The NEC contains 12 articles (320 through 340) covering types of cables. The cable types covered include: armored cable (Type AC); flat cable assemblies (Type FC); Flat conductor cable (Type FCC); integrated gas spacer cable (Type IGS); metal-clad cable (Type MC); mineral-insulated, metal-sheathed cable (Types NM and NMC); instrumentation tray cable (Type ITC); power and control tray cable (Type TC); Type P cable; service-entrance cable (Types SE and USE); and underground feeder and branch-circuit cable (Type UF).Any cable must be listed and any fittings used with it must be listed for use with that type of cable.
Let’s compare two commonly used cable 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), as shown in Fig. 1.
Type AC cable 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 [Art. 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, as shown in Fig. 2.
Type MC cable 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 [Art. 100]. It contains any number of insulated conductors, 18 AWG through 2,000kcmil, 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 [Sec. 320.10] in:
- Feeders and branch circuits (exposed or concealed).
- Cable trays.
- Dry locations.
- Plaster (dry locations).
- Voids in block walls where not exposed to excessive moisture or dampness.
And in plenum spaces per Sec. 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.
- Branch circuits, feeders, and services.
- Power, lighting, and power-limited circuits.
- Indoor or outdoor locations.
- Exposed or concealed.
- Directly buried (if identified for the purpose).
- In cable tray (if identified for the purpose).
- In raceway.
- As aerial cable on a messenger.
- In hazardous (classified) locations as permitted in Sec. 501.10(B)(5), Sec. 502.10(B)(4), and Sec. 503.10(A)(1).
- In plaster in dry locations.
- In damp or wet locations, where a corrosion-resistant jacket is over the metallic sheath.
(B) Specific Uses.
- In a cable tray per Art. 392.
- Direct-buried, if protected per Sec. 300.5.
- As service-entrance cable, if installed per Sec. 230.43.
- Outside buildings, if complying with Sec. 225.10, Sec. 396.10, and Sec. 396.12.
Barred uses
You can’t use Type AC cable [Sec. 320.12]:
- Where subject to physical damage.
- In damp or wet locations.
- In voids of block or tile walls that are exposed to excessive moisture.
- Where exposed to corrosive conditions.
- In plaster finish or concrete in wet or damp locations.
You can’t use Type MC cable where [Sec. 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 Sec. 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 [Sec. 320.15]. For Type MC, the requirements [Sec. 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 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 Sec. 300.4(A), (C), and (D) [Sec. 320.17]. For Type MC, the requirements [Sec. 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 [Sec. 320.23(A)]. For Type MC, the requirements [Sec. 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 [Sec. 320.24]. For Type MC, what you do depends upon whether the cable is smooth sheathed or interlocked, and what its size is [Sec. 320.24(A)(1)-(3) and (B)].
For example, for smooth sheathed cable (up to 3/4 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 41/2 ft [Sec. 320.30(B)]. For Type MC cable with four or fewer conductors sized no larger than 10 AWG, the interval is 6 ft [Sec. 330.30(B)].
Type AC cable must be supported at intervals not exceeding 41/2 ft (6 ft for Type MC). Cables installed horizontally through framing members are considered supported and secured if such support does not exceed 41/2-ft intervals (6 ft for Type MC) [Sec. 320.30(C), Sec. 330.30(C)]].
Either type of cable can be unsupported and unsecured:
- Where fished through concealed spaces [Sec. 320.30(D), Sec. 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 [Sec. 320.40]. There’s no corresponding requirement for Type MC.
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) [Sec. 320.80(A), Sec. 330.80(B)].
Equipment grounding conductor
Type AC cable can serve as an equipment grounding conductor (EGC) [Sec. 250.118(A)(8)], as shown in Fig. 3.
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 [Sec. 320.40]. See Sec. 300.12, Sec. 300.15, and Sec. 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 Sec. 250.118(A)(10)a, as shown in Fig. 4.
The outer metal sheath of all-purpose Type MC cable with an uninsulated aluminum grounding/bonding conductor can serve as an EGC per Sec. 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.
