Plastic boxes and flexible nonmetallic cable (commonly called Romex) put electrical wiring projects within the skill range of every dedicated DIYer. In this article, we'll show you some basics—how to position outlet and switch boxes and run electrical cable between them. We won't cover many other details. For help with circuit design and making connections to your main electrical panel, we recommend you consult a licensed electrician.
Besides standard hand tools, you'll need a special-purpose tool to cut and strip wire. We like the Klein No. 1412 ($18 at hardware stores and home centers). To drill a few holes, use a 3/4-in. spade bit in your electric drill. For larger jobs, rent a heavy-duty right angle drill ($25 per day) and equip it with a 3/4-in. x 6-in. auger bit ($7).
Electrical wiring mistakes can be deadly, so make sure you obtain a permit from your local building department and have the work inspected when you’re finished. Draw a sketch of your room that shows lighting, switch and outlet locations. Review your plan with the inspector and ask whether there are any special requirements.
First mark the box locations on the studs (Photo 1) using symbols to indicate outlets, switches and lights.
O// = Duplex receptacle;
S = Single switch;
S3 = Three-way switch;
O+ = Light fixture.
Mark the height from the floor to the center of the boxes (usually 48 in. for switches and 12 in. for outlets) or line them up with existing boxes. Then nail up the boxes so the face of the box will be flush with the face of the future wall covering (Photo 2). Most boxes have nibs or marks to help you align the box for use with standard 1/2-in. thick drywall. If you're not sure how thick the final wall material will be, use a special adjustable depth box. Paddle fans require a special box assembly that is rated to support the weight and stress of a spinning fan (see “Special Boxes,” at the end of this story).
Calculating Box Sizes
The electrical code limits how many wires you can safely put in an electrical box. To figure the minimum box size required by the National Electrical Code, add:
1 – for each hot and neutral wire entering the box
1 – for all the ground wires combined
1 – for all the cable clamps combined (if any)
2 – for each device (switch or outlet—but not light fixtures)
Multiply the total by 2 for 14-gauge wire and 2.25 for 12-gauge wire to get the minimum box size required in cubic inches. Plastic boxes have their volume stamped inside. Steel box capacities are listed in the electrical code.
With the boxes nailed up, you're ready to drill holes and pull cable. The holes and the cable running through them must be at least 1-1/4 in. from both faces of the stud to prevent nails and screws from hitting the cable (Photo 3).
This means drilling dead center on 2x4 walls. If you have to drill closer to the face of the stud, protect the area with a nail plate (Photo 6). When you drill floor or ceiling joists, drill toward the end rather than the middle of the span, and keep the holes centered on the width of the joist. You can easily pull two 14-2 or 12-2 cables or one 14-3 or 12-3 cable through a 3/4-in. hole. Drill more holes for additional cables.
When you drill through wall plates (the horizontal framing members on the top and bottom of the wall), keep the hole a few inches from the stud to avoid hitting nails.
Straighten about 12 ft. of cable and thread it through the holes from one box to the next. When you reach each new box, follow the stripping procedure shown in Photo 7, push the conductors and about 1/4 in. of sheathed cable into the box, and staple the cable (Photo 8). Then cut the end still connected to the coil and repeat the process at the other box.
When you're done drilling holes, pull the cable between the boxes and to the service panel to complete the circuit. Start by pulling about eight loops from the center of the coil and tossing them away from you. Then pull the 12-ft. length of cable back between your thumb and forefinger to remove the twist and straighten it out. The whole process takes only a few seconds and keeps the cable from twisting and kinking as you pull it through the holes (Photo 5).
Once you've pulled the cable through the holes, push it back a little to leave a small amount of slack. This is handy insurance in case you cut the cable a little short and need extra length, and it also allows other tradespeople a little slack to push your wire out of the way.
Photo 6 shows one method of getting the cable around a corner. But keep in mind that it's often faster and easier to drill up through the double top plate of the wall and route the cable up over the corner and down the other side.
Remove about 12 in. of the white outer plastic sheathing from the cable before you push it into the box (Photo 7). Use a stripping tool like ours for 12-2 and 14-2 cable, or an inexpensive ($2.50) sheathing stripper that works on all cables. You can also use a sharp knife to slit the sheath, but if you nick the insulation on the wires, cut the cable off at that point and try again.
Push the cable into the box so that at least 1/4 in. of sheathing is visible inside the box. The National Electrical Code requires that at least 3 in. of wire protrude beyond the face of the box, but we recommend at least 6 in.
Position outlets so that no point in any wall space is more than 6 ft. from an outlet without crossing a doorway. Install an outlet in every wall section that’s 2 ft. wide or wider.
- Add at least one 15-amp circuit for every 600 sq. ft. of building area. Twenty-amp circuits are required for kitchen, pantry, breakfast, dining room, laundry rooms and bathrooms.
- Add separate circuits for heavy power users such as room air conditioners and electric space heaters.
- When possible, install new circuits by running cable all the way to the service panel rather than connecting to existing circuits.
Punch a hole (or two for two cables) through the knockout area of the box with a screwdriver or the point on your stripping tool. Push the conductors and about 1/4 in. of sheathed cable into the box and staple the cable within 8 in. of the box. The cable must be at least 1-1/4 in. from the face of the framing. Push the first cable aside while you staple the second cable.
Next staple the cable in place. Position the staples in the following locations:
- Within 8 in. of boxes without cable clamps or within 12 in. of boxes with cable clamps. Most plastic boxes for two or more switches have built-in cable clamps.
- Every 4 ft. 6 in. along framing members like joists and studs. This is the maximum distance. Many electricians put them closer as needed.
- Within 12 in. of where a cable runs through a hole and continues along a framing member, like a ceiling joist.
Because cables must be kept at least 1-1/4 in. from the face of studs, you can't staple them side by side along a 2x4. The electrical code also prohibits you from placing more than one cable under standard 1/2-in. staples. Instead, weave the cables (Photo 8) or use special cable stackers.
For safe wire connections, cut all the wires to leave at least 6 in. protruding past the face of the box. Leave one bare copper ground wire an extra 6 in. long. Thread the long ground wire through the hole in the top of the special green wire connector and splice all the ground wires by holding them together and twisting the connector clockwise until it's tight. Strip the ends of the white and black wires and one end of each 6-in.-long pigtail and splice them with wire connectors. Cover the unstripped end of the black (hot) pigtail with a wire connector for safety.
Complete the rough-in phase of the wiring job by connecting the appropriate wires with wire connectors, adding short lengths of wire (pigtails) where they're needed and folding all the wires neatly into the boxes.
Photos 11 and 12 illustrate a number of important concepts and handy tips for making up any box. In addition, follow these guidelines:
- When the circuit continues through a box, connect the wires and add pigtails as shown in Photo 10. This method is easier and safer than connecting both sets of wires to the receptacle.
- Leave one bare ground wire long, and snip the rest to 6 in. beyond the box. Push the long wire through the hole in the top of the special green wire connector and connect the remaining wires to it by twisting the connector clockwise until it's tight. Fold the ground wires neatly into the back of the box (Photo 11).
- Snip all the white neutral wires to the same length (6 in. beyond the box). Strip 1/2 to 3/4 in. of insulation from each wire and join them with the right size connector. The instructions on the connector packaging will list both the length of stripped wire required and the maximum number and size of wires the connector can safely join.
- Identify the white wires you'll be using as hot wires (with switches) by wrapping them with black electrical tape.
- Organize the wires so you can understand them, and label them to avoid confusion. It's much easier now than when the cables are covered by drywall (Photo 11). For labels, slide short scraps of plastic sheathing over the wires.
- To keep track of three-way switch wires (two “travelers” and one common), twist the travelers together and wrap the common wire around them (Photo 11).
- It's good practice to cover the end of a hot wire with a wire connector. This helps to identify the wire later and allows you to safely energize the circuit if you need to (Photo 12).
- Use cable stackers when you don't have room to staple all the cables in the usual way.
- Fold and tuck all the wires neatly into the box so they won't be damaged during the drywall and taping process.
- Neatly fold the wires into the box in this order: grounds, neutrals and then hots. Before calling for a wiring rough-in inspection, look around to make sure you've installed enough cable staples and added nail plates where cables run too close to the face of a stud. Finally, to prevent air leakage, fill with expanding spray foam all of the holes through the top and bottom plates of the wall, and around exterior outlets and lights.
Protect the cable in 1-1/2 in. or thinner walls by running it through 1/2-in. metal conduit. Anchor a 4 x 4 x 1-1/2 in. metal junction box and attach a length of conduit to it with a conduit connector (you'll need two conduits for two cables). Secure another conduit connector to the top of the conduit to protect the cable from the sharp edge of the pipe. Secure the conduit with a conduit strap. Attach the ground wire to the metal box with a ground screw driven into the threaded hole. Finally, cover the box with a plaster ring that matches the wall covering thickness.
For an average-size room, you'll be able to buy all the electrical rough-in supplies you'll need at hardware stores or home centers for less than $100. Here's what to buy:
- Nonmetallic cable (NM)
For a room like ours with 15-amp circuits, buy a 250-ft. coil of 14-2 W/G Type NM-B ($15 to $25). You'll also need some three-conductor cable for the three-way switches.
- Plastic boxes
Plastic boxes are less expensive and easier to use than metal boxes. Buy single 18- or 20.3-cu.-in. nail-on boxes for receptacles and single switches, and double-, triple- or quadruple-switch boxes for multiple switches. Buy round boxes for light fixtures. Use round boxes with a bar hanger when the light fixture location is between studs or joists.
- Wire connectors (“ Wire-nuts”)
Buy a box each of red and yellow wire connectors. We recommend the type with wings. Buy green ground wire connectors if your inspector requires them. Read the instructions on the container to see how many wires each connector will hold and how much insulation to strip off the wire.
- Odds and ends
Buy a bag of fifty 1/2-in. staples. Pick up a half dozen metal nail plates (Photo 6) to protect vulnerable cables and a roll of black electrical tape to mark white wires (Photo 11).