The month of Junes newsletter is the second of a three part series on Residential Electrical Wiring. This months article will discuss the two types of protection systems; Armour coating (BX Cable), or Non-metallic coating (generically referred to as Romex), sheathing systems that are the commonly used methods for protecting residential electrical wiring from external damage. Brief histories of the significant developments of these sheathing methods will lead us up to today’s standards. Also found in the June letter will be information on Aluminum Wiring, and the late 1800’s and early 1900’s method of wring known as Knob and Tube. Knob and tube can still be found in use today so I suggest it is something that a basic knowledge of is helpful.

In performing my research for these articles I found a wire industry trade association. The association has formulated a list of the “Top Ten Residential Electrical Wiring Concerns.” The July newsletter ties the first two newsletters together and discusses and expands upon this “Top Ten” list. The article focuses on concerns and dangers around houses with older electrical wiring systems.

All previous news letters can be found at the Inquiring Eye website, They are located under the “About Us” pulldown at “Newsletters,” and also as an attachment of the specific topic the newsletter addresses, for example this series will be found in Anatomy of a Home in the Electrical Section..

Residential Electrical Wiring Part 2:
The first interior wiring systems used bare wire conductors, or wiring covered in cloth which was stapled to the house framing, or to “running boards.” When the wiring needed to pass through walls the wire was protected by cloth tape. Connections or splices were made using the same techniques as those for telegraph connections. Underground wiring conductors were wrapped in cloth, soaked in pitch, and laid in wooden troughs. These early wiring methods were very labor intense and had a high risk of fire and electrocution.

Knob & Tube
The first standardized residential wiring systems were an “open” wiring system more commonly referred to as “knob and tube.” Knob and tube was in use in North America from about 1880 to the 1930’s. This wiring system consisted of a single conductor that was run through wall and ceiling cavities between the framing members of a house. The individual conductors were spaced apart at least 2-1/2 inches, or on the opposite sides of framing, to protect against short-circuits. In addition to keeping the wires spaced apart, knobs and cleats also separated the wiring from framing members or wet surfaces.

The earliest knob and tube wiring consisted of a solid copper wire that was insulated with a thick wrap of brown paper. The paper insulation was then covered in a thin layer of cotton thread. To protect the wiring as it passed through framing members the wire was passed through a ceramic porcelain “tube.” Ceramic porcelain “knobs” and cleats were nailed to the framing to support the wire. Knob and tube wiring systems began being phased out in the 1930’s, probably because of the increased cost of installing two conductors versus the every increasingly more popular non-metallic and armored cable systems. For home inspection purposes the largest single safety issue is that there is no grounding wire for knob and tube. Without a method of grounding, human contact can risk electrocution. By today’s standards knob and tube is a very unsafe wiring system. Knob-and-tube wiring is STILL allowed in the most recent electrical code

Armored Cable
Armored cable (type AC) or what it is more commonly referred to as, BX, was first listed in 1899 by the Sprague Electric Co. When it was first conceived there were two experimental versions of this product. One version was called “AX” and the other was ”BX” with the “X” standing for experimental. The latter or BX version was the method that eventually got produced and the name BX stuck. The name BX is much like going and Xeroxing a copy. “Xeroxing” or “BX” are names of a specific manufacturer, but the dominance of these companies within their industries lead to these names being adopted industry wide when referring to making a copy or armored-cable. The trade name BX is the commonly used and is the registered trade name for armored cable, by General Electric which purchased Sprague Electric.

Armored cable, or BX, did not start gaining in popularity until the early 1930’s, and is still popular and commonly used today. The early BX did not contain a separate grounding conductor. It was felt that the steel armor itself would provide an adequate grounding path. A thin strip of aluminum running inside the armor was present but not directly connected to an appliance to provide grounding protection. The armor of AC cable systems can provide an adequate grounding path. When a grounding conductor is not present it is possible to “Bond” the armored coating. Bonding is connecting two or more items, armored coatings in this example, that will provide conductivity by connecting to a grounding source. If a break is present any where in these connections it is possible for a branch circuit to not be properly grounded when it is bonded. The internal wiring itself was the same as described earlier. The originally manufactured armor was made of steel, but in the 1980’s the steel was changed to lightweight aluminum.

Romex or Nonmetallic Cable
The earliest history of non-metallic-sheathed cable, or NM for short, indicates its start as the early 1920’s. Created by the General Cable Co. at their Rome, NY plant the cable would be marketed by the trade name “Romex.” Non metallic cable today is still referred to as Romex. The earliest forms of NM cable had the conductors jacket wrapped in a cotton braid that was saturated with either an asphalt based substance or varnish to protect the wiring from moisture penetration.

In the years after 1950 synthetic rayon replaced the cotton thread in the jacket. By the early 1960’s the spun rayon was changed to a thermoplastic, and by about 1970 the thermoplastic on almost all Romex wiring was changed again to a PVC outer jacket. It was still permissible, until 1984, to use the rayon braid for conductor protection. 1984 also was the year that saw the jacket being changed to an “NM-B” rating for the outer jacket of 90 degrees Celsius.

Until the early 1960s most residential NM wiring did not provide a grounding conductor. The changes in the 1962 National Electrical Code, however, now mandated equipment grounding for all branch circuits, so out of necessity NM cable now supplied an equipment ground. The earliest forms of this grounding line were usually smaller than the conductor size itself. The 1969 code changes now required that the grounding conductor match the size of the conductor itself.

Aluminum Wiring
In the mid 1960’s the price of copper increased to a point where alternatives were sought for electrical wiring. Aluminum fit the bill as an alternative and was used extensively until the mid 70’s. At the outset, the only perceived drawback with aluminum wiring was that aluminum wire requires a larger gauge (bigger wire) to carry the same amount of electrical current. So, at the outset aluminum wiring, for branch electrical circuits, was not installed any differently than copper. Terminal connections were made by wrapping the wire around a screw connection, or twisted together. Over time though, many of these aluminum connections began to fail. The problem was a couple fold. First, when electricity passes through aluminum wiring it warms up, and the aluminum wire expands. When the wire cools, it contracts. When aluminum wiring goes through enough of these warming and then cooling cycles it loses a bit of its firmness against its connection as it cools (aluminum and copper have different metallurgical properties. Aluminum will cold-flow much more readily than will copper). Another problem occurred when an aluminum wire was connected to a metal other than aluminum. The issue is that aluminum oxidizes, or corrodes, when in contact with other metals. This corrosion, called galvanic response, in turn creates a greater resistance which may cause the connection to get very hot, and possibly cause a fire. As this cycle continues the corrosion will increase, again, making the cycle worse. When aluminum wiring is identified in a house it is vital that a licensed electrician be brought in to fully evaluate the wiring and its connections. When installed properly, the wiring presents little risk. BUT only a qualified licensed electrician should make these determinations.
To learn more about electrical, visit our website's Anatomy of a Home section.  View part I or part III

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