10.3 Nonmetallic-Sheathed Cable, MC Cable & Armored Cable

10.3 Nonmetallic-Sheathed Cable, MC Cable & Armored Cable

While raceway systems provide maximum physical protection and adaptability, factory-manufactured cable assemblies represent the vast majority of branch-circuit wiring in residential, commercial, and light-industrial facilities. Cable assemblies package phase conductors, neutrals, and equipment grounding paths within a single protective outer jacket or flexible metallic sheath, dramatically accelerating installation speed and reducing labor costs.

However, each cable assembly family possesses distinct mechanical limitations, fire-resistance boundaries, and code-restricted installation environments. On the Kentucky Journeyman Electrician examination, questions frequently focus on Type NM (Nonmetallic-Sheathed / Romex), Type MC (Metal-Clad), Type AC (Armored Cable / BX), Type TC (Tray Cable), and Type UF (Underground Feeder). Electricians must know exactly where each cable is permitted, where it is strictly prohibited, its support spacing rules, and its ampacity termination boundaries.

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1. Nonmetallic-Sheathed Cable (Type NM, NMC, NMS — NEC Article 334)

Type NM cable, universally known in the trade by the brand name Romex, is the dominant wiring method in modern residential construction. Article 334 recognizes three distinct sub-types:

• Type NM: Standard nonmetallic-sheathed cable with a flame-retardant, moisture-resistant nonmetallic outer jacket. Permitted for both exposed and concealed work in normally dry locations.
• Type NMC: Corrosion-resistant nonmetallic-sheathed cable with a flame-retardant, moisture-resistant, fungus-resistant, and corrosion-resistant jacket. Permitted in damp, corrosive, or dry locations.
• Type NMS: Hybrid cable containing power conductors alongside signaling and data communications cables under a common sheath.

+---------------------------------------------------------------------------------------------------+
|                             NONMETALLIC-SHEATHED CABLE (TYPE NM-B)                                |
|                                                                                                   |
|  Outer Sheath: Moisture-resistant, flame-retardant PVC jacket                                     |
|  Color-Coding: White = 14 AWG, Yellow = 12 AWG, Orange = 10 AWG, Black = 8/6 AWG                 |
|                                                                                                   |
|  Internal Conductors: Rated 90°C (Type THHN equivalents)                                          |
|  AMPACITY MANDATE (NEC 334.80): Sized strictly to the 60°C column of Table 310.16!               |
|  DERATING PROVISION: May use 90°C column as starting point for adjustment & correction!           |
+---------------------------------------------------------------------------------------------------+

Uses Permitted (NEC 334.10)

Type NM cable is permitted in the following structures:

1. One- and Two-Family Dwellings: And their attached or detached garages and storage buildings.
2. Multifamily Dwellings: Permitted in multifamily dwellings authorized to be of Types III, IV, and V construction.
3. Other Structures: Permitted in commercial and institutional structures permitted to be of Types III, IV, and V construction, provided that cables are concealed within walls, floors, or ceilings that provide a thermal barrier of material having at least a 15-minute finish rating (such as standard 1/2-inch drywall).

Uses Not Permitted (NEC 334.12)

Type NM cable shall NOT be used under any of the following conditions:

• In Any Dwelling or Structure Not Specifically Permitted: Prohibited in commercial structures of Type I and Type II fire-resistive construction (such as steel-and-concrete high-rises).
• Exposed in Dropped Ceilings (NEC 334.12(A)(2)): Prohibited in dropped or suspended ceilings in commercial buildings, retail spaces, offices, or other-than-dwelling occupancies. (It is permitted in suspended ceilings of residential dwellings).
• Commercial Garages (NEC 334.12(A)(3)): Prohibited in commercial garages having hazardous (classified) locations governed by Article 511.
• Theaters and Similar Locations: Prohibited in theaters and similar locations (Article 520), except where 518.4(B) permits it in assembly occupancies that are not required to be of fire-rated construction.
• Wet or Damp Locations: Standard Type NM cannot be installed in wet or damp locations, outdoors, or embedded in poured concrete or masonry.
• Exposed to Physical Damage: Where subject to physical damage, such as low exposed runs across open wall studs in unfinished basements or workshops.

The 60°C Ampacity Mandate & Derating (NEC 334.80)

One of the most heavily tested provisions in Article 334 is the ampacity rule:

• Final Ampacity Limitation: Under NEC 334.80, the allowable ampacity of Type NM cable shall not exceed that of 60°C-rated conductors in Table 310.16.
• 90°C Derating Starting Point: Modern NM cable is manufactured as Type NM-B, meaning its internal conductors have 90°C-rated insulation. The 90°C rating is permitted to be used for ampacity correction and adjustment, provided the final calculated ampacity does not exceed the 60°C value in Table 310.16.

Bundling Through Framing Holes (NEC 334.80)

Where more than two NM cables containing two or more current-carrying conductors are routed through the same bored hole in wood framing members that is to be sealed with thermal insulation, caulk, or draft-stop foam:

• The allowable ampacity of each conductor must be derated according to NEC Table 310.15(C)(1) bundle adjustment factors.
• Electricians calculate derating using the 90°C rating of NM-B, but the resulting value can never exceed the 60°C column limit.

Securing and Supporting Spacing (NEC 334.30)

Proper mechanical support prevents cable weight from pulling terminations loose:

• Distance from Boxes: Must be secured within 12 inches (300 mm) of every cable box, cabinet, or termination fitting.
• Single-Gang Nonmetallic Box Exception: Where installed in a single-gang nonmetallic box without internal cable clamps, the cable must be secured within 8 inches (200 mm) of the box, and the sheath must extend at least 1/4 inch into the box (the exception in NEC 314.17 for single-gang nonmetallic boxes).
• Interval Spacing: Must be supported at intervals not exceeding 4.5 feet (1.4 m) along runs.
• Framing Penetrations: Passing NM cable horizontally through bored holes or notches in wood or metal studs at intervals not exceeding 4.5 feet constitutes legal support.
• Stapling Restrictions: Flat cables must not be stapled on edge; staples must be driven flat without crushing or piercing the outer jacket.

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2. Metal-Clad Cable (Type MC — NEC Article 330)

Type MC cable is the premier flexible wiring method in commercial, institutional, and industrial construction. Unlike NM cable, MC cable is permitted in almost all construction types, including Type I and Type II fire-resistive buildings.

                  METAL-CLAD (TYPE MC) CABLE ARCHITECTURE

     ┌────────────────────────────────────────────────────────┐
     │  Interlocked Steel or Aluminum Armor Outer Sheath      │
     │  ┌──────────────────────────────────────────────────┐  │
     │  │ Mylar / Polypropylene Clear Wrap                │  │
     │  │  (•) Black Phase Conductor (90°C THHN/THWN-2)    │  │
     │  │  (•) White Neutral Conductor (90°C)              │  │
     │  │  (•) Green Insulated Equipment Grounding Wire   │  │
     │  └──────────────────────────────────────────────────┘  │
     └────────────────────────────────────────────────────────┘

Structural Construction: Interlocked Armor vs. Smooth Tube

• Interlocked Armor: Constructed from a helically wound, interlocking metal tape of galvanized steel or aluminum. Highly flexible and standard for commercial branch circuits.
• Smooth / Corrugated Metallic Sheath: A continuous welded tube of aluminum, copper, or bronze, often utilized in heavy industrial feeds.

Equipment Grounding Path: Standard MC vs. MC-AP

1. Standard Type MC Cable: The interlocking metal armor of standard MC cable is NOT listed as an equipment grounding conductor. Standard MC cable must contain an internal insulated or bare equipment grounding conductor (typically green) sized per Table 250.122.
2. Interlocked MC Listed as an EGC (often sold as MC-AP): This cable has interlocking aluminum armor in continuous contact with an internal bare aluminum grounding/bonding conductor. Under NEC 250.118(A)(10)(b), the combined armor and that conductor are listed and identified as an equipment grounding conductor, so a separate green EGC is not needed.

Uses Permitted and Prohibited (NEC 330.10 & 330.12)

• Permitted: In branch circuits, feeders, and services; exposed or concealed; in cable trays; in dry, damp, or wet locations (when equipped with an impervious outer PVC jacket); embedded in plaster or masonry; in commercial dropped ceilings; and in hazardous (classified) locations as permitted in Chapter 5.
• Prohibited: Where subject to destructive corrosive conditions or physical damage, unless protected by an identified corrosion-resistant outer jacket.

Securing and Supporting (NEC 330.30)

• Secured at intervals not exceeding 6 feet (1.8 m).
• Cables with four or fewer conductors no larger than 10 AWG must also be secured within 12 inches (300 mm) of every box, cabinet, fitting, or other termination.
• Unsupported runs are permitted up to 3 feet (900 mm) from equipment terminals where flexibility is necessary, or up to 6 feet (1.8 m) from luminaire connections in accessible suspended ceilings.

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3. Armored Cable (Type AC / BX — NEC Article 320)

Armored Cable (Type AC), historically patented as "BX" cable, was the original flexible metal-sheathed wiring method introduced at the beginning of the 20th century. While similar in external appearance to interlocked MC cable, Type AC exhibits fundamental engineering differences.

                    TYPE AC (ARMORED CABLE) ARCHITECTURE

     Galvanized Steel / Aluminum Interlocking Armor
     │
     ├── Internal Aluminum or Copper Bonding Strip (Runs full length)
     │   - In direct physical contact with armor throughout run.
     │   - TOGETHER, armor + bonding strip form the Equipment Ground (EGC)!
     │
     ├── Individual Conductors Wrapped in Tough Moisture-Resistant Kraft Paper
     │
     └── ANTI-SHORT BUSHING ("RED HEAD") MANDATORY AT ALL TERMINATIONS!

Key Differences Between Type AC and Type MC

The Anti-Short Bushing Mandate (NEC 320.40)

Under NEC 320.40, at every termination of Type AC cable, an approved fitting must be provided with an identified insulating bushing (colloquially called a red head) installed between the conductors and the cut edge of the metal armor. The cut metal edges of interlocked armor form razor-sharp spirals that will slice through conductor insulation under building vibration unless isolated by an anti-short bushing.

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4. Tray Cable (Type TC) & Underground Feeder (Type UF)

Power and Control Tray Cable (Type TC — NEC Article 336)

Type TC cable consists of two or more insulated conductors under a flame-retardant nonmetallic jacket, designed primarily for industrial raceways and cable tray systems:

• Uses Permitted (336.10): In cable trays in industrial and commercial facilities; in raceways; outdoors where sunlight resistant; and for direct burial where identified.
• Type TC-ER (Extended Run): Features enhanced crush and impact resistance. Permitted to transition directly out of industrial cable trays and extend up to 6 feet (1.8 m) to equipment or motor disconnects without continuous raceway or tray support, provided it is mechanically secured at intervals not exceeding 6 feet.

Underground Feeder and Branch-Circuit Cable (Type UF — NEC Article 340)

Type UF cable is designed specifically for direct burial in earth without a raceway:

• Construction: Conductors are completely embedded in a solid, monolithic thermoplastic PVC jacket that bonds tightly around each conductor and bare ground wire.
• Ampacity Rating (NEC 340.80): The ampacity of Type UF cable shall not exceed the 60°C column of Table 310.16, regardless of the temperature rating of the internal conductors.
• Burial Depth (NEC Table 300.5): Direct-buried Type UF cable requires 24 inches of cover under general soil/lawns, or 18 inches of cover under residential one- and two-family dwelling driveways.
• Uses Prohibited (340.12): Prohibited as service-entrance cable; in commercial garages; in theaters and places of assembly; in storage battery rooms; or embedded in poured concrete or aggregate.