How to Install an Electrical Box in an Existing Wall

Installing an electrical box in an existing wall allows you to add new outlets, switches, and fixtures where you need them. While it takes some effort and know-how to cut into a finished wall, installing a retrofit electrical box is a manageable project for most DIYers. With the right materials and safety precautions, you can add an electrical box in virtually any wall surface.

What You Need to Install an Electrical Box in an Existing Wall

Before cutting into the wall, make sure you have all of the necessary materials on hand:

Electrical box – Choose a new work or old work box that fits the location and application. Plastic retrofit boxes with adjustable wings are common for residential wiring.
Wall plates – Purchase wall plates that properly fit the electrical box and devices.
Cables or conduit – Have the correct NM electrical cable or conduit to run the new circuit wiring.
Wire connectors – Make sure you have plenty of wire nuts to connect the wires.
Wire strippers – A pair of wire strippers will be needed to prep the wire ends.
Drywall saw – Use a drywall or jab saw to cut the hole for the electrical box.
Screwdriver – A basic screwdriver gets the box secured to the framing.
Electrical tape – Use it to wrap bare ground wires and outlet pigtails.
Utility knife – Scores drywall so the saw cuts it cleanly.
Stud finder – Locates wood or metal studs behind the wall.
Goggles and gloves – Wear protective eyewear and gloves for safety.

Other supplies like a voltage tester, fish tape, drywall screws, and wall anchors may also be needed depending on the specific project.

How to Choose the Right Electrical Box

Choosing the correct type of electrical box is the first step to a successful installation:

Metal or plastic – Both can work well. Plastic boxes are common for retrofit projects.
Cube taps – Good option for adding outlets in confined spaces.
New work vs. old work – New work has tabs for attaching to framing. Old work uses wings that clamp to the drywall.
Correct box volume – Make sure the cubic inch rating meets code for the number of wires entering the box.
Adjustable depth – Retrofit boxes allow you to set the depth to match the wall thickness.
Match wall material – Use metal boxes for masonry walls or exterior installations.

Selecting the proper box for the project and location takes a little planning. Using the wrong type of box can make for a difficult and unsafe installation. Take the time to choose a box designed for your specific application.

How to Locate Wall Studs

Installing the retrofit electrical box into a stud provides the most secure mounting option. Use a stud finder to locate studs:

Look for clues – Note any outlets, switches, or trim that may indicate stud locations.
Start high on the wall – Studs are often 16 inches apart. Measure over from known studs.
Run the stud finder – Move it slowly side-to-side at least three times marking stud edges.
Tap to validate – Use a knock test. Solid studs have a sharper thud sound versus hollow spots.
Double check – Confirm the exact stud center prior to cutting into the wall.

Locating a stud may not always be possible depending on the box location. Alternative mounting options using wall anchors are also available.

How to Turn Off the Power

An important early step is to turn off the electricity to the working area at the main breaker panel:

Shut it all off – Turn the main breaker to the OFF position.
Check for power – Test outlets in the area for loss of power before working.
Leave it off – Keep the main breaker turned OFF until the wiring is complete.
Post alerts – Use Caution tape and post warning signs so no one energizes the circuits.
Work safely – Wear protective eyewear and treat all wires as live until tested.

Accidentally contacting live circuits while cutting into walls can result in electrocution. Shutting off the main breaker and confirming power is off prevents tragic mishaps.

How to Cut the Wall for the Electrical Box

With power disconnected, outline the wall cutout for the electrical box:

Position the box – Hold it flat against the wall where needed.
Trace the outline – Run a pencil around the outer edges of the box.
Score drywall – Use a utility knife to cut along the outline.
Cut the opening – Carefully saw within the scored line. Make multiple passes, cutting deeper each time.
Trim to fit – Use the utility knife to trim and square the hole edges.

Go slowly and adjust the box position as needed while cutting the drywall. Be sure wires or blocking are not in the way.

How to Mount an Old Work Electrical Box

Old work or retrofit boxes make installing in finished walls much easier:

Fold the wings back – Rotate and bend the adjustable wings so they clear the drywall edges.
Insert in the opening – Turn the box diagonally and place into the cutout hole.
Tighten the wings – Turn the adjusting screws to extend the wings until the box is snug.
Secure with screws – Drive additional screws through wings into adjacent stud.
Check stability – Verify box cannot be moved easily and is properly supported.

Old work boxes use wings, clamps, or tabs to bite into the drywall. Tightening them down anchors the box firmly in position.

How to Mount a New Work Electrical Box

New construction electrical boxes attach differently:

Position on the stud – Hold the box flat on the exposed framing.
Trace outline – Mark a pencil line around the perimeter of the box.
Drill holes – Bore holes for the mounting screws in at least two corners.
Attach to stud – Drive screws through the pre-punched tabs into the framing.
Verify stability – Check that the box can’t twist or slide once mounted.

New work boxes only have a way to attach directly to studs or framing. Cutting the drywall precisely to the pencil line allows the box to sit flush.

How to Run and Connect New Circuit Wires

With the box installed securely, it’s time to run cable and make the wiring connections:

Route new cable – Run NM cable from the panel into the electrical box. Use conduit for masonry walls.
Prep wire ends – Carefully strip back the outer sheath and remove insulation from wire ends using wire strippers.
Make pigtails – Add 6 inch pigtails for the hot and neutral wires. Use wire nuts to join.
Connect grounds – Twisting bare copper wires together securely connects the ground wires.
Mount the device – Place the outlet, switch, or other device into the electrical box.
Secure with screws – Tighten device and wall plate screws to complete the installation.

Carefully organizing the wires and making solid electrical connections inside the box results in a safe wiring job.

How to Caulk and Patch Drywall

The last steps involve closing up the wall and cleaning the area:

Caulk gaps – Run a bead of caulk around the electrical box edge and cutout opening.
Patch holes – Use drywall compound to spread into any small gaps or cracks.
Let dry – Allow caulk and compound to fully cure per manufacturer directions.
Sand smooth – Lightly sand to blend patched areas with existing wall texture.
Clean up – Vacuum up any drywall dust and debris from the workspace.

Caulk and drywall compound give the installation a seamless finished look and prevent airflow around the electrical box.

Helpful Tips for Electrical Box Installation

Follow these tips and tricks for the best results:

Choose a low-profile box when installing between studs or above a baseboard heater.
Retrofit boxes often require adjusting the depth and tightening the wings as you work to get a snug fit.
Always turn off power at the breaker panel before cutting into walls to avoid contact with live wires.
Wear safety goggles, gloves, and long sleeves while cutting drywall to prevent irritations.
Take time to measure carefully so cutout holes are as small as possible.
New work metal boxes are sturdier for ceiling fans, garage door openers, and heavy fixtures.
Caulk minimizes air leakage for a better energy efficient and fire resistant installation.
Go slow while sawing and do not apply excessive pressure, allowing the tool to do the work.
Remove or relocate insulation that blocks access for mounting the electrical box.

Frequently Asked Questions

Some common questions about installing electrical boxes get asked:

How do I know if the box needs to be grounded?

The National Electrical Code requires all new electrical box installations to be grounded through either a grounded NM cable or a separate ground wire. This provides important protection from electric shock.

What size box do I need?

Select a box that meets NEC code requirements for the number of wires and devices it will contain. This is specified in terms of cubic inch capacity. For example, 14-gauge wire needs 2 cubic inches per conductor.

Can I install the new box right next to an existing outlet?

The NEC prohibits installing boxes back-to-back in the same stud bay. They must be at least 24 inches apart horizontally to avoid overcrowding.

Do I need a permit?

Many residential retrofit electrical projects like outlets and light switches typically do not require a permit. But always check with your local municipality.

How do I fish wires in a finished wall?

Use a drywall saw to cut access holes. Then drill small holes through top and bottom plates in the stud bay. Run a fish tape through to pull cables between access points.

What if there is no neutral in the switch box?

For smart switches needing a neutral, use 14/3 cable instead of 14/2 to add this neutral connection back to the electrical panel.

Conclusion

Adding a new electrical box in an existing wall allows you to upgrade wiring and expand capacity anywhere needed. While cutting into finished walls takes precision and care, the electrical box installation process is straightforward. With proper planning, safety precautions, and the right materials on hand, you can add outlets, switches, and fixtures even in older homes. Just be sure to take your time and double check for electricity before cutting into any wall. Follow basic wiring best practices like neat connections and secure mounts. Then patch the drywall smoothly for a seamless updated look. With the techniques outlined above, you can take on retrofitting electrical boxes like a pro.