How to Run Electrical Wires in a Finished Wall
Adding or extending an electrical circuit is a job that seems intimidating to many DIYers, but in reality, the wire connections are rather easy if you have a basic understanding of electrical work. Fishing the wires through finished walls is the challenging part. It is easy enough to run cables through unfinished basements or attics, but running them inside finished walls is another matter.
During major remodeling projects, you can route the wires where you need them to be by removing the drywall and running cables through studs and ceiling cavities. In other situations, it is neither practical nor desirable to remove drywall—it is a messy, expensive process that is best avoided if you can.
But it is possible to add or extend circuits in finished walls without destroying the walls and without putting yourself through an enormous ordeal. This is also the same process used when old wiring is replaced with new cable during system upgrades.
There are several methods for running cable through finished walls, and the approach you take will depend on the circumstances and how extensive the work will be. Does it involve simply extending a circuit from an existing outlet to a new outlet location? Are you running an entirely new circuit from the main service panel to multiple locations? Or are you replacing an entire house full of knob-and-tube wiring with new NM cable? The approach an electrician takes will depend on the scope of the job, but all retrofit wiring jobs use similar techniques.
In the example described here, we are running a simple loop of wire from one new wall box location to another, such as you might do when extending a circuit. The presumption is that the cable run will be looped from one box location down or up into a basement, a crawlspace, or an attic and then across floor joists before entering the stud cavity where the next box is located.
Consider Code Issues
Remember to consult your local code on requirements for running cable. For example, in exposed locations such as open basements or attics, NM cable usually needs to be run through holes or notches cut in joists rather than stapled to the face of the joists. Thus, when looping a length of NM cable between new wall boxes, the process may involve running it through holes drilled in the floor joists in order to reach the stud cavity.
Also, make sure the wire gauge is appropriate to the amperage of the circuit. For standard 120-volt branch circuits, 12-gauge wire is used for 20-amp circuits and 14-gauge wire for 15-amp circuits. There are also new arc-fault code requirements: When extending a circuit, if the latest National Electric Code requires arc-fault protection, then you must put said protection in place to meet code requirements.
Before You Begin
When adding or replacing wiring in finished walls, most electricians will attempt to make the horizontal portion of the cable run in the unfinished attic, basement, or crawlspace areas, looping the cable down or up through the wall cavity, across the floor or ceiling joists, and then vertically through another stud cavity to the next wall box opening. For the horizontal portion of the cable run, this can involve drilling holes in the joists where the cable will pass.
This is much different than how a home is wired during new construction, when horizontal cable runs are installed directly through studs from one outlet location to the next before the wall surfaces are installed. But when you are running wire in existing construction, the looping method prevents the expensive and time-consuming process of opening up walls and patching them after the wires are run.
When extending a circuit, for example, the electrician may run the cable vertically from an existing outlet box, up to the attic or down into the basement, across joists to a spot directly above or below the new box location, and then into that wall cavity to the new electrical box opening. For a DIYer doing this work, the most difficult part is figuring out a way to punch through the wall plates at the top or bottom of the wall in order to fish cable into the basement or attic.
If the job involves simply extending a circuit—such as when adding an additional outlet location in a room—some electricians will remove baseboard molding, notch out the drywall in the area hidden by the baseboard, then drill access holes through the studs to fish cable from location to location. This is a fairly easy way to run cables from one box location to the next. When the baseboards are reinstalled, the holes will be covered—no need for patching.
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What You'll Need
Equipment / Tools
- Stud finder
- Drywall saw or jigsaw
- Wire coat hanger
- Drill with a flexible shaft and 1-inch spade or auger bit
- Electrician's fish tape
- Cable ripper
- Wire strippers
- NM cable
- Electrical tape
- Old-work electrical box
Cut Box Openings
With the cable routes and box locations planned, cut the openings for the electrical boxes in the drywall.
Start by marking the desired location for the box on the walls. When installing them in finished walls, electrical boxes should be located between studs. To do this, you must find the location of the wall studs, using a stud finder. Once the studs are located, outline the position of the old-work electrical boxes on the wall. Make sure the position of the new wall boxes is consistent with the location of other outlets in your house. Outlets, for example, are typically between 12 and 18 inches above the floor.
Cut out the box openings along the outline, using a drywall saw or jigsaw. It is important to make sure you are not cutting into existing electrical wires, plumbing pipes, or other mechanicals, so be careful as you start cutting to inspect what's inside the walls. The cutout should exactly fit the outline of the electrical box.
Drill Holes in the Wall Plate
Once the box holes are cut in the wall, you will need to drill access holes in the wall plate at the bottom or top of the stud cavity. These holes will be used to fish the cable down into the basement or crawlspace area or up into the attic in order to run the cable from box to box.
If the attic is your only access space for running cable, you may find it necessary to cut another small hole in the wall surface, high on the wall, in order to gain access for drilling through the wall's top plate. This hole can be patched once the wiring work is completed. There may also be fire blocks midway up the wall, in which case you'll need to cut a small hole extending above and below the block to drill through it and feed the new wire through the block.
Insert a spade bit or auger bit into a flexible bit extension mounted in a drill, then insert the drill bit through the wall opening and center the extension in the stud cavity so the bit is pressed firmly against the wall plate. Apply firm pressure as you drill through the plate and into the basement, crawlspace, or attic space. Drill slowly, keeping in mind that there may be gas lines, plumbing pipes, electrical lines, or other dangers in the joist cavity into which you are drilling. Stop immediately when you feel the drill bit penetrate the wall plate.
If you don't have a flexible drill shaft, you may be able to bore holes through the wall plate using a straight spade bit or auger bit while drilling from the attic or basement space. You will need to measure carefully, however, since the wall plate may not be visible through the sheathing on the floor or ceiling.
Locate the Drilled Hole
Although it's rarely a problem in an accessible basement or attic, if your home is built over a crawlspace or has a low, narrow attic, it can be hard to find the hole you've drilled in the wall plate.
Start by inserting a long wire through the hole you just drilled (a disassembled wire coat hanger works well). Enter the attic or crawlspace, and look for the wire extending through the drilled hole.
Thread the Fish Tape
When you find the hole in the wall plate, remove the wire, uncoil the end of an electrician's fish tape, and insert it through the drilled hole in the wall plate. Extend the blade of the fish tape until it is visible in the wall box opening—ideally, you want the end of the fish tape to extend out through the hole you cut in the drywall.
This, and subsequent steps, will be easiest if you have a helper at the other end to retrieve the fish tape through the wall opening as it extends into the wall cavity. A short length of clothes-hanger wire can sometimes make it easier to hook the fish tape and extract it through the wall opening.
Attach the Cable to the Fish Tape
After the blade of the fish tape has been retrieved through the wall opening, uncoil enough NM cable to complete the cable run you are planning. Make sure to allow about 2 feet of excess cable at each end. Stretch out the cable and untwist any kinks in it.
Strip about 6 inches of outer sheathing from one end of the cable, then hook the conducting wires and bare copper grounding wire through the loop at the end of the fish tape blade. Bend the wires over, and wrap several loops of electrical tape around the wires and the end of the fish tape. The goal is to have a smooth head that will easily slide through the hole in the wall plate without getting caught.
Fish the Cable
From the attic or basement, pull steadily on the fish tape while a helper feeds the cable into the wall opening. It may take some finessing as the tip of the fish tape blade passes through the drilled hole in the wall plate. Be gentle as you fish the cable through the hole since it is important not to tear the sheathing on the cable. It helps to pull in short, 2- to 3-foot intervals, so that the cable is being pulled at the same time your helper is feeding from the other end.
Complete the Cable Run
Run the free end of the cable to the next stud cavity location. Make sure to use approved attachment methods—drilling holes through joists or stapling the cable where required.
Strip the end of the sheathing, attach the wires to the fish tape, and tape them in place. Then, pull the cable through the wall plate and out the next wall box opening. Make sure that someone holds the opposite end of the cable securely while you fish it to the next location to avoid pulling the cable into the wall where you can't reach it.
You have now completed one cable run.