4.3 Cable Assemblies: NM, NMC, MC, AC, TC & UF

4.3 Cable Assemblies: NM, NMC, MC, AC, TC & UF

Quick Reference:

• Type NM (Article 334): Permitted only in Types III, IV, and V construction (residential and light commercial wood frame). Prohibited in commercial Types I and II, damp/wet locations, and above suspended drop ceilings in commercial occupancies. Ampacity is restricted to the 60°C column of Table 310.16 per NEC 334.80.
• Type UF (Article 340): Direct burial underground feeder/branch cable; solid PVC extrusion. Must be sized from 60°C column per NEC 340.80. Not permitted as service-entrance cable.
• Type AC (Article 320): Armored cable with paper-wrapped conductors and internal 16 AWG bare aluminum bonding strip in contact with armor. Armor + strip forms the EGC. Anti-short bushings ("red devils") mandatory per NEC 320.40. Dry locations only.
• Type MC (Article 330): Metal-clad cable with polymeric tape wrap and separate insulated green copper EGC (standard MC armor is not an EGC). Permitted in Types I, II, III, IV, and V, dry/damp/wet (with PVC jacket), and cable trays.
• Framing Protection (NEC 300.4): Maintain minimum 1-1/4 inches (32 mm) edge distance in bored wood/metal studs. If less than 1-1/4 in, protect with a 1/16-inch (1.6 mm) steel nail plate.
• Support Intervals:
  • Type NM: Within 12 inches of box; supported every 4.5 feet.
  • Type MC: Within 12 inches of box; supported every 6 feet (or 3 feet for small MC conductors ≤ 10 AWG with 4 or fewer conductors).
  • Type AC: Within 12 inches of box; supported every 4.5 feet.

Factory-assembled multi-conductor cable assemblies streamline electrical installations across residential, commercial, and industrial facilities. However, their mechanical flexibility and specialized sheathing make them vulnerable to physical crushing, moisture ingress, and thermal overheating. For Connecticut E-2 journeypersons, mastering the strict boundary lines between Uses Permitted and Uses Not Permitted across NEC Articles 320, 330, 334, 336, and 340 is essential for both job-site code compliance and passing the licensing examination.

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1. Comparative Architecture of Cable Assemblies

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2. Nonmetallic-Sheathed Cable: Types NM & NMC (NEC Article 334)

Type NM cable (widely recognized by the trade name Romex) is the most common wiring method in residential construction. It consists of two to four insulated conductors (rated 90°C) and an uninsulated bare equipment grounding conductor encased within an extruded flame-retardant nonmetallic outer sheath.

Construction Classifications (IBC vs. NEC)

Under NEC 334.10, Type NM and Type NMC cables are strictly regulated according to building construction types defined by the International Building Code (IBC) and NFPA 220:

• Type I and Type II Construction (Noncombustible): High-rise commercial buildings, hospitals, schools, and institutional steel-and-concrete structures. Type NM cable is strictly PROHIBITED in Type I and II buildings.
• Type III Construction (Noncombustible Exterior, Combustible Interior): Buildings with masonry, brick, or concrete exterior walls and wood-framed interior joists and partitions. Type NM cable is permitted if concealed within walls or floors that provide a thermal fire-resistance barrier.
• Type IV Construction (Heavy Timber): Solid noncombustible exterior walls with massive interior timber columns and beams. Type NM is permitted.
• Type V Construction (Combustible / Wood Frame): Standard wood-framed single-family homes, duplexes, and light multifamily structures. Type NM is permitted.

Uses Not Permitted for Type NM (NEC 334.12)

Licensing exam questions frequently target prohibited installations under NEC 334.12:

1. Commercial Dropped / Suspended Ceilings: Under NEC 334.12(A)(2), Type NM cable is strictly prohibited from being installed in or above dropped or suspended ceilings in commercial occupancies (other than one- and two-family and multifamily dwellings). Running Romex above an office acoustic tile drop ceiling is an automatic failure.
2. Damp and Wet Locations: Type NM cannot be embedded in poured cement, concrete, or aggregate, nor installed in wet or damp locations, exposed outdoor weather, or underground raceways (NEC 334.12(B)(4)).
3. Commercial Garages & Hazardous Locations: Prohibited in commercial repair garages having Class I hazardous classified locations (NEC 334.12(A)(3)).
4. Theaters and Motion Picture Studios: Prohibited in assembly occupancies seating 100 or more persons governed by NEC Article 518.

The 60°C Ampacity Limitation (NEC 334.80)

Modern Type NM cable is stamped NM-B, indicating that internal conductors are insulated with 90°C-rated compounds (such as THHN). However, a unique and heavily tested code rule governs its loading:

NEC 334.80 Mandate: The ampacity of Type NM, NMC, and NMS cable shall be determined in accordance with the 60°C (140°F) conductor temperature rating in Table 310.16.

• Even though the conductors inside NM-B have 90°C insulation, you cannot load #12 NM-B to 25A or 30A. It is limited to the 60°C rating: 20 Amperes.
• The Derating Exception: NEC 334.80 allows the 90°C rating to be used as the starting point for ampacity adjustment and correction calculations, provided that the final adjusted ampacity does not exceed the 60°C column rating.

Calculation Example: NM Cable Bundling Derating

Suppose three 2-wire #12 NM-B cables (total of 6 current-carrying conductors) pass through the same bored hole in a wood top plate sealed with expanding fire-stop foam for a length exceeding 24 inches:

• Base ampacity from 90°C column (Table 310.16) for #12 Copper $= 30\text{ A}$.
• Bundling adjustment factor for 6 conductors (Table 310.15(C)(1)) $= 80% = 0.80$.
• Calculated derated ampacity $= 30\text{ A} \times 0.80 = 24\text{ Amperes}$.
• Compare to 60°C column limit: #12 Copper at 60°C $= 20\text{ Amperes}$.
• Because 24A exceeds the 60°C maximum limit, the cable ampacity is clamped at 20 Amperes, fully compliant for standard 20A overcurrent protection.

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3. Underground Feeder Cable: Type UF (NEC Article 340)

Type UF cable is designed specifically for direct-burial branch circuits and feeders running outdoors to detached garages, post lanterns, outbuildings, and landscape pumps.

Physical Characteristics & Installation Rules

• Construction: Unlike Type NM (which has a loose outer paper/sheath wrapper with air voids), Type UF is manufactured with a solid, monolithic polyvinyl chloride extrusion that completely encapsulates each individual conductor and the bare grounding wire.
• Uses Permitted (NEC 340.10): Permitted for direct burial in earth; installed underground in raceways; exposed to outdoor wet, damp, or corrosive locations; and installed as interior wiring following the same rules as Type NM.
• Uses Not Permitted (NEC 340.12):
  • Service-Entrance Cable: Type UF is strictly prohibited from being used as service-entrance cable.
  • Commercial Garages & Theaters: Prohibited in hazardous locations and assembly halls.
  • Direct Sunlight: Prohibited in direct sunlight unless explicitly stamped and listed as Sunlight Resistant.
• Ampacity (NEC 340.80): Like Type NM, Type UF cable must be sized from the 60°C column of Table 310.16.
• Minimum Burial Depths (NEC Table 300.5):
  • Direct-burial Type UF under general residential turf: 24 inches (600 mm).
  • Direct-burial Type UF supplying a 120V, 20A residential branch circuit with GFCI protection: 12 inches (300 mm).
  • Direct-burial under a minimum 2-inch concrete slab: 18 inches (450 mm).
  • Direct-burial under commercial driveways and parking lots: 24 inches (600 mm).

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4. Armored Cable (Type AC / BX) vs. Metal-Clad Cable (Type MC)

Electricians often confuse Type AC and Type MC because both feature flexible interlocking spiral metal armor. However, their internal construction, grounding mechanisms, and termination rules are fundamentally different.

   TYPE AC CABLE (Article 320 - "BX")
   +-------------------------------------------------------------+
   | [Armor] ===> Flexible Interlocking Steel or Aluminum Armor   |
   | [Bond]  ===> 16 AWG Bare Aluminum Bonding Strip (Touches Armor)|
   | [Wrap]  ===> Individual Kraft Paper Wrap on Conductors      |
   | [Ground]===> COMBINED Armor + Internal Bonding Strip = EGC  |
   | [Bush]  ===> Anti-Short Bushing ("Red Devil") MANDATORY     |
   +-------------------------------------------------------------+

   TYPE MC CABLE (Article 330)
   +-------------------------------------------------------------+
   | [Armor] ===> Interlocking Metal Tape or Smooth/Corrugated   |
   | [Wrap]  ===> Clear Polypropylene / Polyester Mylar Tape     |
   | [Ground]===> FULL GREEN INSULATED COPPER EGC (Mandatory)    |
   | [Armor EGC?]===> Standard Interlocking Armor is NOT an EGC  |
   | [Bush]  ===> Anti-Short Bushing NOT required by NEC         |
   |              (unless required by manufacturer/connector)    |
   +-------------------------------------------------------------+

Detailed Structural Comparison

The Anti-Short Bushing Rule (NEC 320.40)

When cutting spiral armor with a rotary hacksaw (Roto-Split), sharp metal burrs are left on the inside cut edge. Under NEC 320.40, an approved terminating fitting for Type AC cable must provide an insulating anti-short bushing (trade slang: "red devil") between the conductors and the armor. The internal 16 AWG aluminum bonding strip is bent back over the outside of the bushing before the connector is tightened, ensuring solid mechanical and electrical contact between the strip and connector body.

Exam Watch: Does the NEC require anti-short bushings on standard Type MC cable? No. NEC 330.40 does not mandate anti-short bushings for MC cable because MC connectors are listed with integral insulated throats. However, if the manufacturer includes bushings and specifies them in installation instructions, NEC 110.3(B) makes them mandatory.

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5. Power and Control Tray Cable: Type TC (NEC Article 336)

Type TC cable consists of two or more insulated conductors under an overall flame-retardant nonmetallic jacket, engineered for installation in industrial raceways and cable trays.

• Uses Permitted (NEC 336.10): Industrial establishments where servicing conditions ensure qualified maintenance; power, lighting, control, and signal circuits; installed in cable trays, raceways, or supported by messenger wire.
• Type TC-ER (Exposed Run): Specially listed and marked TC-ER cable possesses enhanced crush- and impact-resistant jacketing. Under NEC 336.10(10), TC-ER is permitted to transition out of cable tray and run exposed to equipment or devices, provided it is mechanically protected and supported at intervals not exceeding 6 feet (1.8 m).
• Uses Not Permitted (NEC 336.12): Prohibited where exposed to physical damage; outside of cable trays or raceways unless listed as TC-ER; and prohibited in direct sunlight or direct earth burial unless explicitly marked.

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6. Framing Protection, Bored Holes & Nail Plates (NEC 300.4)

When nonmetallic-sheathed cables or metal-clad cables are routed through wood or light-gauge metal framing members, they must be protected against puncture by drywall screws and finish nails.

          WOOD STUD CROSS-SECTION (NEC 300.4(A)(1))
          +-----------------------------------+
          |                                   |  ^ 1-1/4 in (32 mm)
          |         ( Bored Hole )            |  | MINIMUM EDGE DISTANCE
          |             (Cable)               |  v
   ======>+===================================+====== Drywall Surface
          |  If edge distance < 1-1/4 inches: |
          |  MUST INSTALL 1/16-INCH STEEL     |
          |  PROTECTIVE NAIL PLATE HERE!      |
          +-----------------------------------+

The 1-1/4 Inch Rule for Wood Framing (NEC 300.4(A)(1))

• Bored Holes: Holes bored in wood joists, rafters, or studs must be positioned so that the edge of the hole is not less than 1-1/4 inches (32 mm) from the nearest edge of the wood member.
• Notches in Wood (NEC 300.4(A)(2)): Where framing members are notched to lay cables, a steel plate must be installed before applying the wall finish.
• Steel Protective Plates: Where the 1-1/4 inch distance cannot be maintained, the cable must be protected from nail and screw penetration by a steel plate at least 1/16 inch (1.6 mm) thick and of appropriate length and width to cover the area of the wiring.

Metal Framing Members (NEC 300.4(B))

In commercial light-gauge metal stud construction:

1. Bushings and Grommets: Where cables pass through factory-punched or field-punched holes in metal studs, a listed bushing or grommet must be inserted into the hole prior to pulling cable to prevent sharp sheet-metal edges from slicing conductor insulation.
2. Steel Nail Plates: If the hole in the metal stud is positioned within 1-1/4 inches (32 mm) of the stud edge, a 1/16-inch steel plate must be installed to protect the cable.

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7. Support Intervals & Minimum Bending Radii

To prevent cable assemblies from sagging, straining under their own weight, or suffering internal insulation shearing around sharp corners, the NEC mandates strict fastening distances and minimum bending radii.

Cable Support and Securement Table

Field Note: Under NEC 314.17(C) Exception, where Type NM cable enters a single-gang nonmetallic box not larger than 2-1/4 × 4 inches mounted in a wall or ceiling without cable clamps, the cable must be secured within 8 inches (200 mm) of the box, and the cable sheath must extend into the box at least 1/4 inch (6 mm).

Minimum Bending Radii (NEC 320.24, 330.24, 334.24)

Bends made in cable assemblies must not damage the armor, sheath, or inner conductors. Bending radius is measured to the inner edge of the curve:

• Type NM and Type NMC (NEC 334.24): Minimum bending radius = 5 times the cable diameter (5 × d). For flat multi-conductor cable, the radius is calculated using the major (widest) diameter.
• Type AC Cable (NEC 320.24): Minimum bending radius = 5 times the cable diameter (5 × d).
• Type MC Cable (NEC 330.24):
  • Interlocking Armor / Corrugated Sheath: Minimum bending radius = 7 times the external cable diameter (7 × d).
  • Smooth Metallic Sheath (≤ 3/4 in diameter): Minimum bending radius = 10 times external diameter (10 × d).
  • Smooth Metallic Sheath (> 3/4 in to 1-1/2 in): Minimum bending radius = 12 times external diameter (12 × d).
  • Shielded Conductors: Minimum bending radius = 12 times overall cable diameter.

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8. Flexible Cords and Flexible Cables (NEC Article 400)

Every cable assembly covered above is a wiring method — a permanent means of getting power from one point of a structure to another. Flexible cord is something else entirely: it is a connection method, intended for equipment that moves, vibrates, or gets swapped out. The Connecticut E-2 exam tests that distinction directly, because misusing cord as building wiring is one of the most common field violations an inspector writes up.

Decoding Cord Type Letters (NEC Table 400.4)

Cord designations are built from stacked suffix letters. Read them left to right:

Worked example: SOOW decodes as extra-hard usage (S), oil-resistant jacket and conductor insulation (OO), rated for outdoor and wet locations (W) — the standard choice for portable industrial equipment on a wet or oily floor. SJTW is junior hard service (300 V), thermoplastic, weather-resistant — a lighter-duty outdoor cord.

Uses Permitted (NEC 400.10)

NEC 400.10 lists eleven permitted uses. The recurring theme is movement, interchange, or isolation, including:

• Pendants and the wiring of luminaires;
• Connection of portable luminaires, portable or mobile signs, and appliances;
• Elevator cables, and the wiring of cranes and hoists;
• Connection of utilization equipment to facilitate frequent interchange;
• Prevention of the transmission of noise or vibration;
• Appliances whose fastenings and connections are specifically designed to permit ready removal for maintenance and repair, where the appliance is identified for flexible cord connection;
• Connection of moving parts.

Uses NOT Permitted (NEC 400.12) — The High-Yield Seven

This is the list the exam draws from. In the 2020 NEC, flexible cords and cables shall not be used:

1. As a substitute for the fixed wiring of a structure;
2. Where run through holes in walls, structural ceilings, suspended or dropped ceilings, or floors;
3. Where run through doorways, windows, or similar openings;
4. Where attached to building surfaces (exception: as permitted in 368.56(B) for busway taps);
5. Where concealed by walls, floors, or ceilings, or located above suspended or dropped ceilings;
6. Where installed in raceways, except where specifically permitted elsewhere in the Code;
7. Where subject to physical damage.

The Trap Worth Memorizing: Items 1, 2, 3 and 5 all describe the same field violation from different angles — running an extension cord or SO cord through a stud bay, over a drop ceiling, or through a doorway to feed a permanently installed load. If an exam scenario has cord passing through any part of the building structure or hiding above a ceiling grid, the answer is a violation, regardless of how well the cord is protected.

Strain Relief, Splices & Identification

• Pull at Joints and Terminals (NEC 400.14): Cords must be connected so that tension is never transmitted to joints or terminals. Acceptable means include knotting the cord, winding it with tape, or — the professional method — a listed strain-relief (cord-grip) fitting.
• Splices (NEC 400.13): Hard-service and junior hard-service cords 14 AWG and larger may be repaired or spliced, provided the splice retains the electrical, mechanical, insulating, and outer-sheath properties of the original cord. Smaller cords may not be field-spliced.
• Equipment Grounding Conductor (NEC 400.23): The EGC within a flexible cord is identified by a green or green-with-yellow-stripe insulated conductor, matching the general rule of 250.119.
• Minimum Size: Per Table 400.4, 18 AWG is the smallest conductor size for the common listed cord types.