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Introduction
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,
http://www.inquiring-eye.com.
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. |
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To learn more about electrical, visit our
website's
Anatomy of a Home section. View
part I or part III
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