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Common Electrical Conductor Types

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                Common Electrical Conductor Types




by Nick Gromicko, Rob London and Kenton Shepard




Poorly installed and maintained electrical cables are a common cause of electrical fires in homes. Many
older homes contain wiring that is now
considered obsolete or dangerous. Inspectors
should understand the basic distinctions between
the different types of cable systems so that they
can identify unsafe conditions.

Romex Cables

Romex is the trade name for a type of electrical
conductor with non-metallic sheathing that is
commonly used as residential branch wiring. The
following are a few basic facts about Romex
wiring:

                                                  •   Romex ™
                                                      is a common type of residential wiring that is categorized by the
                                                      National Electrical Code (NEC) as underground feeder (UF) or non-
                                                      metallic sheathed cable (NM and NMC).

                                                  •   NM and NMC conductors are composed of two or more insulated
                                                      conductors contained in a non-metallic sheath. The coating on NMC
                                                      cable is non-conducting, flame-resistant and moisture-resistant.
                                                      Unlike other cables commonly found in homes, they are permitted in
                                                      damp environments, such as basements.




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                                   •   Underground feeder conductors appear similar to NM and NMC
                                       cables except that UF cables contain a solid plastic core and cannot
                                       be “rolled” between fingers.

The following NEC regulations apply to Romex conductors:

                                   •   They are not permitted in residential construction higher than three
                                       stories, or in any commercial construction.

                                   •   They must be protected, secured and clamped to device boxes,
                                       junction boxes and fixtures.

                                   •   Support devices that may damage the cables, such as bent nails and
                                       overdriven staples, are not permitted.

                                   •   NM and NMC cables should be secured at intervals that do not
                                       exceed 4½ feet, and they should be secured within 12 inches of
                                       junction boxes and panels to which they are attached. Cables that do
                                       not comply with this rule can sag and are vulnerable to damage.

                                   •   They are intended as permanent wiring in homes and should not be
                                       used as a substitute for appliance wiring or extension cords.

Note: Some communities have never allowed the use of Romex wiring in residential construction. Armored
cable is typically used in these communities.

Armored Cables (AC)

Armored cable (AC), also known as BX, was developed in the early 1900s by Edwin Greenfield. It was first
called “BX” to abbreviate “product B – Experimental,” although AC is far more commonly used today. Like
Romex cables, they cannot be used in residences higher than three stories, and the rules for protection and
support of AC wiring are essentially the same as the rules for Romex. Unlike Romex, however, AC wiring
has a flexible metallic sheathing that allows for extra protection. Some major manufacturers of armored
cable are General Cable, AFC Cable Systems, and United
Copper Systems.

Service Entry (SE) Conductors

These cables begin at the splice and enter the meter. They
are not permitted inside homes, with the exception of “style
R” SE cable that can serve as interior wiring in branch
circuits for ovens and clothes dryers. Style R cables should
be clearly marked on their jacket surfaces.

Knob-and-Tube (KT) Wiring

Most houses constructed prior to World War II were wired
using the knob-and-tube method, a system that is now
obsolete. They are more difficult to improve than modern
wiring systems and are a fire hazard. Knob-and-tube wiring

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is supported with ceramic knobs, and runs intermittently though ceramic tubes beneath framing and at
locations where the wires intersect. Whenever an inspector encounters knob-and-tube wiring, s/he
should identify it as a defect and recommend that a qualified electrician evaluate the system. The following
are a few reasons why inspectors should be wary of this old wiring system:

                                   •   The dissipated heat from knob-and-tube wiring can pose a fire hazard
                                       if the wires are enveloped in building insulation. A possible
                                       exception is fiberglass insulation, which is fire-resistant, although
                                       even this type of insulation should not cover knob-and-tube wiring.
                                       The homeowner or an electrician should carefully remove any
                                       insulation that is found surrounding KT wires.

                                   •   Knob-and-tube wiring is more vulnerable to damage than modern
                                       wiring because it is insulated with fiber materials and varnish, which
                                       can become brittle.

                                   •   Some insurance companies refuse to write fire insurance for houses
                                       with this type of wiring, although this may be remedied if an
                                       electrician can verify that the system is safe.

                                   •   Disregarding any inherent inadequacies, existing KT cable systems
                                       are likely to be unsafe because they are almost guaranteed to be at
                                       least 50 years old.

In summary, inspectors should understand the different types of conductors that are commonly found in
homes.




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