NM Cable vs Armored Cable

wire cable with rubber sleeve open to reveal colored sub wires
  • Beginner

If you are thinking of running wiring in your home, you will have several options for the type of wire to use and how to protect it; however, two of the most common and popular choices are NM cable and armored cable.

Both types of cable are very popular with DIYers because they are easy to work with and can be used in many applications without the need for metal conduit (if allowed by the building code and inspectors).

NM (Non-Metallic) Cables

Romex cables (two insulated, one bare metal)

Sometimes referred to as “Romex”, NM cable stands for “non-metallic” cable. Romex cables are composed of 2 or more insulated wires and one bare wire used for the ground connection.

The label Romex comes from “Rome Cable Corp” which originally produced the cable for the first time. Its outer jacket is a 30 mil PVC jacket that protects its wire conductors with tough sheathing to offer maximum durability and enough flexibility to work with when passing the wiring through holes in the studs, wallplates, and joists.

In its most basic form, it contains a live wire which is the one with black insulation, a neutral wire which is the white insulated wire, and a bare wire which is used to connect every appliance on a specific circuit to the ground.

It can however contain more wires with different colors such as red which is meant to identify a second live line such as for a 220 volts circuit.

More colored wires can also complete an electric cable such as blue, yellow, orange, used as travelers on 3-way and 4-way switches or as switch legs when wiring ceiling fans, lights, and switched outlets.

The NM cables are manufactured in a variety of sheathing colors. The cable jackets are color-coded to permit identifying the cable sizes quickly—black jacket for #6 and # 8 AWG, orange jacket for # 10 AWG, yellow jacket and red jacket for # 12 AWG, and white jacket for # 14 AWG. NMD90 Red sheathing is only used for wiring baseboard or convection heaters.

The cable is very easy to work with and its sheathing can be easily reaped and removed with a cable ripper, scissors, or utility knife, or with the utility knife alone.

Armored or BX Cables

armored wire cable

Brought around in the early 20th century for residential and commercial use, BX or armored cables can still be found in older homes made of different materials and of varying concepts. Nowadays BX cables are also referred to as MC, type AC, armored cable, and Greenfield cables.

At its core, BX cable is made up of several insulated wires and one ground wire which may or may not be insulated and held together with strips of paper wrapped around them in a spiral fashion.

Those wires are then again wrapped around with a metal interlocking ribbon spiraling around the wires to cover the full length of the now shielded cable. This type of sheathing where a specially profiled winding partly wraps over the previous one locking the two together in a loose manner gives the shield some flexibility to make it more pliable to work with.

Inside the sheathing, the cable is made up of one or two power lines or hot lines (one black and one red if there are two), a neutral white wire, and a ground wire—either insulated green or bare copper. It can also contain more wires coded with different colors such as blue, orange, yellow, etc. used as travelers (as needed for 3-way or 4-way switches).

Besides the ground wire to provide GND connection BX cables can also be grounded through the metal sheathing by tightening it inside a metal cable connector providing the junction box is metal.

The fact that BX cables have a metal shield makes them that much more durable than other types of cables. Although it has some flexibility, it is not nearly as flexible as a vinyl cable, and therefore more difficult to work with.

The armored casing makes it also much more difficult to cut and to work with by hand using side cutters or a hacksaw, but easier accomplished with a special armored cable cutting tool.

Although NM cables are better suited for general wiring than BX, there are certain situations where some cables cannot benefit from the wall enclosure to protect them resulting in partial or total exposure to potential hazards. In such situations, the Bx can offer one of the best available solutions.