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Knob and Tube Wiring

Many houses constructed pre 1950's have what is called knob and tube wiring. One can determine if you have this type of wiring in your home, by closely looking at basement or attic and looking up at the joist or down under the insulation in the attic rafters

To determine if your home is wired " knob and tube", look for ceramic knobs or tubes in which the wire gets attached to, or passes through, joists or studs. If the knob and tube wiring is not easily visible, you can usually tell by looking at your electrical outlets and switches. You may only have two prong outlets to plug into.

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Knob and tube wiring is a system that was the best technology had to offer in its day, however, those days have long since gone.  Knob and tube wiring is the original wiring method used from the late 1800’s until approximately 1947.
Knob and tube electrical wiring involved stringing individual insulated electrical wires across porcelain insulators (“knobs”) which were nailed to the surface of wooden framing members. When electrical wires needed to actually pass thru wooden framing members, the electrical wires were strung through porcelain insulators called “tubes” which protected the wires. 


The first insulation was asphalt-saturated cotton cloth called “loom”. Where conductors (electrical wires) entered an electrical wiring device such as a light fixture, receptacle, outlet, switch, or other junction box, they were protected by flexible cloth insulating material (“loom”). Rubber soon followed as an insulating material and became quite common.

Some of the concerns and safety hazards associated with knob and tube wiring are listed below:

  • It is very old wiring (at least 62 years old, at best), and in many cases has deteriorated, cracked or is missing insulation, which can lead to fires.
  • Heat directly above ceiling lights and in un-vented attics can degrade the wire insulation. Some types of insulation used on knob and tube wiring seem to be a delicacy for the critters that find their way into old homes. Rats, mice, and other creatures like this can make short work of the insulation covering the wires. 
  • It is an ungrounded system, which provides a greater chance of shock or electrocution and of damage to sensitive equipment. Additionally, the whole house electrical wiring systems were very often undersized, which just doesn’t hold up to today’s electrical loads.
  • It is common to find that newer wiring has been improperly connected to knob and tube wiring by amateurs, resulting in dangerous conditions in the form of shock hazard, reverse polarity problems, switched neutrals, overloaded circuits, and fire hazards, to name just a few.
  • The electrical service panels supplying the knob and tube electrical wiring were often undersized as well, and in addition, they almost never had any type of main electrical disconnect. The National Fire Protection Association requires that all electrical service panels have a main disconnect switch or some other disconnecting means as a safety and fire prevention feature.

In many cases the contacts inside the original switches and outlets are loose, making for a poor electrical connection and increasing the risk of an electrical fire.

Like other components of your home, such as your roof, furnace, air conditioner and plumbing, there comes a time when replacement is necessary. That time has come for knob and tube wiring.  Replacing and updating your knob and tube wiring is just one of the many residential electrical services we can provide.  With our years of residential electrical installation experience and expertise, we can assist you in bringing your home up to today’s NEC (National Electrical Code) requirements and to the standards of the National Fire Protection Association.\

Nowadays, Home owners with knob and tube wiring may find it difficult or impossible to obtain insurance on their home because most insurance companies are reluctant to insure a house they perceive as risky. Insurance companies usually require a certificate of inspection  and compliance from a licensed electrician, that all knob and tube has been removed and replaced with modern 3 wire grounded circuits before it will insure a home that previously had knob and tube wiring.  After the electrician rewires your home, they  give you a satisfactory assessment of your home, and the  insurance company will consider giving an insurance policy for your house.


ALUMINUM WIRING

When first used in branch circuit wiring, aluminum wire was not installed any differently than copper. Due to increased copper costs in the mid 1960's, aluminum wiring became more prevalent in wiring homes. It was known at the time that aluminum wire requires larger wire gauge than copper to carry the same current.

For example, a standard 15-amp circuit breaker wired with No. 14 gauge copper requires No. 12 gauge aluminum. Typical connections from electrical wire to electrical devices, also called terminals, are usually made by wrapping the wire around the screw terminals and tightening the wire or pushing the wire through the back of the outlet. Over time, many of these terminations to aluminum wire began to fail due to improper connection techniques and dissimilar metals. These connection failures generated heat under electrical load and resulted in overheated connections.

History of Aluminum Wire

Electricity is transmitted from the utility generating stations to individual meters using almost exclusively aluminum wiring. In the U.S., utilities have used aluminum wire for over 100 years. It takes only one pound of aluminum to equal the current carrying capacity of two pounds of copper. The lightweight conductors enable the utility to run transmission lines with half the number of supporting structures. The utility system is designed for aluminum conductors, and utility installers are familiar with installation techniques for the types of aluminum conductors used in utility applications. Prior to 1972, the aluminum wire was manufactured to conform to 1350 series alloy. This alloy was specifically designed for power transmission purpose. Due to its mechanical properties the 1350 alloys were not suitable for use in branch circuitry. At this juncture in time a "new technology" of aluminum wire was developed, known as AA-8000 series which is the current aluminum wire used today for branch circuitry, however it is extremely rare to find in branch circuit wiring. This type of wire when installed properly can be just as safe as copper wire.

Problems with Aluminum Wires

Aluminum wires have been implicated in house fires in which people have been killed. Reports of fires with aluminum wiring generally show that poor workmanship led to failures. Poorly made connections were too often the cause. There were several possible reasons why these connections failed. The two core reasons were improper installation and the difference between the coefficient of expansion between aluminum wire and the termination used in the 1960's.

Feeder and branch circuit wiring systems were designed primarily for copper conductors. Aluminum wiring was evaluated and listed by Underwriters Laboratories for interior wiring applications in 1946; however it was not used heavily until 1965. At that time copper shortages and high prices made the installation of aluminum branch circuit conductors a very attractive alternative. At the same time, steel screw became more common than brass screws on receptacles. As aluminum wire was installed more frequently, the industry discovered that changes were needed to improve the means of connecting and terminating smaller aluminum wire. Installation methods for utility grade aluminum, or series AA- 1350 alloy were also different and workmanship was an important factor in making reliable connections.

The most often identified culprits for poor workmanship involved: incorrectly tightened connections, wires wrapped the wrong way around the binding screws, and aluminum conductors used in push-back connections or with devices meant only for copper. Because the connections were made incorrectly, a chain of events of failures erupted. The connection was loose to begin with due to improper tightening torque, and the physical properties of aluminum / steel interface tended to loose the connection over time. Aluminum and steel have significantly different rates of expansion which would increase the resistance and temperature at the termination point. Similar problems occurred when aluminum conductors were incorrectly terminated in the push-in connections intended only for copper wire.

Corrosion is often cited as a contributing cause of aluminum connections. In 1980 the National Bureau of Standards performed a study to determine what caused the high resistance at aluminum / steel connections in receptacles. The study revealed that the formation of intermetallic compounds (alloys of aluminum and steel) caused the high resistance terminations, not corrosion or aluminum oxide. The thin, protective layer of oxide on aluminum conductors contributes to the excellent corrosion resistance of aluminum. When terminations are made correctly, the oxide layer is broken during the termination process allowing the necessary contact to be made between the conducting surfaces.

One of the most fundamental principles of electrical safety for wiring buildings is that high temperatures are hazardous. Heat is a major contributor to potential electrical hazards. A compromised connection creates additional heat. The additional heat contribution can "snowball" problems. Sometimes if sufficient heat is created, it can start a fire. Even if the heat does not directly start a fire, the heat can melt and or burn away insulation, which can create a short that may arc. Electrical arcs often reach temperatures in excess of 10,000 Fahrenheit. Aluminum wired connections in homes have been found to have a very high probability of overheating compared to copper wired connections.

Upgrading aluminum wired homes

There are several "upgrades" that are commonly done to homes with pre-1974 aluminum branch circuit wiring:

• Ensuring that all devices are rated for use with aluminum wire. Many are not, since they do not meet the CO/ALR specification

• "Pigtailing" which involves splicing a short length of copper to the original aluminum wire for use with devices not CO/ALR rated

• COPALUM a sophisticated crimping system that creates a cold weld between copper and aluminum wire, and is regarded to be a permanent, maintenance free repair. These connections are sometimes too large to be installed in existing enclosures. Surface enclosures or larger enclosures may be installed to remedy this problem.

• Completely rewiring the house with copper instead.

When deciding to repair or replace any electrical installation, a qualified professional should be consulted. The majority of homes wired with the general purpose circuits wired with aluminum are now over 30 years old. The likelihood of experiencing any problems unique to having aluminum is slight.

Any electrical system should be evaluated every 10 years by a qualified electrical professional to determine if it is likely to operate safely under the increased loads in different rooms being used differently, i.e. home office or bathrooms with larger dryers.

 

 

 

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