How to Fish Electric Wire Through a Wall
Pulling a new conduit through a stud partition wall is relatively simple with the right tools and techniques. Here is how to fish electric wire through a wall.
Both residential and commercial renovation projects commonly require installing or relocating electrical outlets inside existing walls. With the right tools and techniques, running the new conduit through an existing stud partition wall, often referred to as “fishing,” is relatively simple.
The process described here for how to fish electric wire through a wall is for a typical commercial application, where a suspended acoustic ceiling allows access to the top of the partition wall.
PROBLEM: Fishing electric wire through an existing wall
Solution: Once you select the new outlet location, verify the depth of the studs, the thickness of the drywall, and where the studs are located. If you want to fish electric wire through a wall, the goal is to hit the space between the studs and avoid any potential conflicts with obstacles within the stud cavity.
Plumbing pipes, wood blocking and horizontal electrical conduit (feeding a nearby wall switch, for example) can make fishing a new conduit difficult or impossible. Horizontal electrical feeds are much more common in residential work. Commercial electrical rough-in is usually fed vertically down the walls from the ceiling plenum.
Check the building code for proper outlet mounting heights, which vary based on the outlet’s intended use. When the new outlet location is determined and marked, hold the outlet box against the drywall with a short level on the top. Trace a mark around the box, once it sits level (1).
Using a jab saw, cut out and remove the drywall. The box should fit snugly if you cut just outside the line you traced (2). Test fit the outlet and mark the drywall where the screw plates are mounted on the top and bottom of the box. Cut small notch “ears” so the box fits flush with the face of the wall (3).
Next, insert a flexible conduit into the stud cavity from above. Photo 4 shows a flex conduit, the connector with set screw, and the locking ring used to attach the connector to the outlet box.
Locate the center of the outlet box on the top of the wall, and using a hole saw, cut a hole through the top track large enough to fit the conduit connecter and set screw through it.
A common diameter for the hole saw bit in this application is 15⁄8 inch (5). Insert the flex conduit, with the connector attached, into the stud cavity from above, while a helper locates the end of the conduit and pulls it though the open outlet hole (6).
The next step takes a little practice to do quickly. Insert the connector through the round knockout in the top of the outlet box, and insert a flat screwdriver into the threaded portion of the connector to hold it in place.
While holding the box, with the connector pinned by the screwdriver, slip the box into the hole in the drywall (7). Thread the locking ring onto the connector. Once it catches, remove the screwdriver and snug the locking ring over the connector until tight.
After the conduit has been attached, the outlet box is secured using a pair of flat, stamped straps, known as Madison straps (8).
Insert the long end of the strap downward, at an angle between the box and the drywall (9).
Once the short (upper) end of the strap can clear the drywall, push the strap into the gap and up, until the two short legs of the strap are horizontal and facing toward you. Repeat this step on the other side of the box, and then bend the four horizontal straps into the box using linemen’s pliers.
This bend is accomplished with a three-step motion. Photo (10)shows the first 45-degree outward bend on the end of the strap. Without letting go of the pliers, push the strap into the box (11). Finally, flatten out the first small bend, so the strap lays straight back into the box. Repeat for the remaining three straps.
With the new outlet box in place, and the conduit extending up through the top of the wall, you can complete the termination of the conduit to a junction box above the ceiling. The installation is ready to pull wires and install the new outlet, switch or other device.
If you are feeding ROMEX into the wall cavity, rather than flex conduit, it may be easier to attach a small weight to help pull the ROMEX down inside the stud space.
PROBLEM: The conduit dropped through the hole in the top track of the wall didn’t feed down to the open outlet box hole.
Solution: Assuming no physical obstructions are present within the wall, the most common cause of this complication is insulation within the wall. This is where the tool and the name “fish” comes from.
A typical steel fish (12) has a bent hook on the end. If insulation is inside the stud cavity, the steel fish can be inserted into the hole drilled in the top track, and down inside the wall. It usually takes a two-man team and a little effort to feed the fish down and out through the outlet box hole.
Once your fish is extended through the opening, wrap a length of wire around the end of the flex conduit, and secure it with electrical tape (13). With the fish down the wall and fed out through the box opening, attach the free end of the wire to the hook on the fish and secure with tape (14). Slowly pull the fish and conduit up the wall until the conduit comes through the top. Once the conduit is through the wall, you can complete the installation.
Just as insulation within the wall makes fishing down the new conduit more difficult, a drywall ceiling also complicates things. A drywall ceiling will require the removal and replacement of portions of the ceiling to allow access to the top of the partition and to complete the overall electric rough-in and wiring.
In some cases, it is more effective to cut out larger sections of drywall and find alternative routes to circuit new devices, instead of fishing conduit down the wall.