6.4 Armored Cable (Type AC) & Metal-Clad Cable (Type MC)

6.4 Armored Cable (Type AC) & Metal-Clad Cable (Type MC)

Quick Answer: While Armored Cable (Type AC, Article 320) and Metal-Clad Cable (Type MC, Article 330) appear similar externally, their internal construction and grounding mechanisms differ fundamentally. Type AC features paper-wrapped conductors and an internal 16 AWG aluminum bonding strip in continuous contact with the flexible metal armor; this combination serves as an approved equipment grounding conductor per NEC 250.118(8). In contrast, standard Type MC carries an independent insulated green copper equipment grounding conductor, because standard interlocking MC armor is not listed as an EGC. Terminating Type AC mandates an insulating anti-short bushing ("red head") per NEC 320.40. Type AC is restricted strictly to dry locations, while jacketed Type MC is permitted in wet and direct-burial environments. Finally, Type AC must be supported every 4.5 feet (NEC 320.30), whereas Type MC may be supported every 6 feet (NEC 330.30).

Flexible metallic cable assemblies provide essential mechanical physical protection while offering routing agility around structural framing, through plenum spaces, and inside commercial drywall partitions. On commercial construction projects throughout Alabama, Type MC cable has largely superseded conduit for branch-circuit drops to lighting and receptacles. However, because Type AC and Type MC cables look virtually identical from the outside, licensing examinations frequently probe an electrician's ability to distinguish their internal design, grounding pathways, fitting standards, and code restrictions.

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Structural and Metallurgical Differences: AC vs. MC

The fundamental distinction between Type AC and Type MC lies in the internal wrapping materials and how the equipment grounding return path is engineered:

+-------------------------------------------------------------------------+
|                   INTERNAL CONSTRUCTION COMPARISON                      |
+-------------------------------------------------------------------------+
| TYPE AC CABLE (Article 320):                                            |
|   [ Flexible Metal Armor ]                                              |
|      |-- Continuous 16 AWG Aluminum Bonding Strip (contacts armor)      |
|      |-- Treated Kraft Paper Wrapping around conductors                 |
|      +-- Conductors (THHN-rated, NO separate green wire)                |
|                                                                         |
| TYPE MC CABLE (Article 330):                                            |
|   [ Flexible Interlocking Metal Armor ]                                 |
|      |-- Clear Mylar / Plastic Binder Tape                              |
|      |-- Conductors (THHN-rated)                                        |
|      +-- SEPARATE FULL-SIZED GREEN INSULATED COPPER EGC                 |
+-------------------------------------------------------------------------+

Type AC Construction (NEC Article 320)

Armored Cable (historically referred to by the trade name "BX") consists of individual conductors insulated with thermoplastic material, each covered with an individual moisture-retardant, flame-retardant kraft paper wrap. Crucially, Type AC contains an internal bare 16 AWG aluminum bonding strip that runs longitudinally throughout the entire length of the cable in intimate, continuous physical contact with the flexible steel or aluminum outer armor.

The Type AC Grounding Mechanism (NEC 250.118(8) & 320.108)

A common misconception is that the 16 AWG aluminum bonding wire in Type AC is an equipment grounding conductor. It is not! An aluminum wire of size 16 AWG is far too small to handle fault currents under NEC Table 250.122. Instead, the bonding strip acts as a magnetic choke reducer:

• The flexible metal armor is wound in a continuous spiral helix. Under high-frequency or alternating-current (AC) ground faults, this spiral geometry behaves like an inductive choke coil, generating high impedance that restricts fault current flow, delays breaker tripping, and causes the armor to glow cherry-red, creating a severe fire hazard.
• The internal 16 AWG aluminum bonding strip shorts out the individual spirals of the armor, converting the helical coil into a continuous, low-impedance cylindrical conductor.
• Under NEC 250.118(8), the combination of the flexible metal armor and the internal aluminum bonding strip is officially listed and recognized as an approved equipment grounding conductor. At terminations, the aluminum strip is simply bent back over the outside of the armor or cut off flush; it is never brought inside the box to connect to a grounding screw.

Type MC Construction (NEC Article 330)

Metal-Clad Cable consists of insulated conductors surrounded by a clear polyester/mylar plastic binder tape, enclosed in an interlocking metallic tape armor or a smooth/corrugated continuous metallic tube. Standard interlocking MC armor alone has high electrical resistance across its convolutions and is not listed as an equipment grounding conductor under NEC 250.118.

Therefore, standard Type MC cable always incorporates an independent, full-sized green insulated (or bare) copper equipment grounding conductor. This green EGC must be brought into every termination box and bonded to the enclosure using a listed grounding screw or clip. (Note: Specialized MC-AP or SmartCable incorporates a full-sized aluminum bonding wire in continuous contact with aluminum armor, which is listed as an EGC under 250.118(10)(a)).

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Anti-Short Bushings ("Red Heads"): Mandatory vs. Best Practice

When flexible metal armor is cut using a rotary cable cutter (Roto-Split®) or hacksaw, sharp spiraled metal burrs remain at the cut edge. These burrs can easily slice through conductor insulation when wires are bent entering a junction box.

• Type AC Mandate (NEC 320.40): The Code is unequivocal: "At all points where the armor of AC cable terminates, a fitting shall be provided to protect wires from abrasion, unless the design of the outlet boxes or fittings is such as to afford equivalent protection. In addition, an approved insulating bushing or its equivalent approved protection shall be provided between the conductors and the armor." Installing an insulating anti-short bushing (the familiar red plastic "red head") is a mandatory legal requirement under the NEC for all Type AC terminations. Connectors designed for Type AC include inspection slots that allow the electrical inspector to visually verify that the red bushing is in place.
• Type MC Fitting Standards (NEC 330.40): NEC 330.40 mandates that fittings used with Type MC cable shall be listed and identified for the use. According to NEMA Engineering Bulletin No. 90 and UL Standard 514B, the NEC does not mandate anti-short bushings for Type MC cable unless the connector manufacturer's published installation instructions specifically require them. Many MC connectors utilize built-in insulated nylon throats. However, supplying and installing red heads on Type MC remains universal industry best practice.

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Uses Permitted and Prohibited: AC vs. MC

Because the internal wrap of Type AC cable is made of moisture-absorbing kraft paper, any exposure to liquid water, outdoor humidity, or concrete sweating leads to insulation breakdown. Type AC is strictly restricted to dry indoor spaces. Standard Type MC is also for dry locations, but Type MC manufactured with an extruded overall polyvinyl chloride (PVC) outer jacket is fully listed for wet locations, parking garages, chemical facilities, and underground direct burial.

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Securing, Supporting, and Bending Radius Rules

Support spacing requirements represent frequent licensing examination calculation questions:

Supporting and Securing (NEC 320.30 & 330.30)

• Distance from Enclosures: Both Type AC and Type MC must be secured by listed cable ties, straps, staples, or hangers within 12 inches (300 mm) of every box, cabinet, fitting, or enclosure.
• Interval Spacing Along Runs:
  • Type AC (NEC 320.30): Must be supported at intervals not exceeding 4.5 feet (1.4 m).
  • Type MC (NEC 330.30): Must be supported at intervals not exceeding 6 feet (1.8 m) for cables with four or fewer conductors of 10 AWG or smaller.
• Support Exceptions for Both Cables:
  1. Horizontal Framing Passes: Cables run horizontally through holes or notches in wood or metal framing members are considered supported and secured provided the framing spacing does not exceed the mandatory support intervals (4.5 ft for AC, 6 ft for MC).
  2. Fixture Whips to Luminaires: An unsupported cable run not exceeding 6 feet (1.8 m) in length is permitted from the last point of support to a luminaire or electrical utilization equipment located inside an accessible suspended ceiling.
  3. Concealed Fish Runs: Where fished in existing walls or finished structures, cable is permitted to be unsupported between access openings.

Minimum Bending Radius (NEC 320.24 & 330.24)

Bends in armored cables must be made smoothly without kinking or opening the convolutions of the metallic armor:

• Type AC Cable (NEC 320.24): The radius of the curve of the inner edge of any bend shall not be less than 5 times the external cable diameter ($5 \times d$).
• Type MC Cable (Interlocking Armor, NEC 330.24(B)): The radius of the curve of the inner edge of any bend shall not be less than 7 times the external cable diameter ($7 \times d$).

+-------------------------------------------------------------------------+
|                    SUMMARY COMPARISON: TYPE AC VS. MC                   |
+-----------------------+------------------------+------------------------+
| Feature               | Type AC (Armored)      | Type MC (Metal-Clad)   |
+-----------------------+------------------------+------------------------+
| NEC Governing Article | Article 320            | Article 330            |
| Conductor Separator   | Kraft Paper            | Plastic / Mylar Tape   |
| Equipment Grounding   | Armor + 16 AWG Al wire | Separate Green Cu wire |
| Anti-Short Bushing    | MANDATORY (320.40)     | Per Fitting Mfr / Opt  |
| Distance to Box       | 12 inches (300 mm)     | 12 inches (300 mm)     |
| Support Spacing       | 4.5 feet (1.4 m)       | 6 feet (1.8 m)         |
| Bending Radius        | 5 × Cable Diameter     | 7 × Cable Diameter     |
| Wet Location Rating   | NEVER PERMITTED        | PERMITTED if jacketed  |
+-----------------------+------------------------+------------------------+