When doing electrical projects on your home, generally you will be using NM, or non-metallic, electrical cable as it is easy to handle and inexpensive. But occasionally you might open up a wall or ceiling and encounter a type of metal-clad cable called BX. 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.
What Are BX Cable and Wire?
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. BX's metal sheathing 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 to 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.
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 removing the outer metal armor of BX cable: with a special BX cutting tool, a hacksaw, or manually with pliers.
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.
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.
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.
Shapiro, David E. Old Electrical Wiring. 2nd ed., Mcgraw-Hill, 2010.