Armoured cable is commonly called "BX", which is a trademark name. The National Electric Code refers to steel armoured cable as AC, or armoured cable. Steel armoured cable is for use in dry locations that never get wet. Typically, the cable contains either two or three wires in 8, 10, 12 or 14 gauge, which are wrapped with paper. A bare, bonding wire or strip is also present. Never wire the bonding strip as a ground.
Cutting Armored Cable
The outer sheathing of steel armoured cable is made of a steel strip that is wound in a spiral, creating a continuous tube. Choose the location you wish to cut the cable and saw perpendicular to the steel strip. A vice to hold the cable is helpful. Once the strip is cut, twist the two sides in opposite directions and pull the cable apart.
A better and easier way is to use an armoured cable cutter available at home centres and hardware stores. The cable is placed in the cutter and a handle rotated until the wheel cuts the cable. The cable will pull apart at the cut if you twist the cable on either side of the cut.
It takes three cuts to have a useful piece of cable for wiring. One to the cable to length, and two more to remove the armoured sheathing and expose the wires to make connections. Short pieces can be done with two cuts. When using a hacksaw, be very careful not to damage the wire insulation or the bonding wire.
Terminating the Steel Armor
A red bushing called a red hat is slipped over the wires and into the steel sheathing. This bushing is required by code and if they can't see it, inspectors will fail the installation. The bushing protects the wires from damage by the sharp steel sheathing edges. Slip the bushing over the wires and down, into the sheathing.
Bend the steel bonding wire back over the bushing and alongside the armoured steel sheathing. Slide an armoured cable connector over the wires down to and over the sheathing. Notice that you can still see the red bushing. Tighten the screw that clamps the connector to the sheathing. The connector will not move, turn or twist if properly installed.
Insert the wires and the connector into the electrical box knockout hole. From the inside of the box, slip the large connector nut over the wires and down to the connector. Turn it clockwise onto the threads of the connector. Once it is hand tight, put a flat head screwdriver against one of the nut lugs and give it a few taps with a hammer to finish tightening it.
Be sure that the "red hat" bushing is visible. Electrical code inspectors will want to see the rough wiring before it is finished and before the walls are closed up. They will inspect the method of running the wire and how it is terminated, and if the correct bushings are installed.
Insure that bond wire makes contact with the metal connector, grounding the box. Devices wired inside the metal box must be grounded to the box, not the bonding strip. Connect the device ground to the box with a green grounding screw and either a bare copper or green insulated wire of the correct gauge.
Steel armoured cable was often used in places where a non-metallic sheathed cable might be easily damaged. Today, armoured cable sees limited use since EMT (steel conduit) has largely replaced it where it was once required. Non-metallic sheathed cable (commonly called romex) has also taken a large share of the market. Be sure to check your local building codes before using armoured cable.