How to Rough-In Electrical Wiring (page 2 of 3)

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Preparation and Getting Started

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.

Drill the Holes
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. This means drilling dead center on 2x4 walls. If you have to drill closer to the face of the stud, rotect the area with a nail plate.

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.

Pull the Cable
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.

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.

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. 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.

Planning Tips
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.

Staple the 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 or use special cable stackers.