Modern Romex wiring has protective plastic insulation around each wire. This insulation is rated to 90 degrees Celsius. For those who don't...

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Modern Romex wiring has protective plastic insulation around each wire. This insulation is rated to 90 degrees Celsius. For those who don’t want to do the conversion, trust me, that’s hot. Hotter than wiring is going to get unless exposed to a fire.

Some of the oldest wiring seen in buildings in our area, called “knob and tube,” has a rubber coating before 1930, which is in turn wrapped in a fire-treated cloth outer covering. With exposure to heat, this rubber coating breaks down. Give it enough heat over a long enough period and it completely crumbles. With the rubber gone, the wire is exposed.

Not wanting to start a panic over knob and tube wiring, let me reassure you that this complete crumbling of insulation is rare. More common is a feeling of brittleness when moving the wire, with the insulation staying completely intact. The insulation damage can occur when heat is trapped, such as behind a receptacle or above a light, and when large amounts of power are being consumed over long periods of time.

Even rarer is damage outside of these limited areas (not near where connected to receptacles and lights). Installing light bulbs of a higher wattage than the rating of the fixture is, in my opinion, the No. 1 cause of these problems.

While insurance companies cast a wary eye toward knob and tube wiring, just about every home built before World War II has some of it, unless the house has been completely gutted and rewired. This is not to say that knob and tube shouldn’t be looked at periodically, and should certainly be replaced during major remodeling when walls are opened up.

The 1924 13-unit apartment building I own and am renovating had all the original wiring still in place. Back in the early 1970s, electricians repurposed this older wiring, before the term was in vogue, running it through metal conduit everywhere in the building. The metal conduit and metal receptacle and light boxes provided a path to the ground. The whole works was hooked to new electric panels in the basement. In effect, they took older wiring, reused it, protected it against physical damage, and grounded a previously ungrounded system via the conduit.

It took a lot of plaster repair, and arguably more money than a simple rewiring. It also provided physical protection from nails in the walls. Problem is, it left the original wiring with its older rubber coating still in place.

Fast-forward 40 years … I am removing and replacing all the outlets and switches, and stripping paint off the antique lights for aesthetic reasons, and rewiring them for safety reasons. In all the other apartments, the wiring is in immaculate condition. In this particular unit, the rubber coating literally fell off in my hand all the way to the back side of the electrical box where the conduit connects. Farther into the wall at the conduit, the wire insulation was fine. Not wanting to wrap multiple strands of wiring with electrical tape several inches back where I can’t see or reach it with my fat hand, I needed an alternative.

Pulling newly painted plaster walls apart all the way through to the brick, patching and providing a junction box was the obvious alternative, but not high on my list. Nor was leaving hot wires dangling inside a metal box. Doing nothing was especially unappealing given the plethora of switched neutrals in this building (“switched neutral” means the hot wire is always powered up, with the switch closing the neutral side of the circuit and completing the circuit).

Yuh huh, you know it — that is illegal as heck today! Beware of switched neutrals when doing electrical work in older buildings — with the breaker on and the switch off you are still dealing with hot wires. Turn off the breaker. If you leave the breaker on and the switch off, you will get hit. Guaranteed. Not that I would know from personal experience …

But there are some lesser-known wire-repair alternatives. Resins and epoxies can be painted on. Great stuff, but I didn’t have the physical access. For about 2 bucks, I picked up heat-shrink tubing. Developed in the electronics industry to cover exposed joints, and to help color-code wires, it has gained acceptance by Underwriters Laboratories and National Electrical Code. Of course, various local codes may not be so welcoming, and should be checked.

Made of different materials for different applications, mine was made of polyolefin. When heated, it shrinks from its original size 33 to 50 percent. It comes in various sizes. I used the smallest (1/8 inch). Slip it over the wire, and heat it with a heat gun, hair dryer, torch or even soldering iron. It shrunk right down, and just like that my wire was newly insulated and ready for a new receptacle.

While I absolutely love the hardwoods, brick, plaster and charm of the old building, I must give a great big tip of the hat to the new technology, keeping it purring along!

Darrell Hay is a local home inspector and manages rental properties. Send e-mail to Sorry, no personal replies.