By Alan Trauger

Knob and tube wiring was a system of open electrical conductors supported on ceramic knobs. It also used ceramic knobs where wires cross each other or pass through framing. This type of wiring was in common use from the 1880′s to 1930′s, in North America.

During the era when knob and tube was in common use, a typical home would have 30 or 60 amp electrical service, sufficient for general lighting and maybe a radio. These were the days of “iceboxes”, wood or gas stoves, coal heat, and no air conditioning. Wiring homes was a new trade, with very few rules.

The first home wiring rules appeared around the turn of the century, when Terrell Croft wrote the American Electricians Handbook. Crofts books defined early wiring practices. One of Croft’s rules was that knob and tube wiring should always be run through, never over the house framing. The problem with running wiring over framing is that it can get bumped, snagged, and damaged. Most electrical experts would agree that wiring run over the framing is a sign of amateur, cobbled-together work, and it never conformed to any electrical code.

Good Points

The theoretical advantage of knob and tube wiring is that it dissipates heat into free air, and therefore has a higher ampacity than cable systems with equivalent wire size. When originally installed in the 1900′s knob and tube wiring was less expensive than other wiring methods. Due to the installation cost, owners and electricians would opt for knob and tube versus conduit wiring and metal junction boxes. The conduit methods were known to be of better quality, but their cost was significantly higher than knob and tube wiring.

Modern wiring methods assume two or more load carrying conductors will lie against each other, for instance the standard non-metallic – 2 cable. Since the load carrying wires are in close proximity, when they heat up, the heating is shared across the wires, limiting the overall current load they can support. Since the load carrying wires in knob and tube wiring are widely spaced, the wires are capable of carrying higher loads without risk of fire.

Bad Points

Knob and tube wiring was commonly insulated with cotton cloth and soft rubber, in addition to the porcelain insulators. Although the actual wire covering may have degraded over the decades, the porcelain insulators have a very long lifespan and will keep any bare wires safely insulated. These same porcelain insulators are commonly used with bare wire electric fencing for livestock. These insulators can carry very high voltage surges without the risk of shorting to ground. Many electricians understand that by running the hot and neutral conductors farther apart from each other than today’s electrical cabling does make shorts between those conductors less likely, at the same time realizing there is not a grounding conductor.

Wiring installation standards and methods were less stringent in the era of knob and tube versus modern day electrical codes and standards. The main shortcomings of knob and tube include the following: lack of a safety grounding conductor, inability to confine switching to the hot conductor, the permitted use of in-line splices in walls without a junction box. Compared to modern thermoplastic wiring insulation, typically known as “romex”, the knob and tube wiring is less resistant to damage.

Knob and tube wiring is very vulnerable to mechanical damage, such as from stored materials in the attic or basement. Bending the wire can cause older insulation to crumble and fall off the wires.

Knob and tube is insufficient for current usage patterns. Contemporary households use a wide range and intensity of electrical equipment that was unforeseen at the time of knob and tube inception. Household power use increased following World War II, first generation wiring systems became susceptible to abuse by homeowners, who would avoid repeated blown fuses by overfusing the circuits, thus subjecting the wiring to heat damage due to higher levels of current.

In many instances deterioration and abuse have rendered the wiring unsafe. During building renovations and remodeling wiring can become damaged. The rubber insulation will become dried out, brittle when handled, damage by rodent, or simple carelessness – for example hanging objects off wiring in accessible places like attics and basements.

Covering knob and tube wiring with thermal insulation is unsafe and prohibited. This is a known fire hazard and it is specifically condemned in the National Electrical Code, Section 324-4. No form of insulation – loose, blown-in, or expanding foam insulation can be placed over the wiring. Knob and tube wiring was designed to let heat dissipate to the surrounding air. As a result, energy efficiency upgrades that involve insulating previously uninsulated walls usually also require replacement of the wiring in affected homes.

Homeowners insurance underwriters may refuse and deny coverage for homes and buildings that have this type of wiring installed.

Alan Trauger is a Building Consultant that performs property condition assessments for residential and commercial properties. An experienced and knowledgeable problem solver, understanding processes and issues related to building structures and their systems. An expert witness, trainer, and educator. To view past newsletters on construction and buildings http://newsletters.alantrauger.com/

To review authors bio, qualifications, and interest in receiving future email newsletters http://www.alantrauger.com

By Alan Trauger

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.

Alan Trauger is a Building Consultant that performs property condition assessments for residential and commercial properties. An experienced and knowledgeable problem solver, understanding processes and issues related to building structures and their systems. An expert witness, trainer, and educator. To view past newsletters on construction and buildings
http://newsletters.alantrauger.com/
To review authors bio, qualifications, and interest in receiving future email newsletters http://www.alantrauger.com