How to Fish Electrical NM Cable 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. What can be much more challenging, though, is routing the cables through finished walls. It is easy enough to run cables through unfinished basements or attics, but the finished walls are another matter.

During major remodeling projects, you can do this by removing the drywall and running cables through studs and ceiling cavities, but removing drywall is not practical (or desirable) in many situations. It is a messy, expensive process that is best avoided if you can. There is also the option of running surface raceway wiring (such as Wiremold products) on the surfaces of the walls, but that can look tacky and look unprofessional—and it may not be allowed by the electrical code in some areas. 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 process used when old wiring is replaced with new cable.

There are any number of 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. Learn how professional electricians do this work.

Tools and Supplies You Will Need 

• Stud finder
• Old-work electrical boxes
• Drywall saw or jigsaw
• Drill with flexible shaft and 1-inch spade or auger bit
• NM cable
• Electrician’s fish tape
• Electrician’s combination tool or wire stripper
• Electrical tape

Plan Your Route 

When adding or replacing wiring in finished walls, most electricians will attempt to make the horizontal cable runs in the unfinished attic or basement/crawlspace areas, looping the cable vertically into the wall cavities at each electrical box location. This much different than how a home is wired during new construction, when horizontal cable runs are installed directly through studs from one location to the next. But in existing construction, the looping method prevents the expensive and time-consuming process of opening up walls and patching them after the wires are runWhen extending a circuit, for example, the electrician may run a vertical length of wire up to the attic or down into the basement, across to a spot directly above or below the new box location, then into that wall cavity to the new electrical box. For a DIYer doing this work, the most difficult part is figuring out a way to punch through the wall plates at the top and bottom of the wall in order to fish cable into the basement or attic.

Tip : If the job involves simply extending a circuit—such as when adding an additional outlet location in 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.

Cutting Box Openings 

With the cable routes and box locations planned, the next step is to cut the openings for the electrical boxes in the drywall.

Start by marking the box locations on the walls. When installing in finished walls, electrical boxes should be located between studs, and 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 electrical boxes on the wall. Make sure the location 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 Hole in the Wall Plate 

Once the hole is cut in the wall, insert a spade bit or auger bit into a flexible bit extension mounted in a drill, then insert the drill bit through the opening and center the extension in the stud cavity so the bit is pressed firmly against the floor or ceiling plate. Apply firm pressure as you drill through the plate and into the basement, crawlspace, or attic space. With any luck, the drill bit should penetrate into the open basement 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.

Tip: If you don’t have a flexible drill shaft, you may be able to bore holes through the wall plate from the attic or basement space, by drilling up or down using a spade or auger bit. 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.

When you find the hole, remove the wire, then uncoil the end of an electrician’s fish tape and insert it through the hole. Extend the blade of the fish tape 1 to 2 feet hrough the hole—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 up into the wall cavity.

Attach the Fish Tape to the Cable 

After the blade of the fish tape has been retrieved through the box opening in the wall, 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, then 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.

Note: When fishing cable to a switch location, you may run into fire blocks—horizontal lengths of framing that block the stud cavity. This may become evident only when you are attempting to run the fish tape through the cavity. In this case, you have no choice but to find another route, or to cut an access hole into the drywall in order to drill through the fire block to run the cable.

Complete the Cable Run

 Run the cable to its destination, making sure to use approved attachment methods—drilling holes through joists or stapling the cable where required. If the cable is being run laterally to another stud cavity where it will run vertically to the next box location, you will need to repeat the process by drilling another hole in the wall plate to fish the free end of the cable.Make sure that someone holds the opposite end of the cable securely while you fish it to the next location, so as to avoid pulling the cable into the wall where you can’t reach it.

Tip: Professional electricians often use a lube product to coat the cable as it’s being pulled through wall plates. The cable lube makes the cable slippery and reduces the likelihood of tearing the sheathing as it passes through the drilled hole. It also reduces the effort required to pull the cable.