8.2 Nonmetallic and Armored Cable Assemblies
Key Takeaways
- Type NM cable (Romex, Article 334) is permitted in one- and two-family dwellings, multifamily dwellings, and other structures of Types III, IV, and V construction, but is strictly prohibited in commercial suspended ceilings, damp or wet locations, and places of assembly seating 100 or more persons.
- Under NEC 334.80, Type NM cable conductors have 90°C insulation (NM-B), but the final allowable ampacity is strictly capped at the 60°C column of Table 310.16; the 90°C rating may only be utilized as the baseline for ambient temperature and bundling derating calculations.
- Type MC cable (Metal-Clad, Article 330) must be supported within 12 inches of every box, cabinet, or fitting and at intervals not exceeding 6 feet; standard interlocking MC armor does not serve as an equipment grounding conductor and requires an internal insulated copper EGC.
- Type AC cable (Armored Cable / BX, Article 320) utilizes the combination of its interlocking armor and an internal bare aluminum bonding strip as an approved equipment grounding path, and NEC 320.40 mandates listed insulating anti-short bushings ('red devils') at all terminations.
- Type SE (Service-Entrance) cable used for interior branch circuits and feeders must comply with the installation and 60°C ampacity rules of Type NM cable when installed in thermal insulation per NEC 338.10(B)(4), whereas Type USE is prohibited inside buildings beyond the terminal point.
8.2 Nonmetallic and Armored Cable Assemblies
Exam Fast Fact: Questions on cable assemblies are heavily represented on the Colorado Journeyman exam. The three most critical testing points are: (1) the 60°C ampacity limitation for Type NM cable under NEC 334.80 (even though NM-B has 90°C rated conductors), (2) the prohibition of Type NM cable in dropped ceilings of commercial buildings under NEC 334.12(A)(2), and (3) distinguishing the equipment grounding mechanism of Type MC cable (insulated EGC required) versus Type AC cable (armor plus internal aluminum bonding strip).
Pre-fabricated factory cable assemblies provide rapid, cost-effective wiring solutions across residential, commercial, and industrial facilities. However, the National Electrical Code imposes strict boundaries on where each cable assembly may be deployed based on building construction types, environmental conditions, and mechanical protection requirements.
Nonmetallic-Sheathed Cable: Types NM, NMC, & NMS (NEC Article 334)
Commonly known by the trade name "Romex," nonmetallic-sheathed cable is the predominant wiring method in modern residential construction.
Cable Classifications
- Type NM: Flame-retardant and moisture-resistant nonmetallic outer jacket. Permitted only in dry locations.
- Type NMC: Flame-retardant, moisture-resistant, fungus-resistant, and corrosion-resistant jacket. Permitted in dry, moist, or damp locations (not wet locations).
- Type NMS: Contains factory-assembled signaling, data, or control conductors along with power conductors under a nonmetallic jacket for home automation and energy management systems.
Uses Permitted (NEC 334.10)
Type NM, NMC, and NMS cables are permitted to be used in the following structures:
- One- and Two-Family Dwellings and their attached or detached garages and accessory storage buildings.
- Multifamily Dwellings permitted to be of Types III, IV, and V construction (wood frame, brick, or masonry construction).
- Other Structures permitted to be of Types III, IV, and V construction, except where explicitly prohibited by other code sections.
- Cables may be installed exposed or concealed in dry locations, and fished in existing wall cavities.
NM CABLE COMMERCIAL CEILING PROHIBITION
RESIDENTIAL (Dwelling Units) COMMERCIAL (Offices, Retail, Clinics)
┌──────────────────────────────┐ ┌──────────────────────────────────────┐
│ Drop / Suspended Ceiling │ │ Drop / Suspended Ceiling │
│ │ │ │
│ NM-B Cable Permitted │ │ NM CABLE STRICTLY PROHIBITED! │
│ (Above ceiling grid) │ │ (NEC 334.12(A)(2)) │
│ │ │ │
│ │ │ Must use MC Cable, EMT, or Conduit │
└──────────────────────────────┘ └──────────────────────────────────────┘
Uses Not Permitted (NEC 334.12) — High-Yield Exam Rules
Type NM cable shall not be used:
- Commercial Dropped Ceilings (334.12(A)(2)): Exposed or concealed within or above suspended or dropped ceiling assemblies in commercial and industrial occupancies (other than one- and two-family and multifamily dwellings).
- Damp or Wet Locations: Embedded in poured concrete, cinder block in contact with earth, or outdoor conduits.
- Commercial Garages & Theaters: In commercial service garages with hazardous atmospheres, motion picture studios, or places of assembly seating 100 or more people (NEC Article 518).
- As Service-Entrance Cable: Prohibited from serving as service-entrance conductors.
Ampacity & The 60°C Rule (NEC 334.80)
Modern Type NM cable manufactured since 1984 is labeled NM-B, indicating its internal conductors have 90°C insulation (such as THHN). However, NEC 334.80 establishes an absolute rule for final circuit sizing:
NEC 334.80 Mandate: "The allowable ampacity of Types NM, NMC, and NMS cable shall not exceed that of a 60°C (140°F) rated conductor."
| Conductor Gauge (AWG) | Table 310.16 (60°C Rating) — Final Ampacity | Table 310.16 (90°C Rating) — Derating Baseline | Maximum Overcurrent Protection (NEC 240.4(D)) |
|---|---|---|---|
| 14 AWG Copper | 15 Amperes | 25 Amperes | 15 Amperes |
| 12 AWG Copper | 20 Amperes | 30 Amperes | 20 Amperes |
| 10 AWG Copper | 30 Amperes | 40 Amperes | 30 Amperes |
| 8 AWG Copper | 40 Amperes | 55 Amperes | 40 Amperes |
| 6 AWG Copper | 55 Amperes | 75 Amperes | 60 Amperes (Next Size Up) |
The Bundling Derating Rule: Where more than two NM cables containing two or more current-carrying conductors are passed through wood framing holes that are fire- or draft-stopped using thermal insulation, caulk, or sealing foam, the ampacity of each conductor must be derated using the adjustment factors in NEC Table 310.15(C)(1). Crucially, the derating percentage is calculated from the 90°C column, but the final derated ampacity cannot exceed the 60°C limit.
Securing and Supporting (NEC 334.30)
- Distance to Box: Must be secured within 12 inches of every outlet box, junction box, cabinet, or termination fitting.
- Nonmetallic Boxes Without Clamps (Single-Gang): Where installed into a single-gang plastic box without internal cable clamps, the cable must be secured within 8 inches of the box.
- Interval Along Run: Must be supported and secured at intervals not exceeding 4.5 feet (1.4 m).
- Bored Holes: Cable run horizontally through bored holes in wood or metal studs is considered supported, provided the 4.5-foot spacing is maintained.
- Unfinished Basements (NEC 334.15(C)): Cables smaller than two 6 AWG or three 8 AWG conductors must be run through bored holes in joists or on running boards; cables 6 AWG or 8 AWG may be secured directly to the underside of joists.
Metal-Clad Cable: Type MC (NEC Article 330)
Type MC cable is a factory assembly of one or more insulated circuit conductors enclosed in an armor of interlocking metal tape or a smooth or corrugated metallic sheath.
CABLE ASSEMBLY COMPARISON
Type MC Cable (Article 330) Type AC Cable (Article 320)
┌───────────────────────────────┐ ┌───────────────────────────────┐
│ ▒▒▒▒▒ Interlocking Armor │ │ ▒▒▒▒▒ Interlocking Steel Armor│
│ (●) Phase Conductors │ │ (●) Phase Conductors │
│ (○) Neutral Conductor │ │ (○) Neutral Conductor │
│ (⊘) Dedicated Green Copper EGC│ │ ════ Bare Aluminum Bonding │
│ │ │ Strip (Must be folded) │
│ Armor ALONE is NOT an EGC! │ │ Armor + Bonding Strip = EGC! │
│ (Unless listed MC-AP) │ │ Red Devil Bushing MANDATORY │
└───────────────────────────────┘ └───────────────────────────────┘
Armor Types and Construction
- Interlocking Metal Tape Armor: Spiral-wound galvanized steel or aluminum tape.
- Smooth or Corrugated Tube Armor: Continuously welded impervious metal tube, offering gas- and vapor-tight seals for industrial and hazardous applications.
Equipment Grounding in Type MC Cable (NEC 330.108)
A critical distinction on the Journeyman exam is the grounding path:
- Standard Interlocking MC Cable: The interlocking metal armor does not qualify as an equipment grounding conductor (EGC) by itself. A separate, insulated green copper equipment grounding conductor must be included within the cable assembly.
- MC-AP (All-Purpose / Smart Ground): Listed MC cables incorporating an aluminum armor with a full-length uninsulated aluminum bonding wire in continuous intimate contact with the armor. This specific listed assembly is recognized as an EGC without an additional insulated copper wire.
Uses Permitted (NEC 330.10)
Type MC cable is one of the most versatile wiring methods recognized by the NEC:
- Branch circuits, feeders, and services in commercial, industrial, and residential occupancies
- Exposed or concealed work
- Cable trays and environmental air spaces / plenums (where listed for plenum use)
- Wet locations only where the metallic sheath is impervious to moisture or covered by a listed PVC outer jacket
Securing and Supporting (NEC 330.30)
- Supported and secured within 12 inches of every box, cabinet, or termination fitting.
- Supported at intervals not exceeding 6 feet along the run.
- Drop to Luminaires (Exception 2 to 330.30(D)): An unsupported drop of up to 6 feet is permitted from an accessible ceiling junction box to a luminaire or equipment.
Armored Cable: Type AC (NEC Article 320)
Type AC cable, historically known as "BX," is a fabricated assembly of insulated conductors enclosed in a flexible metallic enclosure.
Construction and Grounding Continuity (NEC 320.108)
Type AC cable features an interlocking flexible steel or aluminum tape armor containing an internal bare aluminum bonding strip (typically 16 AWG or 14 AWG) running longitudinally throughout the entire length of the cable.
- The Grounding Mechanism: The thin aluminum bonding strip acts in parallel with the flexible metal outer armor to reduce high-frequency AC impedance. The armor and bonding strip together form an approved equipment grounding conductor.
- Installation Detail: The aluminum bonding strip is never terminated to a green grounding screw inside the box. At terminations, the strip is bent back sharply over the outer armor before the cable is inserted into the connector, ensuring firm mechanical and electrical contact against the connector body.
Anti-Short Bushings (NEC 320.40)
NEC 320.40 Absolute Mandate: "At all points where the armor of AC cable terminates, a listed fitting shall be provided with an approved insulating anti-short bushing (or equivalent) placed between the conductors and the armor."
Electricians refer to these bright red fiber or plastic inserts as "red devils." The bushing prevents the sharp jagged edges of cut steel armor from slicing through conductor insulation under vibration or thermal cycling. Connectors for Type AC cable feature an inspection slot through which the inspector can visually verify the presence of the red anti-short bushing.
Securing and Supporting (NEC 320.30)
- Secured within 12 inches of every box, cabinet, or fitting.
- Supported at intervals not exceeding 4.5 feet along the run.
Service-Entrance Cable: Types SE & USE (NEC Article 338)
Service-entrance cable is a factory assembly of one or more conductors with a protective outer covering primarily designed for bringing electrical utility power into a premises.
Type SE vs. Type USE
| Cable Classification | Description & Environmental Rating | Interior Building Use Permitted? | Grounding Conductor Configuration |
|---|---|---|---|
| Type SE (Service Entrance) | Above-ground cable; moisture-resistant, flame-retardant outer jacket. Style U (flat, concentric neutral) or Style R (round, insulated neutral). | YES. Permitted for interior feeders and branch circuits (e.g., ranges, dryers). | Concentric bare neutral or insulated neutral with separate bare EGC. |
| Type USE (Underground Service Entrance) | Direct-burial underground cable; moisture-resistant, NOT flame-retardant. | STRICTLY PROHIBITED. Cannot enter building structure beyond the service disconnect terminal. | Moisture-resistant insulation, bare or covered neutral. |
Interior Feeders & The 60°C Rule (NEC 338.10(B)(4))
When Type SE cable is used for interior branch circuits or feeders (such as supplying an electric clothes dryer, kitchen range, or residential subpanel):
- It must be installed in compliance with Part II of Article 334 (NM Cable Rules).
- Where installed in thermal insulation, its allowable ampacity shall not exceed the 60°C column of Table 310.16.
Comprehensive Comparison Matrix: NM vs. MC vs. AC vs. SE
| Technical Parameter | Type NM (Romex) — Art. 334 | Type MC (Metal-Clad) — Art. 330 | Type AC (BX) — Art. 320 | Type SE — Art. 338 |
|---|---|---|---|---|
| Conductor Temp Rating | 90°C (NM-B) | 90°C | 90°C | 75°C or 90°C |
| Final Ampacity Column | Strictly 60°C | 75°C or 90°C per terminal | 75°C or 90°C per terminal | 60°C in thermal insulation |
| Support from Box | 12 inches (8" plastic box) | 12 inches | 12 inches | 12 inches |
| Support Along Run | 4.5 feet | 6.0 feet | 4.5 feet | 4.5 feet |
| Armor Serves as EGC? | No armor (Bare wire EGC) | NO (Insulated EGC required) | YES (Armor + Bonding strip) | Bare wire or neutral |
| Anti-Short Bushing? | Not applicable | Recommended / per listing | MANDATORY (320.40) | Not applicable |
| Commercial Dropped Ceilings | PROHIBITED (334.12) | PERMITTED | PERMITTED | PROHIBITED |
Practical Jobsite Scenarios & Common Exam Traps
| Jobsite Scenario | Technical Reality & Code Rule | Common PSI Exam Trap |
|---|---|---|
| Office Tenant Finish Ceiling: An electrical contractor runs 12/2 NM-B cable above an acoustic ceiling tile grid in a commercial dental office to feed LED flat panels. | Immediate Violation of 334.12(A)(2): NM cable cannot be installed in commercial dropped ceilings. The contractor must replace with Type MC cable or EMT raceway. | Believing NM cable is permitted in commercial dropped ceilings if supported every 4.5 feet. |
| 10 AWG NM-B Water Heater: An electrician installs 10/2 NM-B for a 30A residential electric water heater. A continuous load requires 125%: $24\text{A} \times 1.25 = 30\text{A}$. 10 AWG NM-B 60°C rating is 30A. | Code Compliant: 10 AWG at 60°C is rated 30A. However, if the water heater drew 25A continuous ($25 \times 1.25 = 31.25\text{A}$), 10 NM-B would violate the 60°C limit. | Attempting to use the 90°C rating (40A) of 10 AWG NM-B for the continuous load sizing. |
| Terminating AC Cable: An apprentice trims the aluminum bonding strip flush with the armor of Type AC cable and throws away the red anti-short bushing before inserting the connector. | Severe Violation of 320.40 & 320.108: Red anti-short bushing is mandatory. The bonding strip must be folded back over the armor to ensure grounding continuity. | Believing the aluminum bonding strip must be terminated under a grounding screw like an EGC. |
| Type USE Run to Main Panel: An electrician pulls Type USE cable directly from an underground utility transformer through a basement wall to an indoor main panel. | Violation of 338.12(B): Type USE cable is not flame-retardant and cannot be run inside a structure. It must terminate at an exterior meter or disconnect. | Assuming Type USE cable is permitted inside buildings because it is rated for service entrance. |
Under NEC 334.12(A)(2), where is Type NM (nonmetallic-sheathed) cable strictly prohibited from being installed?
What is the maximum permitted support spacing for Type MC (metal-clad) cable under NEC 330.30?
Which safety component is explicitly mandated by NEC 320.40 at every termination of Type AC (armored) cable to prevent conductor insulation damage?