How to Use Metal-clad (MC) Cable
When electrical wiring needs extra protection, metal-clad (MC) cable is a great solution, and is often required by electrical codes. MC cable is easier to work with than rigid conduit, and it too offers protection from fire, vibration, gnawing pests and physical harm in general.
What is MC Cable?
Metal-clad cable comes in several varieties, but the type you’ll find at most home centers is three insulated wires (two circuit conductors and a green equipment grounding conductor) protected by a flexible armor usually made from aluminum. MC cable is identified by the gauge of the wire, not the diameter of the armor. The most common sizes are 14 gauge, 12 gauge and 10 gauge.
Metal Boxes Only!
Plastic or fiberglass boxes aren’t designed to be used with MC cable. Even if you’re able to rig up a connection to a plastic box, you will fail the electrical inspection. Make sure the metal box has knockout holes located where you need them, and don’t rely on the spurs on the bracket to hold the box in place—add a couple screws as well.
Use FMC for Multiple Circuits
Flexible metal conduit (FMC) is commonly called “Greenfield.” The main difference between MC cable and FMC is that FMC doesn’t have the insulated wires pre installed; you have to pull them through instead. This requires more work but gives you the option of pulling, and protecting, more than one circuit in the same conduit. It also allows you to add wires in the future, something you can’t do with MC cable. FMC is identified by its diameter. The most common sizes are 1/2 in., 3/4 in. and 1 in.
Use 'Liquid-Tight' Outdoors
Install liquid-tight flexible conduit wherever the wiring will be subjected to wet conditions. Liquid-tight flexible conduit is water resistant and is available without the wires installed and in short lengths (whips) that come prewired. There are two common types: liquid-tight flexible metal conduit and liquid-tight flexible nonmetallic conduit. Ask your electrical inspector which is approved for your project.
Bend and Cut
If you just have one or two cuts to make and don’t want to invest in a cutting tool, bend the MC cable sharply until the armor pops open, and then use that opening to start the cut with a side/diagonal cutter. You only need to cut through one section in the armor. This method will leave a jagged edge that will need to be trimmed after the armor is separated. The cut ends of MC cable are sharp, so be sure to wear gloves.
A Rotary Cutter Works Best
Cutting the armor without damaging the wires is probably the trickiest part of working with MC cable. The best method is to use an armored cable rotary cutter. This tool uses a small cutting wheel powered by a hand crank and will cut only through the armor, leaving the wires undamaged every time. Apply firm pressure to the handle, but don’t squeeze too hard or the crank will be difficult to turn.
Install the Connector Into the Box First
You can attach a connector to MC cable, then join it to the box, but it’s easier to mount the connector on the box before feeding the wires through. Connectors like the one shown here (Speed-Lock is one well-known brand) are popular with electricians because the one screw secures both the connector to the box and the MC cable to the connector. There should be a minimum of 6 to 8 in. of exposed wire inside the box.
Protect Conductors with Bushings
Plastic anti-short bushings protect the wires from being damaged by the sharp edge of the armor when the cable gets clamped down into a connector. And even if the connector you’re using has its own bushing, make sure to add a bushing like the one shown here. It’s a cheap and easy way to achieve an extra level of protection. Some manufacturers supply a bag of anti-short bushings when you buy the cable, or you can buy a pack for a couple bucks.
Save Time with a Whip
Fixture, appliance and air conditioner whips are short sections of cable or flexible conduit that have not only the wires installed, but the connectors as well. Whips cost a lot more per foot but are a huge time-saver because you don’t need to mess around with cutting the armor. Whips are usually available in 4-ft., 6-ft. and 8-ft. lengths.
Twist and Pull
When removing a section of armor to expose the wires, twist the short end of cable counterclockwise as you pull the cable apart. If it feels like it’s taking too much pressure to separate, double-check that the cut went all the way through the armor. Don’t pull too hard or the armor could separate farther down the line. That’s a problem because damaged armor shouldn’t be installed, and you’d have to start over.
Ground the Box
Whether it’s the first box in a line or the only one on an entire circuit, every metal box needs to be grounded. Grounding a box is as simple as connecting the incoming and outgoing ground wires to a pigtail, and then connecting the pigtail to the box with a grounding screw.
Screw the Straps
Single-hole straps make for easy and sturdy supports. Avoid connecting them with nails or you’ll risk smashing the cable with a hammer. Install a strap within 12 in. of each box. All subsequent straps need to be within 6 ft. of each other. When you’re routing cables, be sure to avoid sharp bends that could damage the cable sheath or the conductors.