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It can be helpful to compare two commonly used types: Armored cable and Metal Clad Cable.
A tale of two cables
Article 320 covers the use, installation, and construction specifications of armored cable (Type AC). Figure 01
“Type AC” is a fabricated assembly of conductors in a flexible interlocked metallic armor with an internal bonding strip in intimate contact with the armor for its entire length [100]. It contains up to four phase conductors and one neutral insulated conductor, sizes 14 AWG through 1 AWG, individually wrapped in a moisture-resistant, fire-retardant paper contained within a flexible spiral metal sheath.
Article 330 covers the use, installation, and construction specifications of metal-clad cable, Type MC.
“Type MC” is a factory assembly of one or more insulated circuit conductors (maybe with optical fiber members), enclosed in an armor of interlocking metal tape, or a smooth or corrugated metallic sheath [100]. It contains any number of insulated conductors, 18 AWG through 2000 kcmil, with an overall polypropylene wrap enclosed in a metal sheath of either corrugated or smooth copper or aluminum tubing, or in spiral interlocked steel or aluminum.
Permitted uses
You can use Type AC cable [320.10] in:
1) Feeders and branch circuits (exposed or concealed).
(2) Cable trays.
(3) Dry locations.
(4) Plaster (dry locations).
(5) Voids in block walls where not exposed to excessive moisture or dampness.
And in plenum spaces per 300.22(C)(1). The “Uses Permitted” is not an exhaustive list; other suitable uses are permitted if approved by the authority having jurisdiction.
Where can you use Type MC cable?
(A) General Uses.
(1) Branch circuits, feeders, and services.
(2) Power, lighting, and power-limited circuits.
(3) Indoor or outdoor locations.
(4) Exposed or concealed.
(5) Directly buried (if identified for the purpose).
(6) In cable tray (if identified for the purpose).
(7) In raceway.
(8) As aerial cable on a messenger.
(9) In hazardous (classified) locations as permitted in 501.10(B)(5), 502.10(B)(4), and 503.10(A)(1).
(10) In plaster in dry locations.
(11) In damp or wet locations, where a corrosion-resistant jacket is over the metallic sheath.
(B) Specific Uses.
(1) In a cable tray per Article 392.
(2) Direct-buried, if protected per 300.5.
(3) As service-entrance cable, if installed per 230.43.
(4) Outside buildings, if complying with 225.10, 396.10, and 396.12.
Barred uses
You can’t use Type AC cable [320.12]:
(1) Where subject to physical damage.
(2) In damp or wet locations.
(3) In voids of block or tile walls that are exposed to excessive moisture.
(4) Where exposed to corrosive conditions.
(5) In plaster finish or concrete in wet or damp locations.
You can’t use Type MC cable where [330.12]:
(1) Subject to physical damage.
(2) Exposed to the destructive corrosive conditions in a or b (below), unless the metallic sheath or armor is resistant to the conditions, or is protected by material resistant to the conditions:
a. Direct burial in the Earth or embedded in concrete unless identified for the application.
b. Exposed to cinder fills, strong chlorides, caustic alkalis, or vapors of chlorine or hydrochloric acids.
(3) Exposed work, framing members, and roof spaces
Exposed Type AC cable, except as provided in 300.11(B), must closely follow the surface of the building finish or running boards. If installed on the bottom of floor or ceiling joists, it must be secured at every joist and not subject to physical damage [320.15]. For Type MC, the requirements [330.15] are the same.
Type AC cable installed through or parallel to framing members or furring strips must be protected against penetration by screws or nails by maintaining a 1¼ in. of separation between the cable and the nearest edge of a wood framing member or furring strip, or by a suitable metal plate per 300.4(A), (C), and (D) [320.17]. For Type MC, the requirements [330.17] are the same.
Type AC cable in roof spaces within 6 ft of the nearest edge of the scuttle hole entrance run across the top of framing members must be protected by guard strips that are at least as high as the cable [320.23(A)]. For Type MC, the requirements [330.23] are the same.
Bending, securing, supporting
Never bend any cable in a manner that may damage it. For Type AC, limit bending of the inner edge of the cable to a radius of at least five times the cable diameter [320.24]. For Type MC, what you do depends upon whether the cable is smooth sheathed or interlocked, and what its size is [320.24(A)(1)-(3) and (B)]. For example, for smooth sheathed cable up to ¾ in. in external diameter limit the bending radius of the inner edge of the cable to 10 times the external diameter of the metallic sheath.
You can support and secure either type of cable with staples, cable ties listed and identified for securing and supporting, straps, hangers, similar fittings, or other approved means designed and installed so the cable is undamaged. You can use their respective cable fittings as a means of cable support
Type AC cable must be secured within 12 in. of every outlet box, junction box, cabinet, or fitting and at intervals not exceeding 4½ ft [320.30(B)]. For Type MC cable with four or fewer conductors sized no larger than 10 AWG, the interval is 6 ft [330.30(B)].
Type AC cable must be supported at intervals not exceeding 4½ ft (6ft for Type MC). Cables installed horizontally through framing members are considered supported and secured if such support does not exceed 4½-ft intervals (6ft for Type MC) [320.30(C), 330.30(C)]].
Either type of cable can be unsupported and unsecured:
- Where fished through concealed spaces [320.30(D), 330.30(D)].
- Not more than 6 ft long from the last point of cable support or Type AC cable fitting to the point of connection to a luminaire within an accessible ceiling.
Type AC cable can be unsupported and unsecured where not more than 2 ft long at terminals and where flexibility is necessary (3 ft for Type MC].
Boxes and fittings
Unless the termination fitting provides protection, you must install an insulating anti-short bushing at each Type AC cable termination. The termination fitting must permit the visual inspection of the anti-short bushing once the cable has been installed [320.40]. There’s no corresponding requirement for Type MC.
Conductor ampacities
Use Table 310.14 to determine ampacity for either cable type. Where more than two Type AC (or Type MC) cables touch thermal insulation, caulking, or sealing foam, adjust the ampacity of the conductors per Table 310.15(C)(1) [320.80(A), 330.80(B)].
Equipment grounding conductor
Type AC cable can serve as an equipment grounding conductor (EGC) [250.118(A)(8)].
The internal aluminum bonding strip is not an EGC, but it allows the interlocked armor to serve as one because it reduces the impedance of the armored spirals to ensure a ground fault will be cleared. The combination of the aluminum bonding strip and the cable armor is what creates the EGC. The effective ground-fault current path must be maintained by using fittings specifically listed for Type AC cable [320.40]. See 300.12, 300.15, and 300.10.
The outer metal sheath of traditional interlocked Type MC cable cannot serve as an EGC, so this cable must contain an EGC of the wire type per 250.118(A)(10)a.
The outer metal sheath of all-purpose Type MC cable with an uninsulated aluminum grounding/bonding conductor can serve as an EGC per 250.118(A)(10)b.
Know the requirements
Because a cable is essentially a self-contained wiring system, it could seem cables all have pretty much the same requirements. The reality is those requirements may have big “redo the work” differences, even between two similar types such as Type AC and Type MC. Know and implement the requirements for each specific cable type.
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