BX Cables: What To Know Before You Buy

"BX cable" is the sort of term most homeowners will only hear about in passing. "What you have in there is BX cable," a contractor might say, "so you need all new wiring, plumbing, and a new roof." When you try to look it up, it appears to refer to at least a couple of completely different things. And if you ask for help, 17 licensed electricians will immediately tell you that you're going to burn down your house unless you stop asking questions and hire a professional.

To understand any cable, including BX, the first step is knowing what a cable is in the first place, and where better to look for definitions than a dictionary? Warshauer Electric Supply hosts a very thorough lexicon of electrical industry jargon and slang, from which you learn that electricians are a colorful lot and that they seem to have a tenuous grasp on human anatomy. You also learn that a cable is "two or more insulated conductors wrapped in metal or plastic sheathing." (Note that it can also include a grounding conductor that might not be individually insulated.) Think of a cable as a group of more than one wire. To get a handle on all of this, it's important to know that modern BX cable is a term used interchangeably with AC (armor-clad) cable. Older BX cable is substantially different from the new stuff, and is no longer allowed by code, and this is the source of yet more confusion — but we'll get to that later.

It might help to go back to the basics. Understanding all electrical cables starts with understanding Romex, a brand name of Southwire that is often used to stand for common nonmetallic (NM) electrical cable. The weaknesses of Romex — namely, its lack of protection against damage and environmental factors — are the positive attributes of metal-armored cable. Whereas Romex is easy to both install and cut or puncture, armored cables are more difficult to use, specifically because they provide their wires more protection.

You can think of BX cable as a type of metal-clad (MC) cable in which a kraft paper wrap (via AFC Cable Systems) protects the conductors from shorting on the armor, and which can, but doesn't always, contain a separate ground wire (because of the integral bonding wire mentioned above). MC cable uses its metal armor only for mechanical protection from damage. BX can carry an additional insulated ground wire inside the sheathing, but this is usually used in applications that require redundancy, as in medical facilities.

So, the key differences between BX and MC cable are the use of the armor as a ground path, the material used as protective sheathing,  and the fact that MC can — when specified for damp locations — be used outdoors, but BX/AC cannot. One final distinction is that MC cables don't have a limited number of conductors they can contain, while AC cable can contain no more than four insulated conductors. What MC and BX have in common is their basic armored structure, which means that its individual conductors are better protected than NM and similar cable. Both are sometimes simpler to install that wire in conduits, which reduces the cost. BX cable can, by some estimates, be 35-40% more expensive than NM (via Healthy Building Science), yet still be cheaper to install overall.

As you might have noted from the special applications in the previous section, BX is primarily used for commercial, industrial, and multi-branch residential circuits. BX is also occasionally used for standard single-family homes. AFC Cable Systems describes a number of applications, the most common of which might be exposed and surface-mounted installations. Because the cable is armored, it has enough protection from the usual hazards (toddlers, mountain bikes, throwing knives) that it can be mounted outside of your walls rather than within. This obviously makes retrofitting your bonus room to include electric throwing knife targets much easier. The usual caveat applies, though: "Exposed" doesn't mean "exposed to the weather." Keep your AC/BX cable indoors.

BX can also be embedded in plaster on masonry and run in the air voids of masonry walls, under raised floors, and in air-handling spaces. You might see raised floors, for example, in churches or data centers. And, again, take note that these allowances don't apply if any of those spaces are prone to getting wet.

BX cable is, naturally, rodent-resistant, so if your home is in the country and therefore prone to mice — or if your spouse keeps beavers as pets — you might consider the protection of BX cable. And if your spouse has all but stopped practicing knife-throwing or hanging out with beavers because of unusually high EMF readings, it's worth noting that BX cable reduces both magnetic fields and electrical fields normally created by home wiring systems, whereas MC cable only protects against magnetic fields (via Healthy Building Science). These same properties might make BX beneficial for use by audiophiles, as well.

The very mention of "BX" produces alarm in the eyes and heart of many older electricians and home inspectors, because early versions of BX were deeply flawed. Those flaws have been corrected in the modern product, which is allowed by the NEC as AC cable, while old-style BX is not. According to JP McCurdy Electrical Services, a lot of the negativity surrounding BX cable comes from both the shortcomings of the old-style cable and the misuse or misunderstanding of the cable in the first place. An example of this is the use of the grounding conductor (and therefore the armor itself) as a neutral conductor rather than a ground — an inexpressibly dangerous mistake. McCurdy says that the insulation, wraps, anti-short bushings, bonding wire, and galvanizing process of the early BX cable have all been improved and are as safe today as other approved cables.

Early BX, on the other hand, can be hazardous. For example, the earliest versions didn't include a bonding wire (they used only the armor for a ground path), so its armor could get dangerously hot in a fault condition. The reason for this seems a little arcane, but is actually straightforward enough. Metal armor is spiraled around the wires within. You calculate its properties as a conductor by stretching it out to its uncoiled (and much greater) full length, because the interlocking edges aren't considered a strong enough electrical link. Given the resistance of the steel used in early BX cable, this effectively means you are using a ground wire that isn't of sufficient capacity, as if using a 16- or 18-gauge ground wire for 12-gauge conductors — a clear no-no throughout the industry.

Cable manufacturer Southwire claims BX can reduce installation costs by 50%, as compared to conduit and wire. This is a major advantage for BX, but the savings probably mostly apply to MC cable as well. Let's have a look at the installation process. Typical residential MC and AC installations start after wall-framing with careful planning and layout, beginning with home-runs from the load center (circuit breaker box), says the National Armored Cable Manufacturers Association. The average residential installation is basically similar to standard Romex, except that obviously the support fittings, connectors, etc. are different. Cable may be routed through wall studs and certain dry structural spaces, such as within masonry air space gaps. MC cable must be supported every 4.5 feet and AC every 6 feet, except in a few special circumstances. Both cables may be cut in a number of ways, though the use of a standard rotary cutter makes for the cleanest edges, which is important in all cases and critical for using anti-short bushings as required for AC cable (and sensible for MC cable as well, via WikiHow). 

Termination varies by cable sub-type, but, in general, the processes for AC and MC are basically the same, except that MC will have a grounding conductor that must be terminated properly at each electrical box, adding 50% more wire terminations per box. Keeping expensive AC and MC variants like AC-HCF and MCI-A out of the picture (since they mostly apply to commercial and industrial settings), this means that there will be some minor increase in labor cost associated with installing MC versus AC cable.