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AC VS. MC CABLE

• Published in EC&M in August 1997
• Reprint permission granted
• By George Straniero
• Applications Engineer, AFC Cable Systems, New Bedford, Mass.

Considering their excellent fire safety track record, it comes as no surprise that the National Electrical Code has accepted Types AC (armored) and MC (metal clad) cables for decades. In fact, industry statistics show these products are involved in fewer fires than other accepted wiring methods. Although both types of cable are similar in outward appearance, looks can be deceiving. Recognizing and understanding the differences in their construction and permitted usage will keep you out of hot water with the inspector.

Read on to learn practical tips every electrical contractor, consulting engineer, and plant facility manager should know to effectively differentiate between AC or MC cables.

At a Glance

Although very similar in outward appearance, Type AC and MC cables have very different constructions, including conductor size range, grounding means, and permitted used per the NEC. Type AC cable is limited to four conductors while Type MC can have many more conductors. The former is limited to a smaller AWG size range (14 AWG to 1 AWG) while the later includes sizes from 18 to 200kcmil. Type AC cable has more NEC restrictions on permitted uses the Type MC. Many specialty-type constructions of Type MC cable provide cost savings while eliminating problems such as overheating shared neutrals on 3-phase 4-wire branch circuits powering nonlinear loads.

Basic Differences Between AC and MC Cables

Here's what you should know to distinguish between Types MC and AC cables.

• AC cable is limited to a maximum of four conductors, plus a grounding conductor, in sizes 14 AWG to 1 AWG. MC cable is not limited to the number of conductors and comes in sizes from 18 AWG to 2000kcmil.
• AC cable contains a 16 AWG bonding strip, which is in constant contact with the metal armor, allowing the armor and bonding strip combination to act as an equipment ground. MC cable does not contain a bonding strip, and the armor is not by itself an equipment ground. However, it supplements the internal grounding conductor.
• Moisture-resistant, fire-retardant paper wraps individual conductors of AC cable. Individual conductors of MC cable are not wrapped in paper but do have an overall polyester assembly tape.

Type AC Cable Construction

Number of conductors: As described by the NEC, Type AC cable is a "fabricated assembly of insulated conductors in a flexible metallic enclosure," usually consisting of two to four copper conductors inside an interlocked metal armor of aluminum or steel construction.

Bonding means: A 16-AWG aluminum bonding strip, unique to Type AC cable, is run inside the cable and is in constant contact with the metal armor, providing a low-impedance fault return path for the operation of overcurrent protection devices. The bonding strip allows the outer metal armor, in conjunction with the bonding strip, to serve as an equipment ground.

Individual conductor wrapping: Each copper conductor (most often having a 90¼C insulation) in a Type AC cable is individually wrapped in a moisture-resistant, fire-retardant paper.

Type AC Cable Permitted Uses

Permitted: NEC Article 333 lists permitted uses for Type AC cable; any applicable local codes may introduce additional restrictions. Basically, AC cable is permitted, where not subject to physical damage, for branch circuits and feeders, in both exposed and concealed work, for virtually all types of electrical systems.

It can be used in dry locations and embedded in plaster finish on brick or other masonry, except in damp or wet locations. It can be run or fished in the air voids of masonry block or tile walls, where the walls are not subject to excessive moisture/dampness or are below grade line. Accepted uses include under raised floors, above suspended ceilings, and in other environmental air-handling spaces per Sec. 30022(c).

Not Permitted: You can not install AC cable in the following locations:

• Places of assembly (defined as intended for the assembly of 100 or more persons).
• Motion picture studios.
• Classified hazardous locations.
• Areas exposed to corrosive fumes or vapors.
• Cranes or hoists.
• Storage battery rooms.
• Hoist ways or on elevators.
• Commercial garages.

Available Type AC Products

One new AC product resembles a standard AC cable in construction; however, it features a green grounding conductor. This additional ground allows its use, per Sec 517-13(a), in patient care areas of healthcare facilities (other than hazardous locations) including hospitals, nursing homes, dental offices, outpatient facilities, and medical centers. The separate green ground satisfies the following requirement of Art. 517: "In an area used for patient care, the grounding terminals of all receptacles and all noncurrent-carrying conductive surfaces of fixed electric equipment likely to become energized that are subject to personal contact, operating at 100 volts, shall be grounded by an insulated copper conductor." In addition, the armor and bonding strip combination satisfies the requirement that "... all branch circuits serving patient care areas shall be provided with a ground path for fault current by installation in a metal raceway system or cable assembly. The metal raceway system, or cable armor or sheath assembly, shall qualify as an equipment grounding return path in accordance with Section 250-91 (b)."

This new product is also suited for other applications requiring a separate or isolated ground. In isolated ground applications, where the effects of electromagnetic interference (EMI) on circuits supplying sensitive electronic equipment (as computers, cash registers, test instruments, etc.) are an issue, the separate green ground serves as the isolated ground for the sensitive equipment, and the bonding strip and armor combination as the other grounding means to ground other enclosures of the circuit.

Type MC Cable Construction

As described by the NEC, Type MC cable is a "factory assembly of one or more insulated circuit conductors with or without optical-fiber members enclosed in a metallic sheath of interlocking tape, or a smooth or corrugated tube."

MC cables have two or more solid or stranded conductors in sizes 18 AWG, and larger. The conductors may be copper, aluminum, or copper-clad aluminum. The metal armor may be a smooth tube, corrugated tube, or interlocked metal armor. By far the most common type of MC cable, and the one we'll discuss here, is the interlocked armor type with copper conductors.

The make-up of the typical 600V interlocked armor MC cable consists of copper circuit and grounding conductors covered with thermoplastic insulation, marker tape indicating cable type, an overall mylar cable assembly tape or overall paper wrap, and an outer metal interlocked armor.

Permitted Type MC Cable Uses

The permitted uses for MC cable appear in NEC Art. 334; again, any applicable local codes may introduce additional restrictions. Basically, the NEC permits MC cable, where not subject to physical damage, for services, feeders, and branch circuits, in both exposed and concealed work, for virtually all types of electrical systems. It can be run or fished in the air voids of masonry block or tile walls, where the walls are not exposed or subject to excessive moisture or dampness, or are below grade line. As per Sec. 30()-22(c), usage under raised floors, above suspended ceilings, and in other environmental air-handling spaces is acceptable. Wet locations are also acceptable when the armor has an overall outer, moisture-resistant, PVC jacket.

You can also use MC cable in the following locations and applications:

• Places of assembly (defined as the assembly of 100 or more persons).
• For power, lighting, control, and signal circuits.
• Indoors or outdoors.
• Direct buried where indicated for such use.
• In cable tray.
• In any raceway.
• As open runs of cable.
• As aerial cable supported from a messenger.
• In hazardous (classified) locations as permitted in Arts. 501, 502, 503, and 504.

Available Type MC products

Type MC cable is available in regular MC as well as a variety of specialty configurations.

Fire alarm/control cables: Dual-rated fire alarm control cables are designed for fire alarm wiring and remote control circuits. They are temperature-rated at 105¡C for power limited fire protective (FPLP) signaling cable applications and 900C for non-power limited circuit cable (Type MC) installations. FPLP cable is fully plenum rated, low smoke and fire resistant.

Branch circuit cables: Some special makes of cables come with an over-sized neutral conductor, or one neutral per phase for three-phase, four-wire power supply systems to computers, office machines, programmable controls, electronic discharge lighting, and similar electronic equipment. These non-linear switching loads produce additive third order harmonic current that may create an overloaded shared neutral conductor on a 3-phase, 4-wire circuit.

The oversized neutral conductor(s) are sized 150% to 200% of the phase conductor ampacity to minimize the heating effects of harmonic currents generated by the non-linear loads. The separate neutral per phase accomplishes the same objective.

An additional branch circuit cable is an MC cable used for multi-conductor runs from the panelboard to the junction box. This type of cable provides up to 16 conductors (phase-identified by colors), with separate markings for circuit identification.

Another special branch circuit cable features two grounding conductors for use in isolated ground applications. One serves as the isolated ground and is used to ground sensitive electronic equipment; the other ground is used to ground other metal enclosures of the circuit.

Wet environments have spawned a metal clad cable having an outer moisture-resistant PVC jacket that allows its use in wet locations, including direct burial in earth or embedded in concrete. Typical applications include swimming pool and spa motors and pumps, outdoor lighting, and parking garage wiring.

Another special type MC cable incorporates both current-carrying, data or video conductors and optical fibers, providing for power and voice transmission for industrial robotics, video conferencing, medical imaging, security cameras, etc.

Type MC & AC Installation Criteria

Derating. If you're using a multi-conductor cable assembly of more than three current-carrying conductors, you can calculate aerating per Sec. 310-16, footnote 8. For example, 8-conductor number 12 AWG plus insulated ground MC cable would be aerated in the following way. A 12 AWG, copper, 90¼C insulated conductor is rated at 30A when not more than three insulated conductors are installed in a raceway or cable. When using seven to nine conductors in a raceway or cable (per footnote 8 of Sec. 310-16), the table value of ampacity must be reduced by a 0.70 multiplier. Therefore, a 12 AWG, copper, 90¼C insulated conductor will have an ampacity of 21A(30A x 0.70). Sizing to the nearest overcurrent protection device size, our 8-conductor cable in this example, has a rated ampacity of 20A.

Cable cutting. Always cut Type AC or MC cable with an armored cable rotary cutter. This type of cuter eliminates nicking and cuffing of conductors as well as making quicker and safer cuts.

Cable routing. Usually, you install cable after the rough-in phase of locating and setting all boxes and enclosures. You should determine the best and shortest routes for each cable run because long runs of spider web-type routing not only require more cable but also increase voltage drop.

In general, the cable must hug the surface over which it is wired. The NEC prohibits bridging across open spaces, with the following exceptions.

• When fishing the cable
• When flexibility is required, a short piece of free cable up to 24 in. long run between a box and a motor or appliance {Type AC only).
• Lengths of not more than 6 ft from an outlet for connections within an accessible ceiling to lighting fixtures or equipment.
• Where installed in cable trays.
• For Type AC only, in other than vertical runs through punched holes in metal framing members, where distance between members does not exceed 4.5 ft and where the cable is securely fastened in place by an approved means within 12 in. of each box, cabinet, conduit body or other cable termination.
• For Type MC only, in other than vertical runs through punched holes in metal framing members where the distance between members does not exceed 6 ft. You must secure cables containing four or fewer conductors sized no larger than No. 10 AWG within 12 in. of each box, cabinet, fitting, or other cable termination.

Make sure you don't exceed the bending radius of the cables when routing around corners. For AC cable, the radius of the curve of the inner edge of any bend must be less than five times the external diameter of the cable. For MC cable, the maximum bending radius is seven times the external diameter of the cable. All cable runs must be continuous from outlet to outlet.

AC cable must be supported and secured at intervals of 4.5 ft or less (unless routed through a framing member) and within 12 in. of every termination.

MC cable must be supported and secured at intervals of 6 ft or less (unless routed through a framing member), and cables containing four or fewer conductors sized no larger than No. 10 AWG must be secured within 12 in. of every termination.

As noted, wave AC and MC cable support requirements with fished cable. This is a major advantage of AC and MC cable in remodeling work over other wiring methods that cannot be fished.

You may also install AC and MC cable in cable tray, per NEC Art. 318. You must secure these cables only in vertical runs of tray.

Cable terminations

When terminating or splicing at a junction, outlet, or switch box, cut the cable so 6 in. of free conductor remains for connections or splices. Use an approved connector and ensure a proper bond by firmly tightening the connector to both the box and cable. Caution: Set screw connectors cannot be used with aluminum armor Type AC cable.

For Type AC cable, insert an anti-short bushing and bend back the exposed length of bonding strip. (The bonding strip bends back before or after the bushing inserts.) Follow the same procedure for MC cable, with the exception that there is no bonding strip.

Insert the cable into the connector, and secure the connector into the box. Be sure the anti-short bushing is plainly visible in the connector for easy inspection. (Note: Anti-short bushings are required for Type AC cable and recommended for Type MC cable.)