How to Install Electrical Wiring

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What You'll Need
Neon tester
2-wire cable
3-wire cable
Switch boxes
Outlet boxes
Cable connectors
Wire clamps
Electrical tape
Side cutter pliers
Ripping bar
1/4-inch drill
Extra long bit
Stud finder
Drywall or keyhole saw
Hand or circular saw
Madison hangers
Cable protector plates
Drywall tape
Spackling compound

Plenty of home improvement and remodeling projects require either new wiring or replacements for old, worn, or fried connections. For extensive work, it is obviously best to hire an electrician, but some projects might be within the skill level of the average DIYer. Read this article for advice before taking on your next wiring job.

Basics of Wiring

When working with wiring, always cut the power supply at the breaker box first before beginning any actual work. Then, test the local area with a neon tester to make sure that the power is off and that it is safe to go ahead.

Most wiring in a home is either 12-gauge or 14-gauge. Typically, when attaching existing wiring to a device or fixture, you will follow the color code by matching each color wire with its corresponding twin. Light switches, for example, operate on the black wire and will connect directly to another black wire already in the system.

Electrical wiring in the US follows the same basic color codes: red and black wires denote live wires, white wires serve as the ground, and blue, yellow, or other colors are used for switches or other specific purposes.

Connect to the Outlet

Usually, the wiring which you are installing will be affixed to the final outlet in the current run of cable. This can be determined by finding the outlet with wires connected only to two of the four terminal screws. Make sure the current is shut off to the circuit and then remove the faceplates to each outlet to find the one you’re looking for.

Attaching Cable for New Wiring

Loosen the screws holding the receptacle in place and remove it in order to add wiring. Take care to make sure you attach the correct size of cable. If 12-gauge is being used, continue with 12-gauge cable; the same applies to 14-gauge. White wire will be attached to the chrome terminal on the receptacle while black wire will be attached to the brass one. The ground wire will be attached to the box itself, but only if it is made of metal.

Adding New Wiring From a Junction Box

New wiring can also be tied into a junction box, so long as there is excess capacity. Once the power is shut off, carefully trace the cables connecting to the box to be sure you’re not connecting a 240v supply to a 120v outlet, or vice versa.

To tie in the new wiring, first locate the main supply wire by tracing the white wires, as all of these will be attached to the white wire on the supply line. Take out the unused plug and run the new wiring from the box. Clamp the cable to secure it to the junction box.

Tying in New Wiring at a Ceiling Light

If your light fixture isn't controlled by a switch, you can tie new wiring in order to add it. Make sure the power supply is shut off and then connect the wires according to the color code: white to white and black to black. Connect the ground wires together and attach them to the metal box and the light.

Always Match Connectors to Type of Cable Used

Depending upon the type of box you have purchased, it may or may not come with built-in connectors. There are two main types of cable connectors you will have to differentiate between when making a new connection. Armored types have inner rims to hold fiber bushings at the end of the cable, and nonmetallic kinds have a two-screw clamp that will fasten the installation around the cable. You must always leave at least six inches of wiring inside the box so that you have enough slack to make a connection.

Make All Connections in Approved Boxeswiring diagram

Use only boxes that are approved for your electrical application and never use an open-line splice when adding wiring. Properly position all boxes so that they’re always accessible.

Running New Cable Between Multiple Floors

Drill a hole wide enough to accommodate the hardware from top to bottom through the floor in a recessed area behind the wall. Then thread a cable through the newly drilled hole using a wire with a hooked end and a string with a weight attached, as is shown in the diagram to the right.

Adding New Wiring From Ceiling Boxes

Homes with attics may be more easily wired by using ceiling boxes. This method uses gravity to make the job of adding wiring easier. Start by attaching your cable to the box as previously described. Then, saw a hole at the desired position and install the cable box. Run new cable by adding more holes in places such as the 2x4 board plate and ceiling material.

Adding New Wiring on the Same Wallwiring in wall diagram

Cable can be run between existing and newly installed outlets by running it through the wall. Find the spot (not on a stud) for the new outlet and mark an approximate location. Then, using a stud finder, locate and mark the studs in the wall. Start your marks with the wall stud before the existing outlet and finish with the stud on the opposite side of the new outlet location, as shown.

Now it’s time to mark the exact dimensions of your new outlet. Make sure that it is level with the old one before you use a keyhole or drywall saw to cut the opening in the wall.

With the same drywall saw, cut a three inch strip of drywall starting at the middle of your first marked stud and ending in the center of the last. Switch to a hand or circular saw and made two cuts, about an inch apart and 3/4-inch deep in the exposed studs. Take a hammer and chisel to the wood between the cuts to remove it.

Once you have tested the existing outlet to guarantee that the power is off, take off the cover of the existing outlet box. Add a wire through a knockout, threaded end first, and tighten the clamp if there is one. If there’s not, place a new wire clamp on the cable and tighten the screw to hold it in place. Take the nut off the clamp to feed it through the knockout in the box, then replace the nut and tighten. Attach the wires once the cable is secure and then screw the receptacle back in and put the face plate back on.

Move to the new box and take away one of its knockouts. Self-clamping boxes need only be installed and tightened. Otherwise, use a Madison hanger on each side. Run the wire from the existing outlet behind the wall, using the notches, and up into the new box. Clamp the wire and replace the receptacle into the wall.

Finish reassembling all of the hardware before turning the power back on and testing the current to be sure the wiring is working properly. If everything checks out, it’s time to clean up. Attach cable protectors over the notches in the wall and put the strip of drywall you cut earlier back into place, repairing the damage with spackling and drywall tape.

Information in this article has been furnished by the National Retail Hardware Association (NRHA) and associated contributors.