Cable is installed through metal framing members in about the same way as it is in wooden ones. Either holes or slots are used and they must be cut, punched, or drilled at the factory or in the field. Normally, holes in metal members must have bushings or grommets inserted securely to prevent damage to the cable covering. If holes or slots have been formed so that no metal edge can cut or tear the cable insulation, bushings or grommets are not needed. Use good judgment when deciding whether or not grommets are required. A steel sleeve, plate, or clip must be installed for protection wherever a cable might be punctured or cut by a nail or screw. The first step in wiring a building after the distribution panel or panels have been installed is, in most cases, to mark the location of the outlet boxes. Then mount the boxes in place unless the mounting of a box might interfere with hole boring or cable anchoring. Once the boxes are in place, make the needed holes or notches for the installation of the cable. Install the cable by starting at the free end. Strip, as a minimum, 6 inches of the outside covering from the cable end. If the box has cable clamps, remove one of the knockouts and insert the stripped cable end through the hole under the clamp until one-fourth inch of the cable cover extends beyond the clamp. Tighten the clamp to hold the cable firmly in place, but be careful not to overtighten as this procedure
Figure 5-26. - Cable termination at box.
could cause a short. This procedure will give you conductor ends 6 inches long in the box, as shown in figure 5-26. You need these ends so you can make splices or attach devices or fixtures easily. Remember to anchor the cable within 12 inches of the box, as shown in figure 5-26. Boxes without cable clamps require a little different procedure. After stripping the cable end, remove one of the box knockouts. Then install a cable connector, similar to the ones in figure 5-27, in the knockout hole. Install the locknut so that it points inward to dig firmly into the metal box as shown in figure 5-27. Put the cable end through the connector to expose one-fourth inch of the outer cover, and then fasten it in place, as shown in figure 5-25. After the free end of the cable is in the box, work the cable back toward the other box. Run the cable along the framing members, fastening it at points no more than 4 1/2 feet apart. When you get to the other box, cut the cable to length, allowing for 6 inches of free conductor in the box. Strip the outside covering, insert the cable end, fasten it to the box, and anchor it within 12 inches of the box. To help avoid damage to the wires as wall coverings are being placed and other construction is being done, make sure you tuck back the wires into the outlet box.
Figure 5-27. - Nonmetallic cable box connectors and mounting techniques.Continue Reading