see new panels all the time, it used to be protected by fuses and there's a strong likelihood that it's going to have been pennied more than once.

There's a weird characteristic of copper wiring - if you run it above it's rated ampacity and overhead it for too long by pennying it, you'll lower the wire's ampacity; in other words, 12ga. wiring will overheat with a 12-amp fuse and 10ga wiring is going to overhead with a 30 amp fuse. So, I was taught that electricians, to be safe, will go down one size for all of the branch wiring when replacing the fuse boxes in houses with K & T.

Has anyone heard of this phenomenon?

No. The idea that heating copper reduces its ampacity is not correct.

The limitations in the ampacity tables are, generally, concerned with the insulation on the wire more than on the wire itself.

Overheating a circuit might damage the *insulation* on the wire in that circuit, but it isn't going to reduce the current-carrying capacity of the wire itself unless it's gotten hot enough for the wire to be physically distorted.

Also, running 30 amps through a #12 wire isn't going to cause it to heat up very much. If there are poor connections somewhere, they'll heat up, but they'll heat up at 20 amps too.

- Jim Katen, Oregon

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And, this isn't a shot at EE's or any other engineer, but the goofiest electrical systems I see are invariably in the homes of electrical engineers. I am not aware of any EE education path that teaches about residential wiring peccadilloes.

I would suggest that that is more of a reflection on those engineers who checked their common sense at the door when they first entered college than a reflection on the engineering profession in general. There's one down the street who's wife walked over to ask me to fix their air handler that her husband had spent days trying to do. The husband is a mechanical engineer and a good one, but he disregarded his bounds when he got started on the AC and the wife got tired of a hot house.

The truth is, Kurt is right, mainstream EE curriculums don't teach a dang thing about electrical wiring except for voltage drop calculations, skin effect, magnetic coupling, capacitance, etc, little of which is of any use to tradesmen or inspectors but let's just keep in mind that not all engineers succomb to this 'God' syndrome. Some remain conscious of their bounds and do not lose respect for neighboring professions and trades.

When I learn that the seller of a house that I'm about to inspect is an engineer, I prepare myself for a 'long haul'.

Marc

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There's a weird characteristic of copper wiring - if you run it above it's rated ampacity and overhead it for too long by pennying it, you'll lower the wire's ampacity; in other words, 12ga. wiring will overheat with a 12-amp fuse and 10ga wiring is going to overhead with a 30 amp fuse. So, I was taught that electricians, to be safe, will go down one size for all of the branch wiring when replacing the fuse boxes in houses with K & T.

Copper has no such properties, but connections sometimes do develop undue resistance with age and one way to combat that problem is to reduce the maximum current permitted to pass through them. Perhaps that is what motivated the electricians to derate the conductors on K&T installations.

Marc

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Also, running 30 amps through a #12 wire isn't going to cause it to heat up very much. If there are poor connections somewhere, they'll heat up, but they'll heat up at 20 amps too.

30 amps will develop 2 1/4 times more heat than 20 amps in a conductor. You don't think that's a big enough difference?

Marc

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Also, running 30 amps through a #12 wire isn't going to cause it to heat up very much. If there are poor connections somewhere, they'll heat up, but they'll heat up at 20 amps too.

30 amps will develop 2 1/4 times more heat than 20 amps in a conductor. You don't think that's a big enough difference?

Marc

It's all about balance. If the wire can dissipate heat faster than it can accumulate it, it'll be fine. If it can't, it'll overheat.

I've seen a #12 cable that had fed a 7,000 watt electric heater for 30 years without a sign of a problem. It's not a good thing. It removed a layer or two of safety from the electrical system, but the insulation on the wire was doing ok.

On the other hand, I've seen #6 aluminum SE cable that had 160 amps running through it and the outer sheath of the cable was melting and dripping like mozzerella cheeze from a slice of pizza. And this was when the cable was outdoors in 20-degree weather with ice & snow all around it.

Running too much current through a wire is bad. But running 30 amps through a #12 cable isn't *as bad* as most home inspectors seem to think.

BTW, could you share your math on the 2-1/4 times more heat thing? It isn't that I don't believe you, I'm just curious about your method of calculation.

- Jim Katen, Oregon

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BTW, could you share your math on the 2-1/4 times more heat thing? It isn't that I don't believe you, I'm just curious about your method of calculation.

- Jim Katen, Oregon

Sure Jim,

Most of us know that developed power is the product of the voltage and current, but voltage is equal to the current multiplied by the resistance, Hence:

P=VI

V=IR

Substituting:

P=(IR)I

P=I(squared)R

The resistance for the conductor hasn't changed but the current has changed from 20A to 30A. 30 is 20(1.5):

P=(1.5)(squared)R

1.5 (squared) = 2.25

P=2.25R

So the heat developed is now 2.25 times greater than before.

Pardon the lack of suitable notation. I don't know how to do it on this word processor.

Marc

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Great conversation going on, but all I really know about K&T is that it is old and as said by a few others on this thread it is old...

When I discover a home with K&T, in addition to telling my client that it should be replaced I also tell my client that many insurance providers will not issue a new policy on a home with K&T. Heck, most have heartburn when a home as any electrical that is over 40 years of age.

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