4.3 Cable Assemblies (NM-B, MC, UF)

4.3 Cable Assemblies (NM-B, MC, UF)

Exam Focus: Article 334 (NM-B ampacity & support rules), Article 330 (MC cable installation & fittings), Article 340 (UF-B burial depths & ratings), and NEC 300.4 physical protection (nail plates).

Field wiring often relies on factory-assembled cable packages containing phase, neutral, and equipment grounding conductors bundled inside a protective outer sheath. The three most widely tested cable assemblies on the Michigan licensing exam are Nonmetallic-Sheathed Cable (Type NM-B), Metal-Clad Cable (Type MC), and Underground Feeder Cable (Type UF-B).

---

Nonmetallic-Sheathed Cable (Type NM-B) — Article 334

Type NM-B (commonly referred to by the trade brand Romex) consists of two or more insulated conductors and a bare equipment grounding conductor encased in a flame-retardant nonmetallic outer sheath.

+-------------------------------------------------------------------------+
|                    TYPE NM-B CABLE ANATOMY & RULES                      |
|                                                                         |
|  [ Outer Flame-Retardant PVC Jacket ]                                   |
|   +-- THHN 90°C Phase Conductor (Black)                                |
|   +-- THHN 90°C Neutral Conductor (White)                              |
|   +-- Bare Equipment Grounding Conductor (Cu)                           |
|                                                                         |
|  AMPACITY MANDATE (NEC 334.80): MUST use 60°C Column of Table 310.16    |
+-------------------------------------------------------------------------+

1. Ampacity Limitation — NEC 334.80

• Conductors inside NM-B are manufactured with 90°C rated insulation (THHN).
• CRITICAL CODE RULE: Under NEC 334.80, the allowable ampacity of Type NM cable shall NOT exceed that of a 60°C rated conductor in Table 310.16.
• Derating Exception: The 90°C rating MAY be used as the starting base ampacity ($I_{\text{base}}$) when calculating ambient temperature or bundle derating factors, provided the final adjusted ampacity does not exceed the 60°C rating.

2. Permitted & Prohibited Building Construction Types — NEC 334.10 & 334.12

• Permitted Uses: One- and two-family dwellings, multi-family dwellings, and other structures classified as Type III, IV, and V construction (wood-frame or non-combustible exterior wall construction).
• Prohibited Uses: Commercial structures of Type I and II construction (fire-resistive steel/concrete construction), commercial garages, theaters, places of assembly (over 100 persons occupancy), motion picture studios, damp or wet locations, or exposed inside drop ceiling spaces in commercial buildings.

3. Support & Securing Rules — NEC 334.30

• Must be secured within 12 inches (300 mm) of every cabinet, box, fitting, or termination.
• Exception: Nonmetallic single-gang boxes without internal cable clamps permit securing within 8 inches (200 mm) of the box.
• Must be supported at intervals not exceeding 4.5 feet (1.4 m) along runs.
• Staples must be installed flat and tight without crushing the outer sheath.

---

Metal-Clad Cable (Type MC Cable) — Article 330

Type MC cable features insulated conductors enclosed in a flexible, interlocked armor tape (aluminum or steel) or a smooth/corrugated metallic sheath.

1. Construction & Advantages over NM-B

• Permitted in Type I, II, III, IV, and V construction, commercial drop ceilings, places of assembly, and exposed or concealed runs.
• Conductor insulation is rated 90°C (THHN/THWN-2).
• Ampacity is based on Table 310.16 (75°C column for terminations per 110.14(C), and full 90°C column for ampacity derating calculations under NEC 330.80).

2. Support & Securing Rules — NEC 330.30

• Must be secured within 12 inches (300 mm) of every box, cabinet, fitting, or termination.
• Must be supported at intervals not exceeding 6 feet (1.8 m).
• Horizontal runs passing through wood or metal framing members are considered supported at 6-foot intervals.

3. Connectors & Anti-Short Bushings

• Must be installed with listed MC cable connectors.
• Anti-short bushings (red plastic sleeves, trade name "red devils") must be installed at armor cut ends where required by manufacturer listing and NECA standards.

---

Underground Feeder Cable (Type UF-B) — Article 340

Type UF cable is designed for direct earth burial and wet location installations. Its conductors are solidly embedded inside a continuous, solid moisture-resistant plastic compound.

1. Ampacity Rating — NEC 340.80

• Type UF cable ampacity MUST be determined using the 60°C column of Table 310.16.

2. Direct Burial Depth Requirements — NEC Table 300.5

                     NEC TABLE 300.5 DIRECT BURIAL DEPTHS
+-------------------------------------------------------------------------+
| Wiring Method / Location                                | Minimum Cover |
+---------------------------------------------------------+---------------+
| Direct Burial UF Cable (No raceway or concrete envelope)| 24 Inches     |
| Rigid Metal Conduit (RMC) or Intermediate Metal (IMC)   | 6 Inches      |
| Nonmetallic Raceway (PVC Schedule 40/80) Direct Burial   | 18 Inches     |
| Residential 120V 20A Branch Circuit w/ GFCI Protection  | 12 Inches     |
| Under 4-Inch Concrete Building Slab (No vehicular traffic)| 4 Inches   |
| Under Commercial Driveways & Parking Lots (All methods) | 24 Inches     |
+---------------------------------------------------------+---------------+

[!IMPORTANT] Physical Protection at Grade (NEC 300.5(D)): Where direct buried cables emerge from the ground, they must be protected against physical damage by a raceway (Schedule 80 PVC or RMC) extending from 18 inches below grade to a point at least 8 feet above finished grade.

---

Physical Protection of Cables — NEC 300.4

Cables installed through or parallel to structural framing members must be protected against mechanical damage caused by dry-wall screws and nails.

1. Framing Clearance & Nail Plate Rules (NEC 300.4(A))

• Where cables pass through bored holes or notches in wood studs, joists, or rafters, the edge of the hole must be at least 1-1/4 inches (32 mm) from the nearest edge of the wood framing member.
• If clearance is less than 1-1/4 inches, the cable must be protected by a steel plate at least 1/16 inch (1.6 mm) thick of suitable length and width.

                      NAIL PLATE CLEARANCE REQUIREMENT
         +-------------------------------------------------------+
         | Edge of Stud                                          |
         | | <-- Distance < 1-1/4 in. --> [ Bored Hole w/ Cable] |
         | |                                                     |
         | Steel Nail Plate (Min. 1/16 in. / 1.6mm thick) REQUIRED|
         +-------------------------------------------------------+

2. Metal Stud Installations (NEC 300.4(B))

• Cables passing through factory or field-punched holes in metal studs must be protected by listed bushing or grommets securely fastened in the opening prior to pulling cable.

---

Cable Assembly Comparison Table

---