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Common Electrical Conductor Types
by Nick Gromicko, CMI® 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. InterNACHI inspectors should understand the basic distinctions between the different types of cable systems so that they can identify unsafe conditions.
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.
- 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 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. This article is from InterNACHI and can be found at https://www.nachi.org/conductor-types.htm.
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