BX Cable and Wire: What to Know Before You Buy

BX Cable and Wire: What to Know Before You Buy

Running electric cable from the electric service panel to devices like outlets or switches is at the heart of any electric project. Your choice of electric cable is not only based on the project requirements but on your skill level and experience.

That's why homeowners tend to gravitate to NM, or non-metallic, electrical cable (Romex is a popular brand of NM cable). NM wire is lightweight, easy to handle, and inexpensive. It's simple to cut, rip, and strip, plus it pulls effortlessly through holes in studs.

But occasionally you might open up a wall or ceiling and encounter a type of ribbed metal-clad cable called BX. In some cases, you might elect to use BX cable instead of NM cable.

While widely used in the past, BX cable is not relegated to the past. Even with new projects, you still have the choice of using either metal-armored BX cable or plastic-sheathed NM cable, and there are many good reasons why you might decide to use BX.

What BX Cable and Wire Is

Going under alternative names such as metallic sheathed cable, type AC, MC, Greenfield, or armored cable, BX cable is a collection of plastic-coated insulated wires (typically 14- or 12-gauge), bundled together and protected by a ribbon-like metal sheathing.

At first glance, BX's metal sheathing looks like it is a single ribbed metal tube. It's actually a metal strand that runs in a helix-like or twisted manner around the wires.

BX is contrasted with a newer cable, NM, which stands for "non-metallic." Instead of the metal sheathing, NM has a slick vinyl covering that is easy to rip and pull through holes in studs. Romex is one popular brand of NM electrical cable.

A chief distinction between BX and NM is that BX can achieve grounding through the outer metal casing. This casing needs to be attached to metal boxes.

Another distinction is that some types of BX cable can be installed in exposed locations, either indoors or outdoors. NM cable and wiring must always be installed in an enclosed location (typically within a wall, ceiling, or under a floor). Always be sure to check with your local building and electrical codes as to whether BX cable may be left exposed.

BX Cable Longevity and Replacement

Like any other cable, if the armor is nicked, cut, or shredded, the wires inside can be compromised. BX's armor, while much stronger than NM's vinyl, can still be pierced by a determined and ill-placed nail or screw. However, with the exception of electrical wires that run through rigid metal conduits, no other type of electrical cable has as strong an outer casing as BX cable.

Wires within the armor may display degradation of their rubber insulation. But this may just be at the exposed ends. If you rip back the metal sheathing, you may find that the insulation is still good.

If old BX wiring is in good condition and can carry today's higher power demands, there is usually no reason to replace it. Unlike the older knob-and-tube wires from the early 20th century, the wire sheathing will not turn gummy and degrade over time.

Older BX wiring does not have to be removed unless the wire coating inside the sheathing is in bad condition.

BX Cable vs. NM Electrical Cable

  BX Cable NM Cable Ripping BX is difficult to rip back without a special tool. NM is far easier to rip back. This is accomplished with an inexpensive cable ripper. Cost BX cable is more expensive than NM cable. NM cable tends to run about 25 percent cheaper than BX cable as it ships lighter and uses fewer source materials. Handling BX is heavy and difficult to run through studs. Not only is NM cable light, but the slippery coating makes the cable easy to pull through holes in studs. Safety BX is safer than NM since the metal armor protects well against accidental penetrations. NM cable's vinyl sheathing is easily penetrated. Grounding BX cable is grounded via its metal armor or internal green plastic-coated ground wire. Because vinyl is not conductive, grounding is achieved by a separate bare copper ground wire in the bundle. Cutting BX is cut with a hacksaw. Better yet, use a special armored cable cutting tool. NM cable can be cut with a lineman's pliers or even with the cutter on a wire stripper. Code BX is accepted by the National Electrical Code (NEC). Older BX cables without an internal bonding strip are not accepted by NEC. NM cable is also accepted by the NEC.

How to Rip Back BX Cable

There are three methods of ripping, or removing, the outer metal armor of BX cable: with a special BX cutting tool, a hacksaw, or manually with pliers.

Cable Ripping

Ripping an electrical cable means to sever, pull back, and remove the outer sheathing protecting a set of bundled wires. Ripping is contrasted with wire stripping, To strip a wire means to remove the plastic coating on an individual electric wire.

BX Cutting Tool

If you expect to be doing a lot of cutting, you may wish to invest in a special BX cutter, such as the Roto-Split. This tool costs between $20 and $50 and makes the job of splitting and ripping back BX cable far easier and safer than by hand.

After you insert the cable into the tool's groove, you turn the handle to cause the cutting wheel to cut away the metal sheathing. The tool is calibrated to cut the metal but stops short of touching the inner wires.

Cutting BX by Hand

It is possible to cut and rip the armor without a BX cutting tool. You can cut the outer armor with a hacksaw, assisted with a strong pair of wire snippers or pliers.

With this method, there is the danger of nicking the insulation on the inner wires, not to mention lacerating your fingers on the sharp metal armor.

BX Cable Development and History

BX is one of the earliest types of electrical cable developed for both residential and commercial uses in the early part of the 20th century.

Early forms of BX can still be found by homeowners renovating their homes. It is not certain how the term "BX" came to represent metal-armored cable, but it may have something to do with the product first being produced in the Bronx borough of New York.

Early accounts of BX cable being used date back to 1910. Widespread production and use of BX cable began in the 1920s when BX was promoted as being a modern, safe, fireproof improvement over older cloth-sheathed wiring.

Should You Buy BX Cable or Wire?

As a do-it-yourself residential electrician, you likely will find it easier to handle, rip, and pull NM, or Romex brand, electrical cable. Unless the specifics of the job or the electrical code demand that you use BX cable, your wiring project will go faster with NM, plastic-sheathed wiring.

BX cable is heavy and its surface is corrugated, making it difficult to pull through the holes in studs. BX cable's metal sheathing can be hard to cut without nicking or severing the inner wire. NM cable, too, presents the danger of nicking inner wires. But because the outer sheathing is softer, less force is required to cut it. 

In addition, while home improvement stores still do carry BX cable, do-it-yourselfers will find a far greater selection of NM cables at retail outlets.

The Spruce uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.

Shapiro, David E. Old Electrical Wiring. 2nd ed., Mcgraw-Hill, 2010.

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What’s the Difference Between Shielded and Unshielded Category Cable for AV?

Category cabling is made of twisted pairs, which—when carrying a balanced signal (signals that are equal in magnitude and opposite in phase)—creates its own electromagnetic field within and around the twisted pair. When the signal is balanced, it provides its own immunity to outside noise interference. (Watch for a future Belden blog about how we manufacture the best balanced twisted-pair cable possible using expertise we’ve developed over several years.)

Shielding can be another good way to keep outside electrical interference from impeding a signal. It can also keep the signals inside a cable from impacting other cables, but more on that later. In noisy environments or for sensitive signals—which are often found in AV installations—a shielded Category cable might offer some benefits if chosen and installed correctly.  

So how do you decide whether a shielded version is the better option—or a required solution—for your situation? The best place to start is with the equipment manufacturer. What type of cabling does it recommend? The manufacturer should be designing its system around the requirements. (This isn’t to say you won’t run into an issue. Maybe you want to design a system to support multiple applications and build futureproofing into the project.)

What do we mean by “shielded cable” as it applies to Category cabling? To start, there are two ways to provide a shield for a cable:

Metal foil

Metal wire braid

You can use one or the other—or a combination of both. Foil is most common, provides 100% coverage and is good for high-frequency signals; however, it doesn’t flex well. As a result, you’ll occasionally see braid used in specialty applications where cable is often moved around. A braid consists of multiple small strands of wire woven around a cable. Due to spacing between the strands, the braid is good only for low-frequency signals. For this reason, a braid is usually used along with a foil.

Category cabling typically consists of four twisted pairs. Shielding can be placed on the outside of all pairs, on each pair individually or both. Do you want to shield from outside noise or from noise between each pair within a cable? Either way, the continuity of the shield throughout the cabling system must be maintained, including within the connectors (which is often overlooked). Later, we’ll discuss why outside noise is usually the biggest interference factor in AV projects.

For most common cables—Category 5e, Category 6 and Category 6A—there are two choices:

Unshielded twisted pair (UTP)

Aluminum foil tape around all twisted pairs (foiled/unshielded twisted pair, or F/UTP), which is commonly referred to as shielded Category cable

Some specialty shields exist, but those are generally for specific harsh environments, such as those requiring the high flexibility we mentioned earlier.

A shield works by transferring induced currents from external noise signals to ground and away from the twisted pair. This requires not only the cable shield to be bonded to ground, but also the connectors. In addition to shielded cable, Belden offers shielded jacks, plugs, patch panels and patch cords.

Our recommendation is that the shield be bonded to ground at both ends of the cabling system per TIA standards; however, this isn’t followed by everyone and could result in additional issues. Systems are often installed with telescoping ground, which is only bonded on one end—and this is not ideal, either. If you don’t ground both ends, then you could be creating an antenna (an invitation to external noise), making things even worse.  

Alien Crosstalk: A High-Speed Signal Killer

One of the biggest sources of noise for a twisted pair are the twisted pairs next to it. They can be internal and external to the cable. Through predictive processing, chip makers figured out how to filter noise from other pairs within the cable. For signals outside the cable, however, they don’t know the signal and can’t filter it out. This type of crosstalk is referred to as “alien crosstalk” or “crosstalk alien to the cable.”

Alien crosstalk is what separates Category 5e and 6 from Category 6A cabling. Category 6A cabling features special twists and isolators to dramatically reduce crosstalk from adjacent cables. This is another great reason to consider upgrading to Category 6A cabling. You might not need 10 Gb/s speed, but you will benefit from the cable’s enhanced design to improve immunity to external noise within your network. 

Doing Bonding and Grounding Right

Earlier, we mentioned that the shield needs to be continuous and bonded to ground at both ends for best results.

Belden recently completed testing with 10 Gb/s signals and Category 6A F/UTP and UTP cables. (More about that in a future blog.) For this testing, we measured the integrity of a 10 Gb/s signal when subjected to external noise signals of between 0 MHz and 500 MHz.

Not surprisingly, we found that the most effective solution was the shielded F/UTP cable with the shield bonded to ground at both ends. What was surprising was the UTP cable outperforming the F/UTP cable when the shield bond to ground was only at one end. This helps reinforce our stance that shielded cable needs to be bonded to ground on both ends to be most efficient.

Avoiding a Grounding Loop

Of course, one possible downside of connecting a cable’s shield at two points is the fact that, under certain conditions, it can create a grounding loop (current on the shield due to two ground points having different potentials).

This is especially true in very long runs or runs between buildings. It’s important to note: Grounding is a critical safety concern within a building. Please rely on a qualified professional to address grounding issues. Ground loops can cause a large amount of noise and hum on cables and may be a good reason to avoid shielding and use a UTP cable instead.

Shielded cables also come with an increased cost. In general, shielded cables, connectors, patch panels and patch cords have a higher cost than UTP versions. In addition to increased material costs, extra labor is required to properly bond the shielding to ground. Combined, these additional expenses could increase the installation cost of a shielded system by as much as 25% per drop when compared to a UTP system.

If a UTP system isn’t an option for you, then the REVConnect® shielded system is a good option. It minimizes additional labor, reducing added costs from 25% per drop to less than 15% when compared to a UTP system. In addition, shielded cables have larger diameters and require more conduit cable tray space; some shielded solutions also have a large bend radius. 

In the end, the real question comes down to this: Do the additional costs and steps warrant the use of an F/UTP solution? Can you be assured that the installation will be done correctly? How much electrical interference is at the site? Has noise been a problem in the past? Like most situations, there isn’t one answer for all. When answering this question, you’ll want to consider:

What’s recommended by the equipment manufacturer?

How much electrical noise is in your environment?

Do you have access to a common ground at both ends?

Are system requirements going to change in the future?

Belden offers both options, and we would be more than happy to help you address whatever your job needs. To learn more, watch a recent Shielded vs Unshielded Cabling webinar we led with PSNI Global Alliance.

P.S. Don’t miss our upcoming blog, where we take a look at well-balanced cables, what they are and how they compare to shielded cables. We’re here to help you make the right decision when it comes to mitigating noise.