5.2 Cable Assemblies: NM, NMC, MC, AC & UF

5.2 Cable Assemblies: NM, NMC, MC, AC & UF

Quick Answer: Factory-manufactured cable assemblies provide rapid branch-circuit and feeder installation without on-site conduit bending. Type NM (Article 334) is restricted to residential and light commercial Type III, IV, and V buildings, capped at the 60°C ampacity column (NEC 334.80), and secured within 12 inches of boxes and every 4.5 feet. Type AC (Article 320) utilizes an internal aluminum bonding strip that works in combination with the metal armor to form the equipment ground, requiring an anti-short bushing at terminations. Type MC (Article 330) is permitted in all construction types and uses an equipment-grounding construction recognized by 250.118(10)—often an internal wire-type EGC, or a specifically listed sheath/bonding design—and is generally supported every 6 feet. Type UF (Article 340) is a solid-jacketed cable for direct earth burial, capped at 60°C ampacity.

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

Nonmetallic-sheathed cable (commonly referred to by the trade brand name Romex®) is the dominant wiring method for residential wood-frame construction in the United States and Massachusetts.

                    TYPE NM-B NONMETALLIC-SHEATHED CABLE
    +----------------------------------------------------------------+
    |  Flame-Retardant, Moisture-Resistant Outer PVC Jacket (90°C)   |
    |   +--------------------------------------------------------+   |
    |   |  Kraft Paper Wrapping / Separator                      |   |
    |   |   +-------------------+  +-------------------------+   |   |
    |   |   | Phase Conductor   |  | Bare Equipment          |   |   |
    |   |   | (Black PVC/Nylon) |  | Grounding Conductor     |   |   |
    |   |   +-------------------+  +-------------------------+   |   |
    |   |   +-------------------+                                |   |
    |   |   | Neutral Conductor |  (All internal conductors      |   |
    |   |   | (White PVC/Nylon) |   have 90°C THHN insulation)   |   |
    |   |   +-------------------+                                |   |
    |   +--------------------------------------------------------+   |
    +----------------------------------------------------------------+
    * MANDATORY AMPACITY CAP: Limited to 60°C Column per NEC 334.80!

Cable Classifications

• Type NM-B: The standard modern variant. Incorporates 90°C rated conductor insulation (designated by the "-B" suffix) encased in a flame-retardant, moisture-resistant nonmetallic outer jacket. For interior dry locations only.
• Type NMC-B: Corrosion-resistant. Encased in a solid, non-absorbing outer jacket that resists moisture, fungus, and corrosion. Permitted in damp interior environments (such as agricultural barns or dairy facilities under Article 547).
• Type NMS-B: Hybrid cable containing power conductors alongside signaling, home automation, or Class 2/3 data communication cables under a single overall jacket.

Uses Permitted (NEC 334.10)

Type NM cable is permitted in:

1. One- and two-family dwellings and their attached or detached garages and accessory storage sheds.
2. Multifamily dwellings permitted to be of Types III, IV, and V construction (as defined by the International Building Code / Massachusetts State Building Code 780 CMR).
3. Other structures permitted to be of Types III, IV, and V construction, where the cables are installed concealed within walls, floors, or ceilings that provide a thermal barrier of material that has at least a 15-minute finish rating (such as 1/2-inch gypsum drywall).

Uses NOT Permitted (NEC 334.12)

Type NM cable is strictly prohibited in the following locations—a primary source of state exam questions:

• In any dwelling or structure exceeding the building construction types permitted in 334.10 (e.g., prohibited in Type I and Type II fire-resistive commercial steel/concrete high-rises).
• As service-entrance cable.
• In commercial garages having hazardous (classified) locations (Article 511).
• In theaters and places of assembly designed for 100 or more occupants (Article 518).
• In motion picture studios and storage battery rooms.
• In hoistways or on elevators.
• Embedded in poured concrete, adobe, or aggregate.
• In wet or damp locations (plain Type NM).
• Exposed in Dropped Ceilings (NEC 334.12(A)(2)): Prohibited exposed within dropped or suspended ceilings in other than one- and two-family and multifamily dwellings. Electricians cannot run exposed NM cable above lay-in acoustical tile ceilings in commercial retail or office spaces!

The 60°C Ampacity Limitation (NEC 334.80)

Although modern Type NM-B cable contains individual conductors with 90°C insulation (THHN), NEC 334.80 contains a strict statutory limitation:

"The ampacity of Types NM, NMC, and NMS cable shall be determined in accordance with Table 310.16. The ampacity shall be in accordance with the 60°C (140°F) conductor temperature rating. The 90°C (194°F) rating shall be permitted to be used for ampacity adjustment and correction calculations, provided the final calculated ampacity does not exceed that for a 60°C (140°F) rated conductor."

Practical Exam Consequences:

• 14 AWG NM-B Copper: 60°C ampacity = 15A (Protected by 15A OCPD).
• 12 AWG NM-B Copper: 60°C ampacity = 20A (Protected by 20A OCPD).
• 10 AWG NM-B Copper: 60°C ampacity = 30A (Protected by 30A OCPD).
• 8 AWG NM-B Copper: 60°C ampacity = 40A (Even though 8 AWG THHN is 55A in the 90°C column, you CANNOT place 8 AWG Romex on a 50A electric range breaker! You must pull 6 AWG NM-B or use conduit).
• 6 AWG NM-B Copper: 60°C ampacity = 55A (Cannot be protected by a 60A breaker for continuous heating loads exceeding 44A).

Securing, Supporting & Physical Protection (NEC 334.30 & 300.4)

• Support Intervals: Type NM cable must be secured by staples, cable ties, or straps at intervals not exceeding 4.5 feet (1.4 m) along runs.
• Distance to Boxes: Must be secured within 12 inches (300 mm) of every cable entry into a cabinet, panelboard, or box equipped with internal cable clamps.
• The 8-Inch Exception for Nonmetallic Boxes (NEC 314.17(C) Exception): Where NM cable enters a single-gang nonmetallic outlet box (not larger than $2-1/4 \times 4$ inches) mounted in a wall or ceiling without internal cable clamps, the cable must be secured to the framing member within 8 inches (200 mm) of the box, and the outer cable sheath must extend into the box not less than 1/4 inch (6 mm).
• Protection from Nails and Screws (NEC 300.4(A)): Where cable passes through bored holes in wood studs, rafters, or joists, the edge of the hole must be not less than 1-1/4 inches (32 mm) from the nearest edge of the wood member. If this 1-1/4 inch setback cannot be maintained, the cable must be protected from drywall nails and screws by a steel plate at least 1/16 inch (1.6 mm) thick of appropriate length and width.

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2. Armored Cable (Type AC - Article 320)

Type AC cable (historically marketed as "BX") is an armored cable assembly engineered specifically to provide mechanical protection and an integrated ground-fault current path.

                      TYPE AC (ARMORED CABLE / BX)
    +----------------------------------------------------------------+
    |  Flexible Interlocking Galvanized Steel or Aluminum Armor      |
    |   +--------------------------------------------------------+   |
    |   |  Internal Aluminum Bonding Strip (16 AWG)              |   |
    |   |  (In intimate contact with armor along entire length)   |   |
    |   |   +------------------------------------------------+   |   |
    |   |   |  Individual Kraft Paper Wrap over conductors   |   |   |
    |   |   |   +-------------------+  +-----------------+   |   |   |
    |   |   |   | Phase Conductor   |  | Neutral Wire    |   |   |   |
    |   |   |   | (60°C/75°C/90°C)  |  | (White Jacket)  |   |   |   |
    |   |   |   +-------------------+  +-----------------+   |   |   |
    |   +---+------------------------------------------------+---+   |
    +----------------------------------------------------------------+
    * CRITICAL: Armor + Bonding Strip = Listed Equipment Grounding Conductor!

Internal Bonding Strip & Grounding Mechanism (NEC 320.108)

Type AC cable does not contain a conventional green-insulated equipment grounding wire. Instead, grounding is accomplished through a patented interaction between two physical components:

1. The outer flexible interlocking metal armor.
2. An internal 16 AWG bare aluminum bonding strip that runs longitudinally inside the armor, touching the metal spirals across its entire length.

The Engineering Principle of the AC Bonding Strip: Flexible interlocking metal armor has high inductive reactance. When high-frequency AC ground-fault current travels down the coiled spirals of the armor, the spiral acts like a choke coil (inductor), choking off fault current and preventing the upstream circuit breaker from tripping. The internal aluminum bonding strip shorts out the individual spirals, transforming the high-impedance coil into a low-impedance tubular path. Under NEC 250.118(8) and 320.108, the combination of the outer armor and the internal bonding strip constitutes the listed Equipment Grounding Conductor (EGC).

Exam Application Warning: The internal aluminum bonding strip is NOT an equipment grounding conductor by itself! It cannot be landed under a ground screw inside an outlet box. When terminating Type AC cable, the bonding strip is simply folded back over the outer armor or cut flush with the connector collar.

The Anti-Short Bushing Requirement (NEC 320.40)

When metal armor is cut with a rotary hacksaw or side cutters, jagged metal edges remain that can pierce the paper-wrapped conductors. NEC 320.40 mandates that an approved insulating anti-short bushing (commonly known in the trade as a "red devil") must be installed between the conductors and the armor cut edge at every termination point. Connectors listed for Type AC cable feature an inspection window allowing the electrical inspector to visually verify the presence of the anti-short bushing after assembly.

Securing and Supporting (NEC 320.30)

• Must be secured within 12 inches (300 mm) of every outlet box, junction box, or fitting.
• Must be supported at intervals not exceeding 4.5 feet (1.4 m).
• Permitted unsupported lengths: Up to 2 feet (600 mm) at terminals where flexibility is necessary, and up to 6 feet (1.8 m) from an accessible ceiling luminaire fitting to a box.

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3. Metal-Clad Cable (Type MC - Article 330)

Type MC cable is the heavy-duty commercial and industrial standard for branch circuits, feeders, and services.

                      TYPE MC (METAL-CLAD CABLE)
    +----------------------------------------------------------------+
    |  Interlocking Aluminum / Steel Armor OR Corrugated Smooth Tube  |
    |   +--------------------------------------------------------+   |
    |   |  Overall Clear Mylar Wrap (No kraft paper)             |   |
    |   |   +-------------------+  +-------------------------+   |   |
    |   |   | Phase Conductors  |  | Dedicated Green         |   |   |
    |   |   | (90°C THHN/XHHW)  |  | Insulated Equipment     |   |   |
    |   |   +-------------------+  | Grounding Conductor     |   |   |
    |   |   +-------------------+  +-------------------------+   |   |
    |   |   | Neutral Conductor |                                |   |
    |   |   +-------------------+                                |   |
    |   +--------------------------------------------------------+   |
    +----------------------------------------------------------------+
    * VERIFY THE LISTING: ordinary interlocked armor alone is not automatically an EGC.

Key Distinctions: Type MC vs. Type AC

Listed MC Grounding Constructions

Type MC is available in more than one grounding construction. Common products contain a wire-type EGC. Other listed designs use a metallic sheath alone where the applicable 250.118(10) provision recognizes it, or use an interlocked armor in combination with one or more uninsulated conductors identified as the EGC. Read the cable marking, listing, and tested-code provision; visual similarity to another MC product does not establish the grounding path.

Luminaire Whip Exception (NEC 330.30(D))

Type MC cable is permitted to be installed unsupported from an accessible junction box to a luminaire (light fixture) inside an accessible suspended ceiling for lengths not exceeding 6 feet (1.8 m). This allows commercial fluorescent and LED troffers to be relocated during tenant office fit-outs without pulling new home runs.

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4. Underground Feeder Cable (Type UF - Article 340)

Type UF cable is an engineered nonmetallic cable assembly designed specifically for underground burial and exterior moisture-laden environments.

Construction & Listing

Unlike Type NM cable, where insulated conductors are loosely sheathed inside a hollow PVC jacket with paper fillers, Type UF cable is manufactured as a solid, monolithic plastic extrusion. Each conductor and the bare equipment grounding wire are individually embedded within continuous, solid moisture-resistant polyvinyl chloride. There are zero paper separators or air voids to wick water.

Uses Permitted (NEC 340.10)

• Direct burial in the earth with appropriate cover depths per NEC Table 300.5 (typically 24 inches for residential circuits, or 12 inches for residential 120V 20A GFCI-protected branch circuits).
• As interior premises wiring in wet, damp, or corrosive locations (e.g., stables, car washes).
• Exterior runs exposed to the sun, provided the cable outer jacket is marked "Sunlight Resistant".

Uses NOT Permitted (NEC 340.12)

• As service-entrance cable.
• In commercial garages, theaters, motion picture studios, or storage battery rooms.
• In hoistways or embedded in poured concrete/aggregate.
• Ampacity Limitation (NEC 340.80): Exactly like Type NM cable, the ampacity of Type UF cable is strictly limited to the 60°C column of Table 310.16, regardless of the insulation rating of the internal conductors.

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5. Comprehensive Cable Assembly Comparison Table

+--------------------------------------------------------------------------------------------------+
|                             CABLE ASSEMBLY SPECIFICATION MATRIX                                  |
+-----------+--------------+------------------+------------------+-----------------+---------------+
| Cable Type| NEC Article  | Temp Column Cap  | Support Interval | Box Support Dist| Allowed Types |
+-----------+--------------+------------------+------------------+-----------------+---------------+
| Type NM   | Article 334  | 60°C (334.80)    | 4.5 feet (1.4 m) | 12 in (8 in NM) | Types III-V   |
| Type NMC  | Article 334  | 60°C (334.80)    | 4.5 feet (1.4 m) | 12 in (8 in NM) | Types III-V   |
| Type AC   | Article 320  | Per conductor and terminal rules | 4.5 feet (1.4 m) | 12 inches | Types I-V |
| Type MC   | Article 330  | 75°C / 90°C      | 6.0 feet (1.8 m) | 12 inches       | Types I-V     |
| Type UF   | Article 340  | 60°C (340.80)    | 4.5 feet (1.4 m) | 12 inches       | Direct Burial |
+-----------+--------------+------------------+------------------+-----------------+---------------+

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6. Common Massachusetts Exam Traps: Cable Assemblies

• Trap 1: Confusing Grounding in AC vs. MC. Question: "An electrician cuts Type AC cable. What must be done with the internal 16 AWG aluminum bonding strip?" Answer: Wrap it back or cut it off. It is NOT connected to the green ground screw! Do not confuse the AC bonding strip with a wire-type EGC in a common MC construction.
• Trap 2: Sizing 8 AWG NM-B for a 50A Range. Contractors routinely install 8/3 NM-B on a 50A circuit breaker because 8 AWG THHN is rated 55A at 90°C. Under NEC 334.80, Type NM is legally capped at the 60°C column: 40A. Installing 8 AWG NM-B on a 50A breaker is a severe Code violation!
• Trap 3: Running NM Cable Exposed Above Commercial Drop Ceilings. Under NEC 334.12(A)(2), NM cable cannot be run exposed in dropped ceilings of commercial buildings (retail stores, offices, clinics). In commercial spaces, you must transition to MC cable, EMT, or other Chapter 3 raceways.
• Trap 4: Anti-Short Bushings on MC Cable. While anti-short bushings are universally supplied by manufacturers and considered trade best-practice for Type MC cable, NEC 320.40 specifically mandates them for Type AC cable. The NEC does not explicitly mandate anti-short bushings for standard MC cable unless specified in the listing instructions of the connector fitting.