Electrical-box lineup. The plastic electrical boxes on the left mount ei- depth in the wall. Metal boxes are available in a variety of depths, and ther with built-in nail holders or a bracket that attaches to a stud. The the utility box (far right) has rounded corners for mounting in exposed plastic box with the face-mount bracket is automatically set at the right inside areas. Installing Electrical Boxes and Receptacles Keep boxes straight and splices tight for safe, trouble-free circuits by Rex Cauldwell ecky and Tyler were con- and so forth (photo above). The vinceB d that their house was haunt- main factor to consider when buy- ed. Calling me was their last resort. ing an electrical box is volume, or The demon in residence was caus- cubic inches of space inside the ing lights to go on and off mysteri- box. The National Electric Code ously, and several of those child- calls this space "cable fill" and protector inserts had actually specifies the necessary amount of melted in receptacles. A few hours cable fill based on the size of the before the couple called, sparks wires, the number of wires enter- had flown from an outlet in the ing the box and the type of device nursery; nothing was plugged into the box will hold. If a box is too it. If I couldn't help them, they small, the wires will be over- were moving out. crowded, resulting in broken, When I got there, I could feel shorted wires and possible dam- heat radiating from a number of age to the box's receptacle- outlets. In fact, one kitchen outlet Correct box volume is the law. Plastic boxes have their volume clear- holding threads. was so hot that it had deformed ly stamped on the outside, but a manufacturer's chart is needed to deter- Here are the NEC cable-fill re- the plug on a blender. It didn't take mine the volume of a metal box. Box volume indicates how many wires quirements for wiring a standard long to find the problem. Cheap can safely and legally be run into it. 120v receptacle. A typical cable for receptacles had been installed and wiring receptacles, such as 12/2 wired using the push-in connec- NM (nonmetallic) cable with tions in the back. These connections were be- Receptacle boxes: a question of volume— ground, has three conductors: black, white and ginning to fail, causing heat in the outlets and The first step in correctly installing a receptacle ground. Each conductor adds one volume unit the intermittent flow of current. I exorcised the is choosing the right box. The array of electrical (2.25 cu. in. for 12-ga. conductors), except for evil spirit by correctly wiring in new outlets in boxes can be bewildering even to an electri- the grounding conductors. Regardless of the the house, and my head didn't swivel around cian: metal vs. plastic, integral nail vs. bracket number of cables entering or leaving the box, even once. mount, square vs. rectangular, deep vs. shallow the grounding conductors together equal one volume unit for the total of the box. Add a vol- ume unit for cable clamps or support fittings in the box. Add two volume units for the recepta- cle. Add nothing for NM external connectors. In a typical situation of a 12/2 cable with ground entering and exiting the receptacle box, the two cables add up to 9 cu. in. Add another 2.25 cu. in. for the ground wires and 4.5 cu. in. for the receptacle itself. The total cable-fill re- quirement for this very basic receptacle instal- lation is 15.75 cu. in. for boxes without hold- down clamps and 18 cu. in. for boxes with hold-down clamps, which add 2.25 cu. in. A third cable entering the box brings the total to a minimum 20.25 cu. in., and a fourth cable puts the minimum volume requirement at 24.75 cu. A screw changes the mounting position. Veco's Adjust-A-Box mounts on the face of a stud in. for plastic. The largest single-gang box cur- but can be adjusted in or out to match any wall surface. rently available is around 23 cu. in., so legally there is no single-gang box that accommodates four 12/2 cables with a receptacle without be- ing in violation of cable-fill requirements. In this situation I usually install a larger 4-in. square box with a -in. cover plate, also called a mud ring. After you've figured out how much volume you'll need, take a look at a manufacturer's chart to figure out which size box matches or exceeds those requirements. If you go with a nonmetallic box, the size is usually stamped di- rectly on the box (bottom photo, facing page). Nonmetallic boxes are inexpensive and easy to install—Nonmetallic boxes have be- come the most commonly used boxes for resi- dential applications. They're easy and fast to in- stall and usually cost less than metal boxes. Metal boxes can be ganged together. Interlocking notches and flanges allow metal boxes to be joined to increase the volume of the box or to accommodate multiple receptacles. Nonmetallic boxes are also nonconductive, made of PVC plastic, fiberglass and thermoset. Nonmetallic rough-in boxes are more fragile than their metal cousins, so these boxes are to be used inside walls and not in exposed areas where they can be bumped and broken. There are many different varieties of non- metallic boxes, but the most commonly used for receptacles is the single-gang integral-nail box. These boxes have nail holders molded into the top and bottom of each box, and usually come with nails already in the holders. Installa- tion of these boxes seems simple enough—just nail them to the stud. However, other consider- ations can make installing integral-nail boxes a little tricky. First, the box has to project out from the stud so that it's flush with the finished wall. If the box sticks out too far, the receptacle wall plate won't lie flat against the wall. If the box doesn't stick out far enough, spacers have to be added un- der the drywall ears on the receptacle, or a box extension has to be installed to make the box flush with the finished wall. Cut-in boxes go into existing walls. Old-work, or cut-in, boxes let you add a receptacle with- Another problem is keeping the front of the out tearing out the wall. The wallboard is sandwiched between plaster ears on the front of the box box parallel to the front of the stud while it's be- and a bracket on the back. Better receptacles have wraparound yokes. The heavy-duty re- 15-amp vs. 20-amp receptacles. The 20-amp receptacle on the left has ceptacle (left) has a metal yoke for support. The inexpensive receptacle a horizontal slot for the neutral side of the outlet to accommodate the (right) has no metal yoke, and the plastic housing has broken. special plugs on appliances that draw a lot of power. ing nailed and at the same time making sure the Because the metal yoke on a receptacle is at- In addition, adapters are available to make box- nail doesn't bend or deflect, which can break or tached to the box, metal boxes need to be es with drywall ears work like cut-in boxes. But distort the box. All of these problems are solved grounded. The best way to ground the box is by take care if installing a cut-in box in a wall with a by choosing a box with a side-mount bracket adding a pigtail or a short length of ground wire receptacle directly on the other side of the wall that nails or snaps onto the front of the stud, au- to the ground-wire splice inside the box. The cavity. The new box needs to be offset a bit be- tomatically positioning the box parallel to the other end of the pigtail is attached to the box cause the boxes won't fit back to back. stud at the proper depth. with a green grounding machine screw in the The problem with all cut-in single-gang boxes Nonmetallic boxes don't vary much between back of the box. These green screws cannot be is again volume. The common sizes are usually manufacturers. However, Veco (Veco Products used for any purpose other than grounding. A no larger than 18 cu. in., which severely limits Inc., P. O. Box 692, Lyle, Wash. 98635; 800-688- common but illegal trick is wrapping the ground the number of cables the box can hold. Howev- 0890) makes the Adjust-A-Box, which attaches to wire around the screw that attaches the box to er, if you have a single cable running to a box in the stud with a bracket and can be adjusted eas- the wall. Another option for metal boxes is a a narrow wall cavity, the 10-cu. in. and 12-cu. in. ily to any finished wall depth (top photo, p. 59). grounding clip that retains the grounding wire boxes are ideal because they are shallow. If the homeowner decides to change a wall from and slides onto the edge of the metal box, but panelboard to tile, a turn of a screw moves the grounding clips don't stay put unless a mud ring Better receptacles are worth the extra box in or out flush to the new finished wall.