1. FYI: ROMEX is a generic electrical term for nonmetallic sheathed cable. The name "Romex" is a registered trademark of General Cable Corporation.

  2. NEC, Article 336 covers the use, installation and construction specifications for nonmetallic-sheathed cable.

    Nonmetallic-sheathed cable may be used for either exposed or concealed wiring. Where exposed, it should not be subject to physical damage. The basic advantages of nonmetallic-sheathed cable are that the outer sheath provides continuous protection in addition to the insulation applied to the conductors; the cable is easily fished in partitions of finished buildings; no insulating supports are required; and only one hole need be bored that can accommodate more than one cable passing through a wood cross member.

  3. Definition: Nonmetallic-sheathed cable is a factory assembly of two or more insulated conductors having an outer sheath of moisture-resistant, flame-retardant, nonmetallic material. NEC 336-2.

  4. Romex Use Limitation, NEC 336-5: Not to be used in buildings that are more than 3 stories in height or as Service-Entrance Cable.

    Note! Some State and city jurisdictions limit, or even prohibit, the installation of nonmetallic sheathed cable. Always check with the local building department- just in case.

  5. Romex "let through" withstand current rating is less than 10,000 amp. If the AIC rating, supplied by the Utility Company for the building exceeds 10,000 amps, then wiring should be in conduit or "current-limiting fuses" should be installed. Circuit breakers are not adequate (too slow opening the circuit). See Table of "Withstand Current Ratings for Wiring":

  6. Conductors, NEC 336-30(b): Shall be one of the insulated types listed in NEC, Table 310-13. The ampacity of Types NM & NMC cables shall be that of 60C (140F) conductors.

  7. Cable shall be continuous for outlet to outlet. All splices should be made in light boxes, junction boxes, receptacle boxes or switch boxes.

  8. See NEC, Table 310-16 for wires current carrying capacity in amps.

  9. Bundled Cables: Where stacked or bundled longer than 24" without maintaining spacing, and not installed in raceways, the allowable ampacity for each conductor shall be reduced per NEC, Table 310-16.

  10. Markings, NEC 336-11: Distinctive marks specifying cable type shall be on the exterior of cable's entire length. Cables that are flame-retardant and have limited smoke features can be identified with the suffix "LS".

  11. Romex has 2, 3 or 4 conductors with a green insulated or bare conductor used for grounding equipment.

  12. Type "NM" cable has a flame-retardant and moisture-resistant outer jacket and is restricted to inside wiring, either in exposed or concealed locations. NEC 336-4(a).

  13. Type "NMC" cable has a flame-retardant and moisture-resistant outer jacket and also is fungus-resistant and corrosion-resistant. NMC cable may be used for outside wiring. NEC 336-3(b).

  14. Protection Against Physical Damage:
  • Romex installations shall be protected from physical damage. See NEC 336-6(b).
  • See also NEC 300-4 (protection against physical damage).
  • Nail plates are required when the cable is less than 1-1/4" 
    from stud edge. The nail plates shall be at least 1/16" thick. NEC 300-4(a) 1.
  • Romex wiring shall be clamped to electric boxes with approved NM clamp. NEC 370-17 (b, c). 
  • Bends in Romex, NEC 336-36: Shall be made in a manner that does not damage the protective covering of the cable. Bends radius should not be less than 5 times the diameter of the cable.

  • AC cable installed in accessible attics (protection):

15. Romex Wiring Support, NEC 336-18: Stapled, in a manner that does not damagethe cable, within 12" of electric boxes; every 4-1/2 ft. intervals, and clamped to box. Cables run through holes in studs, rafters of floor joists are considered supported at those points.

* Grounding with NM cable. See sketch .

* Multi-conductor cables. See sketch .

Common thermoplastic-insulated conductors
Type Max. operating temperature Application
TW 60 C (140 F) Dry and wet locations
THW 75 C (167 F) Dry and wet locations
THWN 75 C (167 F) Dry and wet locations
Conductor Type Designations:
H= 75
, 15C temperature rise over 60C
HH= 90C, 30C temperature rise over 60C
N= Outer nylon jacket or equivalent coating
R= Rubber insulation
T= Thermoplastic insulation
U= Underground use
W= Moisture resistant
X= Cross-linked synthetic polymer insulation