ELECTRICAL BOXES INTRODUCTION
Wherever wires are stripped of insulation and hooked onto terminal screws, such as on a receptacle or switch or joined to a fixture, or where they are joined for the purpose of rerouting the direction of the current, they must be housed in a metal or plastic box for safety. No raw wires should be left exposed, not only because of the possibility of fire from electrical malfunctions, but also to protect anyone working on the wires in the future.
The same applies outside: any switch, receptacle, or lamp must have its electrical mechanism protected by a box, in this case a weatherproof box.
Boxes are available in various shapes and sizes. The National Electric Code --which many municipalities follow -- puts restrictions on how many wires may be in a box of a given size, because crowding of wires can be a safety hazard.
All localities allow metal boxes, but some don't allow plastic. Boxes are for "new" work or new construction, where framing members are exposed, or "old" work, where framing is not exposed. Metal boxes are designed to be used on both old and new work, whereas plastic, except for one box noted below, is strictly for new work.
For new work inside the house, use boxes that have nailing brackets on them. Simply locate the box where you want it on the framing, and nail it in place to studs or other framing members. The brackets are notched or otherwise constructed so that they tie in well with wall or ceiling material -- they will bring the housed electrical device flush with the material. If you are installing a box in old work, first cut a hole the size of the box. Then slip the box into the hole. The plaster ears on it -- brackets at top and bottom -- stop it flush with the wall. To keep the box from falling out, it may have clamps that can be opened, somewhat like a Molly bolt, to grip the back side of the wall. Or there may be built-on screws, devices called Madison clips to grip it, or, if the wall is plaster, the plaster can be chipped away so that screws can be run through screw holes on the box and into the lath. Boxes with clamps or Madison clips can be used for any kind of installation -- plaster or drywall -- but drywall is not solid enough to accept screws.
All boxes have screw-hole tappings in front to accept 6x32 machine screws to secure switches and receptacles, and all have "knockouts, " which are holes plugged with either metal or plastic disks. To get the cable through on metal, pry out the disks with a screwdriver and feed the cable through. (If you make a mistake and take out the wrong disk, you can get toothed replacements to plug up the holes again.)
On plastic, the disks are pushed open and remain in place. "hinged" at the top and pushed up when the cable is passed through.
Cable or conduit (the other ways wires can come to a box) is secured to the box in a variety of ways, as detailed below. The main idea, though, is to prevent the cable from moving, which can lead to loose connections and electrical hazards. Indeed, unwanted movement is the bane of any electrical installation.
Weatherproof boxes are made of cast aluminum or an alloy and have either plates -- on alloy types -- inside with screw-hole tappings to screw switches or outlets to, or, on cast aluminum, built-in screw threads. Use stainless steel screws on aluminum boxes. Regular steel screws do not react well to aluminum and the reaction can lead to the screws' corroding. A dab of silicone paste on the screws helps too.
There are also covers for boxes. You can get metal or plastic covers with knockout slots for receptacles or switches, blank covers that screw on for a rarely used receptacle, or covers for junction boxes where wires terminate and are connected to one another.
Screw-on covers are made for weatherproof boxes. Sometimes the covers are solid -- when the box is not used for a long time -- and sometimes the covers have knockouts for switch toggles and receptacles and flip covers for them -- or you unscrew the entire cover to get at the receptacle. The covers are weatherproof not only in the material they're made of, but also in their gasketed edges, which provide a watertight seal between them and the box.
Sometimes, too, covers have 1/2-inch threads to accept fixtures (lampholders). These come prewired, ready for hookup inside the box.
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