Knob and tube upgrade

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karl riley

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A question for anyone who has added on to a knob and tube residence, particularly in the CA Bay area:
As an EMF consultant I need to advise a homeowner as to how important it is to replace the knob and tube circuits in a 1923 luxury home which they are adding to.

Since I need to make a recommendation for the electrician looking at the job, my question is what factors should he weigh (as against the cost)? The magnetic fields generated are one factor which the client is now aware of, but I am looking at the electrical safety questions: 1923 insulation condition, conductor size for the increased loads, replacement of all receptacles with GFCI.

I would appreciate any input from anyone who has dealt with this situation.

Karl
 
Re: Knob and tube upgrade

I have done a bit of SF and Bay area K&T and am of two minds about it. It seems to me that most fires are caused by pinched hot to grd. or neut., hot to grded box etc. Since the neutrals are run seperately, there is less arcing even with damaged wire. hence, fewer fires. However, The circuits tend to be overloaded both as a "global neutral" and as over fused circuits. Much of what I saw was 14 guage wire.

If the K&T is in very good condition and has not been added on to too much, the primary complaints are limited circuit capacity and to those who are concerned about emf, very broad emf fields. NM is much harder to detect buried in walls than K&T.

I haven't been in the actual biz for a bit, but the costs are driven by a few factors, the least is NOT whether the wall covering is original plaster, or wallboard. Before rotozips, the only tool that worked to cut old lathe and plaster was a dremel with this tiny circular saw blade, otherwise one small catch of the recip. blade and viola, instant cracks the length of the lathe. Now rotozips have made it less painfull.

If the walls are open, replace the K&T. If original plaster and you can't cut it open, the cost per device has got to be 2 to 3 bills a pop. Figuring out how to do legal splices to partial runs of K&T is an art. It takes a patient and persistent examination of the wiring and how the added devices can be fed, whether by new circuis or with the K&T without overloading either the hot or neutral.

Each application is unique as most K&T has been added on to, adapted.

As you can see I am not a big EMFer, but I am big on the hazards.

I would need more specifics before I could say anything more of use to you.

paul
 
Re: Knob and tube upgrade

This raises an interesting question: How can you legally splice something new, say NM-B or THHN, into existing K&T? I would assume a j-box is required, but I've never seen any clamps designed to accept K&T.

Personally, I replace K&T when I run into it, but I'm curious as to how it's done by people who choose to leave the K&T in place.
 
Re: Knob and tube upgrade

Karl,

Based on my hands-on experience with a lot of different K&T installations I suggest starting with #1 below and working down the list until the budget is exhausted.</font>
  • <font size="2" face="Verdana, Helvetica, sans-serif">Shed load </font>[list:355b11addb]</font>
  • <font size="2" face="Verdana, Helvetica, sans-serif">New wiring method to outlets likely to serve grounded loads (such as computer, cable converter, window AC, etc.)</font>
  • <font size="2" face="Verdana, Helvetica, sans-serif">Accessible armored cable extensions of the core K&T many times can simply be refed with a modern method and (depending upon local code) be grandfathered as a grounding means</font>
<font size="2" face="Verdana, Helvetica, sans-serif">[*]<font size="2" face="Verdana, Helvetica, sans-serif">Replace single conductor switch legs with new feed and switch loop</font>[*]<font size="2" face="Verdana, Helvetica, sans-serif">Rebuild any threeway wiring completely</font>[*]<font size="2" face="Verdana, Helvetica, sans-serif">Replace as much remaining K&T as the budget can tolerate, starting with: </font>
  • <font size="2" face="Verdana, Helvetica, sans-serif">attics,</font>
  • <font size="2" face="Verdana, Helvetica, sans-serif">basements,</font>
  • <font size="2" face="Verdana, Helvetica, sans-serif">insulated walls, then, finally,</font>
  • <font size="2" face="Verdana, Helvetica, sans-serif">interior walls.</font>
<font size="2" face="Verdana, Helvetica, sans-serif"></font>[/list:u:355b11addb]<font size="2" face="Verdana, Helvetica, sans-serif">In my area of Minnesota, there has been a steady pressure to improve the original wiring, incrementally, by removing exposed lengths of K&T in attics and basements. The environmental stresses on the K&T insulation has been shown to be greatest in those areas. Curiously, K&T that has been insulated around is overlooked until it is exposed by remodeling.

In my experience, a chunk of the K&T is wired in pairs of conductors that are separates by no more than eleven inches and, while operating, have balanced currents, the removal of which will produce minimal EMF improvements. Concentrating on #1, #2 & #3 above will yield progressively smaller improvements, in my opinion.
 
Re: Knob and tube upgrade

Originally posted by jeff43222:
This raises an interesting question: How can you legally splice something new, say NM-B or THHN, into existing K&T? I would assume a j-box is required, but I've never seen any clamps designed to accept K&T.

Personally, I replace K&T when I run into it, but I'm curious as to how it's done by people who choose to leave the K&T in place.
Click to expand...
Jeff this is spelled out in 310.16(A) :)

300.16 Raceway or Cable to Open or Concealed Wiring.
(A) Box or Fitting. A box or terminal fitting having a separately bushed hole for each conductor shall be used wherever a change is made from conduit, electrical metallic tubing, electrical nonmetallic tubing, nonmetallic-sheathed cable, Type AC cable, Type MC cable, or mineral-insulated, metal-sheathed cable and surface raceway wiring to open wiring or to concealed knob-and-tube wiring. A fitting used for this purpose shall contain no taps or splices and shall not be used at luminaire (fixture) outlets.
Click to expand...
 
Re: Knob and tube upgrade

Why would you even think of adding K +T ? far far easier to run romex and cheaper.Where would we even be able to buy it? I would highly suggest rewiring the house and certanly not add and K T to it.
 
Re: Knob and tube upgrade

Jim,

I understood Karl's question to be about a legacy K&T system with decades of additions to it, that is staying in the core building that is having an addition added. The addition will, I assume, be wired with contemporary methods, NM, MC, whatever. The tweeking of the existing structure required to integrate the addition provides certain opportunities to get at the existing K&T during the remodeling process that will add some economy to improving the K&T. Adding careful EMF analysis will yeild better results
 
Re: Knob and tube upgrade

Thanks to all. Al, that info is very helpful. Yes, this is not a case of extending K&T, but adding NM for the additions. The info on cutting tools is good also.

Any more comments will be appreciated. This discussion should be preserved or reprinted to help others who have to deal with the problem. This is still very common in the Bay area, and in some very upscale mansions.

Karl
 
Re: Knob and tube upgrade

a few more thoughts on the subject.

if the walls are plaster and you are not removing the old electrical boxes, GFCIs barely if at all fit into those old boxes.

Never assume that any of the old K&T is unused.

never use auger bits in old wood with lathe and plaster, speed bores work well. about the biggest bit you can stab into a remote diagonal is a half inch without it risking the plaster.

try to figure out every hole that has to be opened into the plaster and you might find a way of overcoming impossible pulls.

When I was completely baffled by some unseen problem, I drilled a second hole , one for flashlight and one to look into. If the hole is large enough, stick a small flashlight down one hole and you can look past thin shank bits to put the cutting head where you want.

Another trick for diagonal bracing: drill initial hole large enough for emt. If the brace isn't too far, the emt can be held to an extremely sharp diagonal, and a bit driven down inside the emt, and it will start the hole instead of sliding along or down the diagonal.

Electricians who specialize in old work have tons of tricks.

paul
 
Re: Knob and tube upgrade

In my area, bracing, in balloon framing, and even the more modular framing of the '20s, is uncommon. But, on occasion, one will find diagonal blocking that is installed a lot like a fire block (completely filling the void between studs), except it is tilted at a 45? angle. These diagonal blocks are not placed by a logic that is intuitive to the finish of the structure, in my opinion.

Another characteristic of balloon framing is that headers tend to extend to either or both sides of an opening for a door, window or medicine cabinet. Any fishing close to existing openings will teach about the carpenter's choices, and fishing, in general, will also uncover the header for the closed opening that was removed decades ago.

Given that this thread is about a "luxury home", the client will many times pay extra, gladly, for time consuming installation tricks that avoid damaging existing wall surfaces. This is best handled Time and Material with a cap.
 
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