9.2 Nonmetallic-Sheathed (NM), Metal-Clad (MC) & Armored (AC) Cables
Key Takeaways
- Type NM cable (Romex) is permitted in 1- and 2-family dwellings and Types III, IV, and V structures, but is prohibited in commercial drop ceilings, wet/damp locations, theatres, and commercial buildings of Type I/II construction unless enclosed in 15-minute thermal barriers.
- Per NEC 334.80, conductors in Type NM cable are rated at 90°C for ampacity derating and temperature correction, but the final allowable ampacity is capped at the 60°C column of Table 310.16.
- Type MC cable (Article 330) is approved for virtually all construction types and occupancies, requires support within 12 inches of boxes and every 6 feet along runs (10 ft for vertical runs), and utilizes either an insulated wire EGC or listed interlocking armor with bonding strip (MC-AP).
- Type AC cable (BX - Article 320) utilizes flexible metallic armor with an internal 16 AWG bonding strip as its EGC, and NEC 320.40 strictly mandates listed anti-short insulating bushings ('red devils') at all terminations.
- Bundling more than two NM cables through a single framing hole draftstopped or firestopped with insulation or sealant for more than 24 inches triggers ampacity derating per Table 310.15(C)(1).
9.2 Nonmetallic-Sheathed (NM), Metal-Clad (MC) & Armored (AC) Cables
Factory-manufactured multiconductor cable assemblies represent the primary wiring methods utilized in modern residential and light commercial construction. In the 2023 National Electrical Code, three distinct cable assemblies dominate the trade: Nonmetallic-Sheathed Cable (Type NM/NMC/NMS - Article 334), Metal-Clad Cable (Type MC - Article 330), and Armored Cable (Type AC - Article 320).
While these cable types may appear similar in basic application, their structural composition, allowed building construction types, equipment grounding configurations, supporting rules, and ampacity derating procedures differ substantially.
1. Type NM, NMC & NMS Nonmetallic-Sheathed Cable (Article 334)
Commonly known by the trade name Romex, Nonmetallic-Sheathed Cable consists of insulated conductors enclosed within an overall nonmetallic outer jacket.
+-----------------------------------------------------------------------------+
| NONMETALLIC-SHEATHED CABLE TYPES (334.2) |
| |
| [TYPE NM] ---> Flame-retardant, moisture-resistant nonmetallic sheath. |
| Permitted for EXPOSED and CONCEALED work in DRY locations.|
| |
| [TYPE NMC] ---> Flame-retardant, moisture-resistant, FUNGUS-resistant, |
| and CORROSION-resistant. Permitted in dry, moist, damp, |
| and corrosive locations (e.g., dairy barns). |
| |
| [TYPE NMS] ---> Hybrid cable containing power conductors alongside fiber- |
| optic / network communication signaling conductors. |
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Uses Permitted & Prohibited (NEC 334.10 & 334.12)
| Permitted Uses (NEC 334.10) | Prohibited Uses (NEC 334.12) |
|---|---|
| One- and two-family dwellings and their accessory structures | In any dwelling or structure not classified as Type III, IV, or V (i.e., prohibited in Type I & II steel/concrete commercial high-rises unless concealed within 15-min thermal barriers) |
| Multifamily dwellings permitted to be of Types III, IV, and V construction | As open runs in drop or suspended ceilings in commercial or other non-dwelling buildings |
| Other structures permitted to be Types III, IV, and V construction | In damp or wet locations (Type NM is strictly dry only) |
| Exposed or concealed in normally dry locations | In commercial garages, repair hangars, storage battery rooms, or hoistways |
| Installed in cable trays in industrial establishments (where listed) | In theatres and places of assembly (except as permitted in 518.4) |
| Embedded in masonry plaster / dry adobe (Type NMC only) | Embedded in poured concrete, aggregate, or direct buried in earth |
The 60°C Ampacity Rule & 90°C Derating (NEC 334.80)
One of the most heavily tested topics on the journeyman exam is the dual-temperature ampacity calculation for Type NM cable:
- Conductor Insulation Rating (90°C): Conductors inside modern NM-B cable are insulated with 90°C-rated material (e.g., THHN). This 90°C rating must be used as the starting ampacity when applying ambient temperature correction factors (Table 310.15(B)(1)) and bundle adjustment factors (Table 310.15(C)(1)).
- Final Ampacity Limitation (60°C Column): Regardless of the calculation, the final allowable ampacity of Type NM cable shall not exceed that of a 60°C rated conductor in Table 310.16.
Final Allowable Ampacity = min(Ampacity_90°C x Adjustment Factors, Ampacity_60°C)
Bundling Rules (NEC 334.80)
Where more than two NM cables containing two or more current-carrying conductors are installed through the same wood framing hole that is sealed with thermal insulation, firestop caulk, or draftstop foam for a length exceeding 24 inches (600 mm), the ampacity of each conductor must be derated per Table 310.15(C)(1) (e.g., 4–6 current-carrying conductors derated to 80%; 7–9 conductors derated to 70%).
2. Type MC Metal-Clad Cable Architecture & Applications (Article 330)
Type MC cable contains one or more insulated circuit conductors enclosed in an armor of interlocking metal tape (steel or aluminum) or a smooth or corrugated metallic tube.
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| TYPE MC CABLE ANATOMY & EGC |
| |
| [STANDARD TYPE MC] |
| * Interlocking Aluminum or Steel Armor (Mechanical protection only) |
| * Insulated Phase Conductors (THHN / XHHW 90°C) |
| * Insulated or Bare Green Full-Size EQUIPMENT GROUNDING CONDUCTOR (EGC) |
| * Armor alone CANNOT serve as the equipment grounding path! |
| |
| [TYPE MC-AP / SMARTGROUND (ALL-PURPOSE)] |
| * Interlocking Aluminum Armor + Bare Aluminum Bonding Strip (in contact) |
| * Armor + Bonding Strip combination IS listed as an EGC (250.118(10)) |
| * No separate green wire needed for standard circuits! |
+-----------------------------------------------------------------------------+
Key MC Cable Requirements (NEC 330.10, 330.30)
- Uses Permitted: Permitted in all occupancies, including commercial high-rises (Type I and II), industrial plants, cable trays, exposed or concealed, and in wet locations if jacketed with sunlight-resistant PVC and conductors are listed for wet locations.
- Support Spacing (NEC 330.30(B)): Must be secured within 12 inches (300 mm) of every box, cabinet, or fitting, and supported at intervals not exceeding 6 feet (1.8 m) along the run.
- Vertical Riser Support (NEC 330.30(E)): Vertical runs of MC cable with interlocked armor can be supported at intervals up to 10 feet (3.0 m) if listed for vertical support.
- Unsupported Drops: Unsupported runs up to 3 feet (900 mm) are permitted from the last support to a luminaire or device where flexibility is necessary.
3. Type AC Armored Cable ("BX") & Anti-Short Bushings (Article 320)
Type AC cable consists of insulated conductors individual-wrapped in fibrous moisture-resistant paper, enclosed inside a flexible, interlocking galvanized steel or aluminum armor.
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| TYPE AC CABLE ANATOMY & ANTI-SHORT BUSHING |
| |
| +---------------------------------------------------------------------+ |
| | Interlocking Armor | Paper Wrap | Conductors (THHN) | 16 AWG Bond | |
| +---------------------------------------------------------------------+ |
| \ / | |
| [ANTI-SHORT BUSHING ("RED DEVIL")] | |
| NEC 320.40 Mandate: Must be placed at every 16 AWG Aluminum or |
| termination between conductors and armor to Copper bonding strip|
| prevent sharp armor edges cutting insulation! in intimate contact |
| with outer armor. |
+-----------------------------------------------------------------------------+
Critical AC Cable Code Rules (Article 320)
- Grounding Mechanism (NEC 250.118(8)): Type AC cable does NOT contain an insulated green ground wire. Instead, the outer interlocking armor in combination with the internal 16 AWG aluminum or copper bonding strip acts as the Equipment Grounding Conductor.
- Mandatory Anti-Short Bushings (NEC 320.40): An approved anti-short bushing (red devil) or its equivalent must be inserted between the conductors and the outer armor at every cable termination. The connector must have an inspection window allowing the inspector to visually confirm the bushing is in place.
- Uses Prohibited (NEC 320.12): Prohibited in damp/wet locations, direct burial in earth, commercial garages, theatres, places of assembly, and hazardous locations.
- Support Spacing (NEC 320.30): Supported within 12 inches (300 mm) of boxes and every 4.5 feet (1.4 m) along runs.
4. Cable Securing, Supporting & Protection in Attics (320.23, 334.23, 334.30)
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| ATTIC CABLE RUN PROTECTION RULES (NEC 320.23) |
| |
| ACCESSIBLE BY STAIRS OR PERMANENT LADDER: |
| * Cables across tops of floor joists or face of rafters within 7 ft of |
| floor MUST be protected by substantial GUARD STRIPS or RUNNING BOARDS. |
| |
| ACCESSIBLE BY SCUTTLE HOLE / HATCH ONLY: |
| * Guard strips required ONLY within 6 FEET (1.8 m) of the entrance hatch. |
| * Beyond 6 feet, cables can be run directly across joist tops. |
+-----------------------------------------------------------------------------+
Nonmetallic Box Support Exception (NEC 334.30(A) Ex. 1)
Where Type NM cable enters a single-gang nonmetallic outlet box without an internal cable clamp (maximum volume 100 cm³ / 6.1 in³), the cable must be secured within 8 inches (200 mm) of the box (rather than 12 inches), and the outer cable sheath must extend into the box at least 1/4 inch (6 mm).
5. Master Cable Assembly Comparison Matrix
| Technical Parameter | Type NM (Romex) | Type MC (Metal-Clad) | Type AC (BX / Armored) |
|---|---|---|---|
| Governing NEC Article | Article 334 | Article 330 | Article 320 |
| Conductor Rating | 90°C rated conductors | 90°C rated conductors | 90°C rated conductors |
| Final Ampacity Column | 60°C column only (334.80) | 75°C or 90°C column (terminal dependent) | 75°C or 90°C column (terminal dependent) |
| Outer Armor / Jacket | PVC nonmetallic sheath | Interlocking Al/Steel or corrugated tube | Interlocking galvanized steel or aluminum |
| Equipment Ground Path | Bare/insulated copper EGC | Dedicated EGC wire OR listed armor + strip | Armor + internal 16 AWG bonding strip |
| Anti-Short Bushing | Not applicable | Recommended by MFR (not NEC mandated) | MANDATORY by NEC 320.40 (Red Devil) |
| Distance to Box Support | 12 in. (8 in. for non-clamp box) | 12 in. (330.30) | 12 in. (320.30) |
| Maximum Support Interval | 4.5 feet (1.4 m) | 6.0 feet (1.8 m) (10 ft vertical) | 4.5 feet (1.4 m) |
| Drop Ceilings (Commercial) | PROHIBITED (334.12(A)(2)) | PERMITTED | PERMITTED |
| Wet / Damp Locations | Prohibited (except NMC in damp) | Permitted if jacketed & listed | PROHIBITED (320.12) |
What is the rule regarding the allowable ampacity and temperature rating of Type NM-B nonmetallic-sheathed cable under NEC 334.80?
Which installation requirement is strictly mandatory for Type AC (Armored) cable under NEC 320.40, but is not an explicit universal Code mandate for standard Type MC cable?
What are the securing and supporting requirements for Type MC (Metal-Clad) cable along a horizontal run in commercial construction?