6.2 Cable Assemblies: NM, NMC, MC, and AC

6.2 Cable Assemblies: NM, NMC, MC, and AC

Prefabricated cable assemblies represent the vast majority of branch-circuit wiring in modern residential, commercial, and multi-family construction. The National Electrical Code divides these assemblies into distinct articles with fundamentally different physical properties, allowable building occupancies, thermal ratings, and grounding mechanisms. For the Wisconsin Journeyman Electrician examination, distinguishing between Type NM (Article 334), Type AC (Article 320), and Type MC (Article 330) is one of the most heavily tested areas of Chapter 3.

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1. Nonmetallic-Sheathed Cable: Types NM, NMC, and NMS (NEC Article 334)

Type NM cable (commonly referred to by the trade brand "Romex") consists of insulated conductors enclosed within an overall nonmetallic, flame-retardant, moisture-resistant jacket. Modern installations universally utilize Type NM-B, introduced to withstand higher ambient temperatures in attics and recessed luminaires.

Construction and Conductor Insulation Rating

Under NEC 334.112, the insulated conductors in Type NM-B cable must be rated at 90°C (194°F). Conductors are typically manufactured with THHN insulation and a bare copper equipment grounding conductor.

The 60°C Ampacity Ceiling Rule (NEC 334.80)

Despite having 90°C insulation, NEC 334.80 enforces a strict thermal limitation that is tested on almost every electrical licensing exam:

The allowable ampacity of Types NM, NMC, and NMS cable SHALL NOT EXCEED the 60°C column of Table 310.16.

This means that for the final selection of overcurrent protective devices and load capacity:

• 14 AWG Copper NM-B: Rated at 15 Amperes (60°C ampacity is 15A)
• 12 AWG Copper NM-B: Rated at 20 Amperes (60°C ampacity is 20A)
• 10 AWG Copper NM-B: Rated at 30 Amperes (60°C ampacity is 30A)
• 8 AWG Copper NM-B: Rated at 40 Amperes (60°C ampacity is 40A, NOT the 50A listed in the 75°C column!)
• 6 AWG Copper NM-B: Rated at 55 Amperes (60°C ampacity is 55A, NOT the 65A listed in the 75°C column!)

The 90°C Derating Advantage (NEC 334.80)

While the final ampacity is capped at 60°C, NEC 334.80 explicitly permits the installer to use the 90°C rating as the starting ampacity for adjustment (bundling) and correction (ambient temperature) calculations, provided the final calculated ampacity does not exceed the 60°C value.

Worked Example: Bundling NM-B Cables Through a Fire-Stopped Hole

Problem: Four 12/2 NM-B copper cables pass through a single 1-inch bored hole in a wood top plate that is sealed with thermal fire-stopping foam insulation. Determine the allowable ampacity of each cable under NEC 334.80 and NEC 310.15(C)(1).

1. Count Current-Carrying Conductors: Four 12/2 cables contain 4 × 2 = 8 current-carrying conductors (grounding conductors do not count).
2. Identify Bundling Adjustment Factor: NEC Table 310.15(C)(1) mandates a 70% adjustment factor (0.70) for 7 to 9 current-carrying conductors bundled without maintaining spacing for more than 24 inches.
3. Select Starting Ampacity: Per NEC 334.80, start with the 90°C rating of 12 AWG copper from Table 310.16: $\text{Base Ampacity (90°C)} = 30\text{ Amperes}$
4. Apply Adjustment Factor: $\text{Derated Ampacity} = 30\text{ A} \times 0.70 = 21\text{ Amperes}$
5. Apply 60°C Ceiling: Table 310.16 lists the 60°C ampacity of 12 AWG copper as 20 Amperes. Because the derated value (21A) is greater than the 60°C rating (20A), the final allowable ampacity is capped at 20 Amperes.
6. Conclusion: Each circuit may remain protected by a standard 20-ampere circuit breaker! If the code had forced the calculation to begin at the 60°C column (20A × 0.70 = 14A), these cables would have been illegal on a 20A breaker.

Uses Permitted (NEC 334.10)

Type NM, NMC, and NMS cables are permitted in:

1. One- and two-family dwellings and their attached/detached accessory structures.
2. Multifamily dwellings permitted to be of Types III, IV, and V construction (wood-framed structures).
3. Other structures permitted to be of Types III, IV, and V construction, except where explicitly prohibited.
4. Cable trays in industrial establishments where cables are identified for the use.

Uses Prohibited (NEC 334.12)

Type NM cable is STRICTLY PROHIBITED in:

• Any dwelling or structure not permitted to be of Types III, IV, or V construction (e.g., commercial high-rises of Type I or Type II steel/concrete construction).
• As service-entrance cable.
• In commercial garages having hazardous (classified) locations (NEC Article 511).
• In theaters, assembly halls, and similar locations having an occupant load of 100 or more (NEC Article 518).
• In motion picture studios or storage battery rooms.
• In hoistways or on elevators or escalators.
• Embedded in poured cement, concrete, or aggregate.
• Exposed to wet or damp locations (Type NM cannot be installed outdoors, underground, or embedded in masonry walls exposed to weather).
• In dropped or suspended ceilings in commercial or multi-family buildings (other than one- and two-family dwellings).

Securing and Supporting (NEC 334.30)

Type NM cable must be secured and supported by listed staples, cable ties, straps, or hangers:

• Within 12 inches (300 mm) of every cable entry into a box, cabinet, or fitting equipped with internal cable clamps.
• Within 8 inches (200 mm) of a nonmetallic box without cable clamps (e.g., standard single-gang plastic nail-on residential boxes where cable enters through flexible knockout tabs). In this case, the cable sheath must extend at least 1/4 inch into the enclosure.
• At intervals not exceeding 4.5 feet (1.4 m) along framing members or horizontal runs.
• Exception (Concealed Fishing): Cable is permitted to be fished between access points in finished walls or ceilings without securing or supporting.

Bending Radius (NEC 334.24)

Bends in Type NM cable must be made without damaging the outer sheath or conductor insulation. The minimum bending radius is 5 times the diameter of the cable:

$\text{Minimum Bending Radius} \ge 5 \times \text{Cable Diameter}$

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2. Armored Cable: Type AC (NEC Article 320)

Type AC cable (historically called "BX") consists of individually insulated conductors wrapped in treated kraft paper, enclosed within a flexible, interlocking metal armor (galvanized steel or aluminum).

Construction and the Critical Bonding Strip (NEC 320.108)

Every roll of Type AC cable contains an internal, uninsulated 16 AWG aluminum or copper bonding strip running longitudinally underneath the metal armor.

Exam Trap & Life Safety Rule: The aluminum bonding strip in Type AC cable is NOT an equipment grounding conductor by itself! It is designed to be in intimate, continuous physical contact with the interior spiraled convolutions of the steel or aluminum armor.

Under high alternating fault currents, the spiral coils of interlocking armor act like a high-impedance electrical choke (inductor), which would choke down fault current and prevent circuit breakers from tripping instantly. The bonding strip shorts across each spiral convolution, drastically lowering the AC impedance of the armor sheath:

$\text{Equipment Grounding Path in Type AC} = \text{Metal Armor Sheath} + \text{Internal Bonding Strip in Direct Contact}$

No separate green equipment grounding conductor is required inside Type AC cable! The armor/strip combination is approved as an EGC under NEC 250.118(8).

Anti-Short Bushings (NEC 320.40)

At every termination of Type AC cable where the outer metal armor is cut, a listed insulating bushing (universally called an "anti-short bushing" or "red head") must be inserted between the conductors and the armor. This protects the paper-wrapped conductors from being sliced by the sharp, burred edges of the severed metallic armor. A visual inspection opening in listed AC connectors allows the inspector to verify that the anti-short bushing is present.

Securing and Supporting (NEC 320.30)

• Supported and secured within 12 inches (300 mm) of every box, cabinet, or fitting.
• Supported at intervals not exceeding 4.5 feet (1.4 m).
• Exception 1: Cable lengths up to 24 inches (600 mm) are permitted without support at terminals where flexibility is necessary (e.g., motor vibration joints).
• Exception 2: Fixture whips not exceeding 6 feet (1.8 m) from an accessible junction box to a recessed luminaire above an accessible lay-in suspended ceiling require no intermediate support.

Uses Permitted and Prohibited (NEC 320.10 & 320.12)

• Permitted: Feeders and branch circuits in exposed and concealed work in dry locations; commercial buildings, residential framing, embedded in plaster finishes on masonry in dry areas.
• Prohibited: Damp or wet locations; direct burial in earth; commercial garages with hazardous areas; theaters and assembly halls over 100 persons; corrosive environments.

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

Type MC cable consists of one or more insulated circuit conductors enclosed within an interlocking metal tape armor or a smooth/corrugated metallic tube sheath. It is the dominant branch-circuit wiring method in commercial and institutional construction.

Grounding Mechanism: Standard MC vs. MC-AP (NEC 330.108)

Here lies the single biggest distinction between Type AC and Type MC on electrical licensing examinations:

1. Standard Type MC Cable: The interlocking metal armor of standard MC cable is NOT listed as an equipment grounding conductor! Standard MC uses an internal wire-type equipment grounding conductor—bare, covered, or insulated as permitted—because its ordinary interlocked armor alone is not the EGC.
2. Type MC-AP (All-Purpose / Smart MC): Specialty MC cable manufactured with an aluminum sheath in direct contact with an oversized bare aluminum bonding wire running outside the conductor bundle. This specific assembly is listed as an equipment grounding path without requiring a separate internal green wire.

TYPE AC CABLE:                            TYPE MC CABLE (STANDARD):
┌───────────────────────────────────┐     ┌───────────────────────────────────┐
│ Interlocking Metal Armor          │     │ Interlocking Metal Armor          │
│ (Acts as EGC when combined with   │     │ (Armor alone CANNOT act as EGC!)  │
│ internal bonding strip)           │     │                                   │
│                                   │     │ [Black Phase Conductor]           │
│ [Conductor with Kraft Paper Wrap] │     │ [White Neutral Conductor]         │
│ [16 AWG Aluminum Bonding Strip]   │     │ [DEDICATED GREEN INSULATED EGC]   │
└───────────────────────────────────┘     └───────────────────────────────────┘

Anti-Short Bushings in MC Cable (NEC 330.40)

Unlike Article 320 (which explicitly mandates anti-short bushings for AC cable), Article 330 does not explicitly use the words "anti-short bushing." However, NEC 330.40 mandates that fittings be listed and identified for use with Type MC cable. Under NEC 110.3(B), manufacturer installation instructions and listing standards (UL 1569) dictate whether anti-short bushings or connectors with built-in insulated throat liners are required.

Securing and Supporting (NEC 330.30)

• Secured within 12 inches (300 mm) of every box, cabinet, or fitting for cables with four or fewer conductors sized 10 AWG or smaller.
• Supported at intervals not exceeding 6 feet (1.8 m) along runs for cables containing conductors up to 10 AWG.
• Vertical runs in hoistways and shafts supported per Table 330.30(C).
• Unsupported runs permitted: up to 6 feet from an accessible junction box to luminaires/equipment above accessible ceilings; up to 3 feet at equipment terminals where flexibility is required.

Uses Permitted (NEC 330.10)

Type MC cable is approved for virtually all construction types:

• Permitted in commercial, industrial, institutional, and residential buildings.
• Permitted exposed or concealed; in cable trays; in hazardous locations where permitted by Articles 501, 502, 503, and 505.
• Permitted in wet locations if provided with an overall PVC moisture-impervious outer jacket (commonly called "jacketed MC" or "parking deck MC") and listed for wet locations.

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4. Comprehensive Master Comparison Table: NM vs. AC vs. MC

The following reference table summarizes the critical code distinctions tested on the Wisconsin Journeyman Electrician exam:

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5. Common Exam Traps & Real-World Pitfalls

• Exam Trap #1: Over-Protecting NM-B Cable. You will see a question: "What is the maximum overcurrent protective device rating for an 8 AWG Copper NM-B feeder supplying an electric subpanel?" Looking at Table 310.16, the 75°C rating is 50A. Many candidates choose 50A. The correct answer is 40 Amperes! NEC 334.80 mandates the 60°C column (40A) for all NM-B final selections.
• Exam Trap #2: Grounding with Standard MC Armor. When asked whether standard interlocking Type MC cable armor can serve as the sole equipment grounding conductor without an internal bonding wire or green conductor, the answer is an emphatic NO. Only Type AC (with bonding strip) or listed Type MC-AP can serve as an EGC without an internal green conductor.
• Exam Trap #3: Support Spans for MC vs. AC. AC and NM cable share a maximum support span of 4.5 feet. MC cable has a maximum support span of 6.0 feet. Confusing the 4.5 ft and 6 ft rules is an easy way to lose points on raceway and cable support questions.
• Exam Trap #4: Unclamped Plastic Boxes. When securing NM cable to a single-gang plastic nail-on box without internal cable clamps, the cable must be secured within 8 inches of the enclosure, not 12 inches (NEC 334.30).