When doing electrical projects on your home, you often encounter a type of metal-clad cable called BX. BX cable is not just a vestige of the past. Even with new projects, you still have the choice of using either metal-armored BX cable or plastic-sheathed NM cable.
BX Cable and Wiring Basics
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 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).
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 well-placed nail or screw.
Wires within the armor may display degradation of their rubber insulation. However, this may just be at the ends. If you rip back the armor, 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.
BX 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.|
Choosing Electrical Cable for DIY Work
As a do-it-yourself residential electrician, you likely will find it easier to handle, rip, and pull NM, or Romex-style, cable.
BX cable is heavy and its surface is corrugated, making it difficult to pull through the holes in seams. 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.
How to Rip Back BX Cable
There are two methods of removing the outer metal armor of BX cable. 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. It is possible to cut and rip the armor without that tool. You can cut the outer armor with a hacksaw, assisted with a very strong pair of wire snippers or pliers. With this method, though, it is very easy to nick 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.