Electrical Conductor Types

Common Electrical Conductor Types

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 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.
• 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.

Knob-and-Tube (KT) Wiring

• 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.