Stripping and Clamping NM-Nonmetallic Cable

Intro

Nonmetallic (NM) cable is easy to work with and inexpensive, so it's not surprising that it is the most common type of cable used in household wiring.

NM cable is sold in lengths of 25, 50, or 100 feet, or more. When in doubt buy the larger package -- it doesn't cost much more and it may come in handy later.

NM's plastic sheathing does not protect the wires much, so keep it out of harm's way. If the cable might get wet, install UF (underground feed) cable, which encases wires in molded plastic. Wherever cable is exposed -- in a garage or basement -- many local codes call for armored cable or conduit.

Codes call for running NM through the center of studs so drywall nails cannot damage it. If the cable is 1-1/4 inches or less from the edge of a framing member, install a protective nailing plate. Some codes require metal plates even if the cable is in the center of a stud.

Take care not to damage wire insulation when working with NM cable. Slit the sheathing down the middle using a sharp utility knife. To avoid slicing the wire insulation, don't cut too deep. Or use a sheathing stripper.

When cutting cable to length, leave yourself an extra foot or two. If you make a mistake while stripping, you can recut the cable and try again.

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Step 1

Lay the cable on a flat work surface such as a small sheet of plywood. Starting 8-10 inches from the end, insert the tip of a utility knife blade into the center of the cable, pushing just hard enough to cut through the sheathing.

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Step 2

Slice the sheathing, exerting even pressure. You feel the tip of the knife rubbing against the bare ground wire as you slice. With practice you can cut evenly and quickly without damaging wire insulation.

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Step 3

Pull back the plastic sheathing, as well as the paper that wraps the wires, exposing 8-12 inches of wire. Snip the sheathing and paper with side cutters. If you use a utility knife, cut away from the wires to avoid cutting or nicking the insulation.

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Step 4

Insert the wires into the box. With this type of plastic box, push the wires through a hole, which has a tab that grabs the cable. Check that about 1/2 inch of sheathing is visible inside the box. Other types of boxes use other clamping methods.

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Other Clamping Methods

Remove a knockout slug from a metal box and fasten an NM clamp in the hole. Insert the clamp and tighten the locknut. Insert the cable through the clamp; tighten the screws.

Remove the locknut from the cable clamp and fasten the clamp to the cable. Then insert the cable with clamp body into the box, slide on the locknut, and tighten it.

Some boxes have built-in clamps. Slide the cable behind the clamp and tighten the screw.

Step 5

Wherever possible, staple NM cable firmly to a framing member, out of reach of nails. Staple cable within 8 inches of the box and every 2-4 feet along the run of the cable. Check your local building codes.

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What If You Have Three-Wire Cable?

NM cable that holds three wires (plus the ground) is round rather than flat. If you cut through the sheathing too deeply, you hit insulated wire rather than the ground wire. Practice cutting through the sheathing. Always examine the wires for damage after removing the sheathing.

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