All About MC Cable
Written by Nicholas Lloyd
*Disclaimer: for the purposes of this article, we will be using the most recently published cycle of the NEC (2020). Some material may be revised from other versions of the code.
Most of us are familiar with metal-clad cable, more commonly known as MC, or UL 1569. Today, we are going to be taking a more in-depth look at the most popular cable assembly of commercial electrical systems: how it is made, the allowances permitted in the code, some trivial information you may or may not know about the cable assembly, and unique characteristics of this material as used in our industry. So let’s “strap” in and divulge ourselves into everything you ever wanted to know about metal-clad cables.
Let us begin by taking a quick peek at Article 330 in our NEC. Metal-clad cable is defined by “a factory assembly of one or more insulated circuit conductors with or without optical fiber members encased in an armor of interlocking metal tape, or a smooth or corrugated metallic sheath”. Typically, we will see an interlocked aluminum or steel jacket spiraled around three or four THHN/THWN-2 solid wire. There are many, many different variations of this, from aluminum conductors to oversized and added neutrals, to isolated ground, fire alarm systems, etc., and in both aluminum armor or steel armor, ranging sizes from 18 AWG to 750 kcmil. Keep in mind, all MC and fittings used SHALL BE listed and identified for such use. This is not something a typical installer would have to lose sleep over; just keep an eye on those greenhorns so that they do not skimp out on a walk to the material trailer or connex and just use a flex connect, or worse a romex connector, or worse yet no connector at all! Need to refer you back to Article 110.12 (C), but let’s trudge on.
Now that we know what MC cable is and the many forms it comes in, let us take a look at Part II of Article 330, the installation. 330.10 and 330.12, respectively, are “Uses Permitted” and “Uses Not Permitted”. Under 330.10, we have the following permissions, in which we can use MC cable:
- “For Services, Feeders, and Branch Circuits”
- For power, lighting, control, and signal circuits
- Indoors or Outdoors
- Exposed or Concealed
- To be direct buried where identified for such use
- In cable tray where identified for such use
- In any raceway
- As aerial cable on a Messenger
- In hazardous (classified) locations where specifically permitted by other articles of Code
- In dry locations and embedded in plaster finish on brick or other masonry except in damp or wet locations
- In wet locations where a corrosive-resistant jacket is provided over the metallic covering and any of the following conditions are met
- The metallic covering is impervious to moisture
- A jacket resistant to moisture is provided under the metal covering
- The insulated conductors unter the metallic covering are listed for use in wet locations
- Where single conductor cables are used, all phase conductors and, where used, the grounded conductor shall be grouped together to minimize induced voltage on the sheath.
All that means is that we can use MC pretty much anywhere at any time, right? Yes, and no. Let me point out, again, that MC comes in all different sizes, shapes, and forms. Specifically, direct your attention to #4 and #11 in the list of “Uses Permitted”. Manufacturers make an MC cable that offers a layer of PVC jacketing that protects the sheath from corrosion and thus protects the wires inside. These wires are THHN/THWN-2 due to the probability of being exposed to damp or wet locations at some point in their lifespan. Therefore, you would not use standard aluminum armored MC for a direct burial installation. I will refer you to a video titled “This is why Electricians do not use Romex in wet environments”. The video hits on the major issue of Nonmetallic Sheathed cable, or NM-B, being exposed to a wet location; the same can apply for MC cable, being it is the Romex of commercial. Although the wires in MC cable are rated for wet or damp locations, the armored jacket is not. Over time, it will degrade and oxidize, even rust through, exposing wires to not only raw elements, but rendering it subject to physical damage.
Moving right along to 330.12 “Uses Not Permitted”, the conditions are as follows:
- Where subject to physical damage
- Where exposed to any of the destructive corrosive conditions in (a) or (b), unless the metallic sheath or armor is resistant to the conditions of is protected by material resistant to the conditions:
- Direct buried in the earth or embedded in concrete unless identified for direct burial
- Exposed to cinder fills, strong chlorides, caustic alkalis, or vapors of chlorine or of hydrochloric acids
This is a crucial article to familiarize yourself with. “Where subject to physical damage” could be objectionable, so it is always in the best interest to check with either a foreman, supervisor, or AHJ (Authority Having Jurisdiction) with what they deem to be subject to physical damage. A good rule of thumb is if it is questionable, it is subjectable. The NEC is a MINIMUM safety standard, it is always ok to protect above and beyond what the NEC requires. Being that Article 330.15 gives us an allowance to run MC exposed, it is important to determine the safety and longevity of that cable run. This is sounding like a “Dummy’s Guide” section, but our first and foremost concern should always be safety. Being subject to physical damage not only encompasses the integrity of that particular circuit, but the safety and wellbeing of others that may pass by or come in contact. In the highly unlikely event that an MC cable gets squashed, flattened and, somehow, does not trip the circuit protection, it could still be leaving exposed live parts in free air. Just some food for thought.
Another important aspect to consider in the integrity of the cable assembly is the bending radius. Let us take a look at the three materials discussed in the NEC.
- Ten times the external diameter of the metallic sheath for cable not more than 3/4“ external diameter
- Twelve times the external diameter of the metallic sheath for cable more than 3/4“ but no more than 1 1/2“ external diameter
- Fifteen times the external diameter of the metallic sheath for cable more than 1 1/2“ external diameter
Interlocked-Type Armor or Corrugated Sheath
- Seven times the external diameter of the metallic sheath
- Twelve times the overall diameter of one of the individual conductors, or seven times the overall diameter of the multiconductor cable, whichever is greater
Basically, we need to apply some simple math here. Let’s say, for example, we have a Smooth Sheath MC cable with an external diameter of 1/2“. Since the item would be classified as a #1 under smooth sheath (3/4“ and less), we will use the x10 multiplier here. If we convert 1/2” into a decimal, we get 0.5”, so 10 x 0.5” is 5”. Therefore, our bend radius of a Smooth Sheath MC cable with an external diameter of 1/2” is 5’’. For another example, let’s use a 4/3 with ground, Shielded Conductor MC cable. We will use Table 5 located in Chapter 9 of the NEC to find our approximate diameters for our calculations. The external diameter of the cable assembly is 0.941”, and the overall diameter of the individual 4 AWG stranded THHN/THWN-2 is 0.232’’. If we take the external diameter multiplied by 7, we would get 6.587”; if we took the individual conductor and multiplied by 12, we would end up with 3.888”. Therefore, we would choose the external diameter equation resulting in a bend radius of 6.587”.
Article 330.30 of the NEC deals with securing and supporting MC cable. 330.30 (A) states: Type MC cable shall be supported and secured by staples; cable ties listed and identified for securement and support; straps, hangers, or similar fittings; or other approved means designed and installed so as not to damage the cable. Again the code stresses the importance of not undermining the safety and integrity of the cable assembly. Let’s continue with 330.30 (B): Unless otherwise provided, cables shall be secured at intervals not exceeding 6 ft. Cables containing four or fewer conductors sized no larger than 10 AWG shall be secured within 12” of every box, cabinet, fitting, or other cable termination. In vertical installations, listed cables with ungrounded conductors 250 kcmil and larger shall be permitted to be secured at intervals not exceeding 10 ft. At any point the cable leaves an enclosure, it shall be secured within 12’’ of that enclosure. This does not mean you have to pull out a tape with every single securing means and place it at these exact intervals. Continuing on in 330.30 (C) with Horizontal runs of Type MC cable installed in wooden or metal framing members or similar supporting means shall be considered supported and secured where such support does not exceed 6 ft. In a typical install, metal framing members will have holes punched out during the manufacturing process to provide quicker installation of mechanical materials, such as cable assemblies or plumbing. With wood framing, holes have to be bored out by the installer prior to running cable assemblies. It’s important to note that when boring holes through wood framing members or punching out/ drilling holes in metal framing, Article 300.4(A)(1) tells us we have to have no less than 1 1/4” spacing between the edge of the hole and the nearest edge of the framing member, unless we use a steel plate no less than 1/16” thick. The only exception to this rule is when Rigid Metal Conduit is used as a raceway through framing members.
Article 330.30 (D) Unsupported Cables
- Is fished between access point through concealed spaces in finished buildings or structures and supporting is impractical
- Is not more than 6 ft. in length from the last point of cable support to the point of connection to luminaires or other electrical equipment and the cable and point of connection are within an accessible ceiling
- Is Type MC cable of the interlocked armor type in lengths not exceeding 3 ft. from the last point where it is securely fastened and is used to connect equipment where flexibility is necessary to minimize the transmission of vibration from equipment or to provide flexibility for equipment that requires movement after installation
This article mentions that there are times when support is not practical, so here are some provisions that we are granted. Say there is a change order, or a jumper was forgotten – part 1 of this section allows us to run cables through walls without having to strap or support it. Part 2 says that there has to be a way to connect power to lighting fixtures or luminaires suspended from a ceiling. When the luminaire is suspended by aircraft hanger cables or a linked chain, we will run our MC cable along that suspended support and use cable ties to tie it off to the support. Lastly, this comes mostly in part with motor wiring and other equipment that will be vibrating or have to be moved over time. We don’t use hard pipe and rigid connectors with vibrating equipment because over time, that vibration produced by the machine will cause fittings and screwed parts to become loose.
Now let’s get into the desert of our purpose: actually working with MC. We’ll tear off the packaging or cut off the tie wires holding the roll together and prepare to pull the circuit. One of the first things you’ll notice about the construction of the MC is that there appears to be “arrows” manufactured into the corrugation of the armored sheathing. It is in this direction that will be the easiest way to pull the cable assembly. The “arrows” will typically be when pulled from inside the roll instead of just unravelling it normally. Some manufacturers do make a “reverse wound” roll for ease of the pull. Another thing to notice is the pre-punched holes in the metal studs. There will be a slight taper in one direction of the stud, indicating the way a pull will be less physically demanding.
Fact of Fiction? The NEC requires the use of anti-short bushing, or “red-heads”, in it’s installation and termination of Type MC cable when entering an enclosure. FICTION! Neither manufacturers nor NEC require anti-shorts to be used while installing MC. A lot of electricians and AHJs, however, do prefer the use of “redheads” on any metallic-armored cable or raceway. This does not exempt the use of anti-short bushing to be used with Armored Cable, which is essentially the same as MC cable except AC uses a smaller grounding wire or bonding strip as well as it’s jacketing whereas MC uses a full sized grounding wire. The reason we use anti-short bushings on AC cable and not MC cable is simply this: the NEMA requirements for connectors are different. The construction of an MC connector does not allow for the cut edge of the armor to come in contact with the wire (if used properly and in accordance with manufacturers specs).
Speaking of terminating MC into enclosures, there are many ways to strip back the armor to prepare the cable to be fastened down. A tool specifically designed to cut armored cable typically has a handle to clamp the assembly to hold it in place, and a handle attached to a cutting wheel that cuts two of the high ribs of the armor, allowing the user to simply pull off the loose armor to expose the wires within. Another method is bending the cable past it’s bend radius and snapping the armor apart. Taking diagonal cutters, cut the armor at a 45º angle, and pull off the loose armor to expose the wires within. Whatever method you choose, there will be a plastic, oily liner surrounding the wires; this will also need to be cut away to install the cable into a connector.
In conclusion, we have worked our way through Article 330 and all of it’s requirements, uses permitted, uses not permitted, the construction of the cable assembly, the bending radius of different types of MC, tips and tricks when working with this product in the field, securing and supporting with exceptions, and some misconceptions between MC cable and Armored Cable.