Originally Posted By: jbowman
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Old style wiring a home inspector will encounter.
In the years since Edison "invented" electricity, several different wiring "styles" have come and gone. When you inspect an older home you may encounter some of this stuff. This narrative will hopefully assist home inspectors in recognizing the old methods, and some of their idiosyncrasies.
The oldest wiring system you're likely to encounter is called "knob and tube" (K&T). It is made up of individual conductors with
a cloth insulation. The wires are run along side structural members (i.e. joists or studs) using ceramic stand-offs (knobs). Wire is run through structural members using ceramic tubes. Connections were made by twisting the wire together, soldering, and wrapping with tape. Since the hot and neutral were run separately, the wiring tends to be rather confusing. A neutral often runs down the center of each room, with "taps" off to each fixture. The hot wire tended to run from one fixture to the next. In some cases K&T isn't color-coded, so the neutral is often the same color as the hot wires.
You'll see K&T in homes built as late as the 40's.
My opinions on K&T:
- the people installing K&T were pretty paranoid about electricity, so the workmanship tends to be pretty precise.
- The wire, insulation and insulators tend to stand up very well. Most K&T I've seen, for example, is in quite good condition.
- No grounding. Grounding is usually difficult to install. Boxes are small. Receptacle replacement (particularly with GFCI) can be difficult. No bushing on boxes either, so wiring changes need special attention to box entry.
- Sometimes the neutral isn't balanced very well between separately hot circuits, so it is sometimes possible to overload the neutral without exceeding the fusing on any circuit.
- In DC days it was common to fuse both sides, and no harm was done. In fact, it was probably a Good Thing. The practice apparently carried over to K&T where you may find fused neutrals. This is a very bad thing.
- Building code does not usually permit insulation in walls or ceilings that contains K&T. Some jurisdictions will allow it under some circumstances (i.e. engineer's certificate). Consult your local authorities having jurisdiction.
- Connection to existing K&T from new circuits can be tricky. Definitely consult the services of Electrical professionals when in doubt.
- Modern wiring practice requires considerably more outlets to be installed than K&T systems did.
Since K&T tends to be in pretty decent condition it generally isn't necessary to replace it simply because it's K&T. (I should note that several Insurance Agency?s are beginning to refuse coverage of homes with K&T wiring). What you, the inspector, should watch out for is renovations that have interfered with it and be cautious about circuit loading. In many cases it's perfectly reasonable to leave existing K&T alone, and add new fixtures on new circuits using modern techniques.
After K&T, they invented multi-conductor cable. The first type you will see is roughly a cloth and varnish insulation. It looks much like the romex cable of the last decade or two. This stuff was used in the 40's and 50's. Again, no grounding conductor was utilized. It was installed much like modern wiring. Its major drawback is that the protective jacket aged quickly and became extremely brittle. As an inspector you should familiarize yourself with this type of wiring. Inspectors can often immediately recognize it because the protective jacket shows several signs of cracking. Use extreme caution when handling and please use proper protection when handling. I've seen whole systems where the insulation would fracture and fall off at a touch. BX cable of the same vintage has similar problems. It is possible for the hot conductor to short out to the cable jacket. Since the jacket is rusted, it no longer presents a low resistance return path for the current flow, but rather more acts like a resistance heater. In extreme cases the cable jacket will become red hot without blowing the fuse or circuit breaker. The best thing to do with old style BX is recommend evaluation by a licensed electrician for possible replacement with modern cable whenever it's encountered and there's any hint of the sheath rusting.
The prospective buyer should be notified that this stuff is very fragile, and becomes rather hazardous if the wires become bare. This wiring should be left untouched as much as possible and whenever an opportunity arises, replace it. A simple receptacle or switch replacement can turn into a several hour long frustrating fight with electrical tape or heat-shrink tubing.
After this wiring technique, the first style of romex was invented. It's almost an asphalt impregnated cloth that is ?sticky? to the touch. This stuff stands up reasonably well and doesn't present a hazard and is reasonably easy to work with. It does not need to be replaced - it should be considered as safe as the "modern" stuff - thermoplastic insulation wire.