9.3 Nonmetallic-Sheathed Cable (Type NM), Metal-Clad Cable (Type MC) & Armored Cable (Type AC)
Key Takeaways
- Nonmetallic-Sheathed Cable (Type NM / 'Romex') under NEC Article 334 is permitted in one- and two-family and multifamily dwellings, but is strictly prohibited in commercial drop/suspended ceilings (334.12(A)(2)), commercial garages, theaters, and wet/damp locations.
- Under NEC 334.80, Type NM-B cable conductors have 90°C insulation, but their allowable ampacity is strictly limited to the 60°C rating in Table 310.16; however, the 90°C rating is permitted as the starting point for temperature and bundling derating.
- Type NM cable must be supported within 12 inches of metal/clamped boxes, within 8 inches of nonmetallic boxes without clamps, and at intervals not exceeding 4.5 feet along framing members (NEC 334.30).
- Metal-Clad Cable (Type MC - Article 330) uses interlocking tape or smooth/corrugated tube and requires a dedicated insulated equipment grounding conductor; standard interlocked armor alone is not listed as an equipment ground.
- Armored Cable (Type AC / 'BX' - Article 320) incorporates an internal 16 AWG aluminum bonding strip in continuous contact with the armor to form the grounding path, and mandatory anti-short bushings ('red heads') must be installed at all termination fittings (NEC 320.40).
Nonmetallic-Sheathed Cable (Type NM), Metal-Clad Cable (Type MC) & Armored Cable (Type AC)
Cable assemblies are prefabricated wiring systems containing insulated circuit conductors encased in an overall nonmetallic or metallic sheath. They offer substantial labor savings over conduit raceways but are subject to strict code limitations regarding structural fire classification, building occupancy, environmental moisture, grounding return paths, and support spacing. On the Oklahoma Journeyman Electrician examination, questions frequently focus on the precise distinctions between Type NM, Type MC, Type AC, and Type UF cables.
1. Nonmetallic-Sheathed Cable (Type NM / "Romex" - NEC Article 334)
Type NM cable consists of insulated conductors enclosed within an overall flame-retardant, nonmetallic thermoplastic jacket.
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| TYPE NM CABLE (NEC ARTICLE 334) OVERVIEW |
| |
| [Permitted Uses (NEC 334.10)] [Prohibited Uses (NEC 334.12)] |
| - One- and two-family dwellings - Dropped / suspended ceilings in |
| - Multifamily dwellings (Types III, commercial buildings (334.12(A)(2))|
| IV, and V construction) - Commercial garages & theaters |
| - Exposed or concealed in dry locations- Wet or damp locations (outdoors) |
| - Fished into existing walls - Embedded in poured concrete |
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The 60°C Ampacity Limitation Rule (NEC 334.80)
Modern nonmetallic-sheathed cable is manufactured as Type NM-B, featuring conductor insulation rated for $90^\circ\text{C}$ (THHN). However, NEC 334.80 establishes a crucial restriction:
"The allowable ampacity of Types NM, NMC, and NMS cable shall not exceed that for 60°C (140°F) rated conductors and shall comply with 310.14."
- Termination Ampacity: Sized using the 60°C column of Table 310.16 (#14 Cu = 15A, #12 Cu = 20A, #10 Cu = 30A, #8 Cu = 40A, #6 Cu = 55A).
- Derating Advantage: The 90°C rating is permitted to be used as the starting baseline when applying ambient temperature correction and raceway/bundling adjustment factors, provided the final derated ampacity does not exceed the 60°C limit.
Securing and Supporting Intervals (NEC 334.30)
- Distance from Enclosure: Must be secured within 12 inches (300 mm) of every cable entry into a box, cabinet, or fitting with internal clamps.
- Nonmetallic Boxes Without Clamps (314.17(C) Exception): Must be secured within 8 inches (200 mm) of single-gang nonmetallic boxes without internal clamps.
- Interval Along Framing: Supported and secured at intervals not exceeding 4.5 feet (1.4 m) along framing members.
- Framing Support: Passing horizontally through bored holes in studs or joists at $\le 4.5$-foot intervals is considered supported.
- Bending Radius (334.24): Bends must not be less than 5 times the diameter of the cable.
2. Metal-Clad Cable (Type MC - NEC Article 330)
Type MC cable contains one or more insulated circuit conductors enclosed in an interlocking metal tape armor (steel or aluminum) or a smooth/corrugated metallic tube.
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| TYPE MC CABLE (NEC ARTICLE 330) ANATOMY |
| |
| +---------------------------------------------------------------+ |
| / Interlocking Galvanized Steel or Aluminum Armor /| |
| +===============================================================+ | |
| | Phase A (Black) Phase B (Red) Neutral (White) EGC (Green) | | |
| | [90°C THHN/THWN-2 Insulated Conductors] |/ |
| +---------------------------------------------------------------+ |
| |
| GROUNDING PATH: Requires dedicated insulated Green Equipment Grounding |
| Conductor (EGC) inside the armor! (Standard armor is NOT an EGC). |
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Grounding Mechanism (NEC 330.108)
- Standard Interlocked Armor: The interlocking metal tape armor of standard Type MC cable is NOT approved as an equipment grounding conductor by itself due to the high spiral electrical impedance of interlocked turns.
- Dedicated EGC: A separate, insulated green copper equipment grounding conductor must run inside the cable assembly.
- Type MC-AP (All-Purpose / Smart Armor): Modern listed MC-AP cables include an uninsulated aluminum grounding/bonding strip in continuous direct contact with the aluminum armor sheath, permitting the combined armor and bonding strip to serve as an equipment grounding conductor.
Permitted Applications and Support (NEC 330.10 & 330.30)
- Permitted Uses: Permitted in dry, damp, and wet locations (wet locations require a listed outer PVC jacket), exposed or concealed, in cable trays, and in environmental air plenum ceilings (NEC 300.22(C)).
- Support Intervals (330.30): Secured within 12 inches (300 mm) of every box or fitting, and at intervals not exceeding 6 feet (1.8 m) along runs.
- Luminaire / Equipment Whips: An unsupported length of up to 6 feet (1.8 m) is permitted from an outlet box to a luminaire or equipment terminal inside accessible ceilings.
- Bending Radius (330.24): Minimum 7 times the external diameter for interlocked armor.
3. Armored Cable (Type AC / "BX" - NEC Article 320)
Type AC cable consists of insulated conductors enclosed in a flexible interlocking metal armor jacket.
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| TYPE AC CABLE (NEC ARTICLE 320) ANATOMY |
| |
| +---------------------------------------------------------------+ |
| / Interlocking Steel or Aluminum Armor /| |
| +===============================================================+ | |
| | 16 AWG Aluminum Internal Bonding Strip (Continuous Contact) | | |
| | Phase / Neutral Conductors with Individual Paper Wrap |/ |
| +---------------------------------------------------------------+ |
| |
| GROUNDING PATH: Outer Armor + 16 AWG Internal Bonding Strip = EGC! |
| MANDATORY: Anti-Short Bushing ("Red Head") required at all fittings! |
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Grounding Path and the 16 AWG Bonding Strip (NEC 250.118(8) & 320.108)
- Type AC cable contains an internal 16 AWG aluminum bonding strip running longitudinally in intimate physical contact with the outer metal armor throughout its entire length.
- The combination of the outer armor and the 16 AWG bonding strip acts as an approved equipment grounding conductor (NEC 250.118(8)).
- Therefore, standard Type AC cable does NOT contain a separate green insulated ground wire.
Mandatory Anti-Short Bushings (NEC 320.40)
An approved insulating bushing (commonly called a "red head" or anti-short bushing) must be installed between the conductors and the armor at all termination fittings to protect conductor insulation from being cut or abraded by the sharp, spiral edges of the cut metal armor.
Permitted and Prohibited Uses (NEC 320.10 & 320.12)
- Permitted: In dry locations only; exposed or concealed; fished in wall cavities.
- Prohibited: In wet or damp locations, embedded in masonry/concrete, exposed to corrosive fumes, or in commercial garages.
- Support Spacing (320.30): Supported within 12 inches (300 mm) of boxes and every 4.5 feet (1.4 m) along runs.
4. Underground Feeder Cable (Type UF - NEC Article 340)
Type UF cable consists of moisture-resistant, flame-retardant insulated conductors embedded in a solid, monolithic thermoplastic jacket.
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| TYPE UF CABLE (NEC ARTICLE 340) SUMMARY |
| |
| [Permitted Uses (NEC 340.10)] [Prohibited Uses (NEC 340.12)] |
| - Direct burial in earth (Table 300.5(A)) - Commercial drop ceiling plenums |
| - Wet, damp, or corrosive locations - Service-entrance cable |
| - Interior wiring (per Art 334 NM) - Theaters and commercial garages |
| - Solar PV and agricultural buildings - Embedded in poured concrete |
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- Ampacity (340.80): Sized strictly using the 60°C column of Table 310.16.
5. Master Cable Comparison Table
| Cable Feature | Type NM (Romex) | Type MC (Metal-Clad) | Type AC (Armored / BX) | Type UF (Underground) |
|---|---|---|---|---|
| NEC Article | Article 334 | Article 330 | Article 320 | Article 340 |
| Outer Sheath | Nonmetallic PVC | Interlocking Metal Tape / Tube | Interlocking Metal Armor | Solid Molded PVC |
| Equipment Grounding Path | Bare copper wire inside | Dedicated Green EGC (or MC-AP) | Armor + 16 AWG Al Bonding Strip | Bare copper wire inside |
| Anti-Short Bushing Required? | No | No (unless required by fitting mfr) | YES (Mandatory 320.40) | No |
| Allowable Ampacity Column | 60°C Column | 75°C / 90°C Column | 75°C Column | 60°C Column |
| Support Distance from Box | 12 in. (8 in. non-clamp) | 12 in. | 12 in. | 12 in. (8 in. non-clamp) |
| Max Support Interval | 4.5 feet | 6.0 feet | 4.5 feet | 4.5 feet |
| Commercial Drop Ceilings? | PROHIBITED (334.12) | PERMITTED (Plenum-rated) | Prohibited if non-rated | PROHIBITED |
| Wet / Direct Burial Use? | PROHIBITED | Only if marked / PVC jacketed | PROHIBITED | PERMITTED (Table 300.5(A)) |
An electrician is terminating a run of Type AC (armored cable) at a 4-inch square metal junction box. Under NEC 320.40, what mandatory component must be installed at the cable termination?
An electrical contractor is bidding a tenant improvement project for a retail store located in a commercial shopping center. The plans call for running branch circuits above an accessible suspended grid tile ceiling. The apprentice suggests using Type NM-B (Romex) cable to save material costs. How does NEC 334.12(A)(2) regulate this installation?
How is the equipment grounding path provided in a standard run of interlocking metal tape Metal-Clad (Type MC) cable under NEC Article 330?
A journeyman electrician is securing Type NM-B cable runs along wood framing joists in a single-family dwelling. Under NEC 334.30, what are the maximum allowable support intervals and distance from a box equipped with cable clamps?