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Knob and Tube Wiring
Need a knob and tube expert?
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 joists or 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. |
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. You may only have two prong outlets to plug
into. Basically, no ground at each outlet or
fixture outlet means knob and tube wiring is present, likewise if you have older
pushbutton switches, this is also a good
sign you may have knob and tube
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 not likely to insure a house they perceive as high
risk. 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.
B. Overview of Knob-and-Tube Wiring
Knob-and-Tube wiring was the predominant wiring system through the 1920 s and
1930 s; some installations of knob-and-tube
wiring continued in houses up until 1950. There are several distinguishing
characteristics of knob-and-tube wiring in
comparison to current wiring methods:
When running perpendicular to structural components (such as floor joists),
modern wiring runs directly through holes in the
components. Knob and tube wiring used protective ceramic tubes placed in the
holes to prevent the wire from chafing against
the structure.
Modern wiring uses staples to hold the wiring against structural components when
the wire is running parallel to the
component. Knob-and-tube wiring used ceramic knobs to clamp the wire to the
structural member.
Connections between modern wires are completed within enclosed electrical
junction boxes. Knob-and-tube wiring had visible
connections. The wires were spliced and soldered together and then wrapped with
electrical tape. These connections are called
pig-tail connections because one wire is wrapped several times around the other
wire before the two are soldered together.
Ceramic knobs were strategically placed to protect the splice ensuring that
inadvertent tugging on the wire would not stress
the electrical connection.
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In modern wiring, the hot wire (black) and neutral wire (white), along with a
ground wire, are insulated separately and
bundled in a single plastic sheathing. In knob-and-tube wiring, the hot and
neutral were insulated and run through a house
separately, usually several inches apart. (3 inches is the minimum separation
prescribed by the NEC). Knob-and-tube wiring
did not include a ground wire.
In a modern system, many branch circuits use 14 gauge conductors protected by a
15amp circuit breaker. Larger, 12 gauge
conductors are required for 20 amp circuits. Knob-and-tube wiring typically
consists of 12 gauge conductors.
While the differences are considerable, there is nothing inherent in
knob-and-tube wiring that makes it dangerous.
Knob-and-tube wire, properly installed, is not inherently a problem. While
opinions regarding the safety of knob-and-tube
wiring vary widely, the concerns are not with the original wiring, but rather
with what has happened after the fact.
Older homes with knob-and-tube wiring were often supplied with 60-amp service at
the main electrical panel. They were also
subject to limited distribution in two forms: (1) limited number of circuits,
and (2) limited number of electrical outlets
per room. Both of these factors opened knob-and-tube wiring to potential abuses
of the electrical system after the initial
installation.
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