Common Electrical Conductor Types
by Nick Gromicko and Kenton Shepard
- 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.
- 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.
- 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.