Some municipalities require either armored cable or conduit rather than NM. Whether it's required by a local code or you just like the added safety, armored cable is a practical choice for many homeowners—especially wherever cable is exposed. Plus, it's easy to install.
Armored cable looks just as it sounds. The coiled metal sheathing that wraps armored cable protects its wires from puncture by nails, unless a nail hits it dead-center. (Even conduit cannot offer absolute protection against a direct hit.) You may want to run armored cable behind moldings where it comes near nails. Armored cable costs more than NM, takes longer to strip and clamp, and can't make tight turns. With some practice you can install armored cable nearly as quickly as NM cable.
BX cable has no ground wire, is common in older homes, and is still available in some areas. Local code may limit use of BX to no more than 6 feet; then ground wire must be used. MC cable has a green-insulated ground wire, used like the bare ground wire in NM cable.
Bend the cable about 10 inches from its end and squeeze with your hand until the coils of the armor come apart. If you can't do this by hand, use pliers or employ another cutting method.
Firmly grip the cable on each side of the cut and twist until the split-apart armor coil pops out, away from the wires. Use two pairs of pliers if you can't do this by hand.
Using side cutters, cut the exposed coil of sheathing. You may have to grab the coil with the side cutters and work it back and forth to open and make the cut.
If you are cutting a piece to length, slide back the sheathing and cut through the wires. Otherwise slide the waste piece off and throw it away.
Cut off any sharp points of sheathing using side cutters. Remove the paper wrapping and any thin plastic strips. If the cable is BX it has a thin metal bonding strip. Cut it to about 2 inches.
If you don't have the right tools for the above method, you can cut the cable with a hacksaw or a armored-cable cutter. For a hacksaw, pictured, cut through just one of the coils and no more, so you won't damage any wires. Then twist and pull off the waste piece. On the other hand, an armored-cable cutter cuts at just the right depth. Adjust it for the size of cable, slip in the cable, and turn the crank.