How to Finish a Basement Bathroom – wiring the ceiling junction box, vanity light and GFCI wall outlet with wiring diagrams.
This project is continued from How to Finish a Basement Bathroom – Part 13.
Basement Bathroom Vanity Light and Switch
The unfinished basement bathroom did not have vanity light above the sink or light switch. A new vanity light bar and wall switch are installed and wired:
This picture was taken before I started the job – note the absence of the vanity light switch to the right of the GFCI outlet:
Basement Bathroom Vanity Light Electrical Wiring
The electrical supplies needed for this project are (from left to right):
- Light switch – I chose the square rocker style.
- Old work junction box for the light switch.
- Metal square junction box with cover and mounting bracket.
- NM cable connectors for 3/8″ to 1/2″ knockout (brown bag).
- Round old work junction box for the vanity light bar.
Shutoff the electricity at the circuit breaker panel or fuse box before working on the electrical wiring to prevent shock, burns, fire and/or death. If you’re not comfortable working with electricity, please hire a licensed electrician.
Before you begin:
- Identify your the electrical wiring configuration. Know which wires are connected back to the main electrical panel and which are branch circuit extensions.
- What is the existing load on the circuit you plan to use for the new line extension? If there are too many devices (loads) on the circuit, you may need to wire a new circuit and install a circuit breaker in the main electrical panel.
- Make a wiring diagram of the “current state” and “future state” (before and after) to plan and guide your work.
- Shutoff the power at the main electrical panel by switching Off the circuit breaker. Verify the circuit is dead with a voltage detector.
Basement Bathroom Light, Switch and GFCI Outlet Wiring Diagrams
This is the wiring diagram for the bathroom before I started work. I was pleased the GFCI outlet was on a dedicated circuit back to the main electrical panel with only one branch circuit to the second electrical outlet by the bathroom sink:
The new basement bathroom wiring diagram is illustrated below. The wiring additions to be made are:
- Install a steel junction box in the ceiling crawlspace to make the various new circuit connections.
- Remove the original NM-B 14/2 wiring to the GFCI wall outlet and install a new circuit from the ceiling junction box to the GFCI. It’s necessary to remove the original wiring because there wouldn’t be enough slack in the original wiring to make the new splice connections.
- Install an old work wall box for the light switch and install a new run of NM-B 14/2 wire.
- Install a round old work wall box for the vanity light and run a new circuit to the ceiling junction box.
I shutoff the electricity the circuit breaker and verified the power was Off with the non-contact voltage detector. I then disconnected the line side wires (wires from the circuit breaker panel) from the GFCI outlet. The line side wires enter the top of wall electrical box:
The line side wires (top) are disconnected from the GFCI wall outlet. The load side (bottom wires in wall box) feed the second electrical outlet located on the other side of the bathroom sink.
Yellow construction string is duct taped to the NM-B 14/2 line side electrical cable. The line side cable will be pulled out of the 2×4 wall from the ceiling crawlspace and cut to route power to the steel junction box in the ceiling crawlspace:
Install an Old Work Junction Box in the Drywall
Before pulling the line side wire into the ceiling crawlspace, I measured and sawed a hole in the drywall for the light switch junction box. This way I can limit the number of trips to the ceiling crawlspace by combining the work. A carpenter’s level is used to plumb the wall box then the box outline is traced in pencil on the wall:
The old work light switch box is traced on the wall. Cut the hole with either a drywall jab saw or Rotozip power saw. Be mindful that there could be electrical cable or plumbing behind the drywall. Best use the Electrical Scan function on a stud finder to check for concealed wiring and make shallow cuts until you know the area behind the wall is free of obstructions.
The drywall cutout for the light switch old work wall box. The Rotozip saw cuts drywall like butter:
Fishing NM-B 14/2 Electrical Cable in a 2×4 Wall
Important Wiring Note: the line side wires from the GFCI outlet are pulled up from the wall box and out of the newly sawn hole for the light switch wall box (see next photo). Just reach inside the drywall hole and grab the line side wires.
The reason for doing so is the new NM-B 14/2 cable run for the vanity light switch will be fastened and pulled with the old GFCI line side wires to the ceiling crawlspace. If you miss this step, you’ve created significant extra work for yourself because you’ll have to fish the light switch NM-B 14/2 cable separately.
The new run of NM 14/2 electrical cable for the vanity light switch is duct taped to the GFCI line side wires. The new cable is being pulled from a box roll (not shown):
Working in the ceiling crawlspace above the basement bathroom, here’s the GFCI outlet NM-B 14/2 cable that will be pulled from the bathroom along with the new cable for the vanity light switch:
After finding and removing a wire staple inside the wall above the GFCI junction box, I was able pull the new NM-B 14/2 cable for the vanity light switch into the crawlspace with the construction string. The old GFCI outlet line side wire is to the left of the duct taped joint:
Basement Bathroom Ceiling Junction Box Wiring
A metal junction box will be mounted to the ceiling joists to make the basement bathroom wiring connections. NM cable connectors will protect and secure the NM-B 14/2 cable from the sharp edges of the junction box knockout holes. The NM cable connectors are required by the National Electrical Code (NEC) and local Building Code.
Four knockouts are removed from the junction box – one for each of the new wire legs per the basement bathroom wiring diagram. NM connectors are installed and the junction box fastened to a ceiling joist with two wood screws:
Bathroom Ceiling Junction Box Wiring
The old line side cable that provides power from the circuit breaker panel is rerouted from the bathroom GFCI wall outlet to provide power to the ceiling junction box and basement bathroom branch circuits to the vanity light and wall electrical outlets by the sink.
The line side NM-B 14/2 cable cut to length, stripped and mounted to the ceiling junction box. Per NEC 314.17 and 300.14:
NEC 314.17 The outer jacket of NM cable shall extend into the box a minimum of ¼ inch.
NEC 300.14 The minimum length of conductors, including grounding conductors, at all
boxes shall be 6″. At least 3″, of every conductor, shall extend outside the box.
I like about 8 inches of wire inside the junction box because wires never grow longer and it’s easier working with longer wire leads. A new section of NM-B 14/2 cable is duct taped to the construction string so it can pulled down through the 2×4 wall to the bathroom GFCI outlet box:
The new run of NM-B 14/2 cable for the GFCI wall outlet pulled down through wall and out of the wall box. This is why the construction needed to be pulled up with the old line side wire from the GFCI outlet such that the new wire could be pulled back via the same pathway – thereby avoiding the tedious job of fishing a new cable from scratch. The new NM-B 14/2 cable for the bathroom vanity light switch is hanging out of the drywall hole on the right:
Next, a generous length of new NM-B 14/2 cable is pulled the roll in the ceiling crawlspace to reach the round old work electrical box for the vanity light above the bathroom sink.
The four NM-B 14/2 cables are labeled with blue felt tip pen, stripped and mounted to the ceiling junction box. About 8 inches of wire extends inside the junction box to make the wiring connections:
Following the basement bathroom wiring diagram, the ground wires are twisted and nutted together. I generally make the ground connections first for safety. I’m using nuts rated for four (4) to six (6) individual 14 gauge wires. I should’ve installed the green ground screw in the metal junction box with a bare copper ground wire pigtail at this step. Metal junction boxes must be grounded per the NEC. See a bit further down the page for the junction box ground screw wiring.
Per the wiring diagram, the black hot wires for the 1) line side, 2) GFCI outlet and 3) vanity light switch are twisted and nutted together (next photo).
You can see why labeling the wires with permanent marker is so important to keep track of everything. The white (neutral) wire from the vanity light switch is relabeled as hot by coloring the wire with a black permanent marker. You can also relabel a neutral white wire as as “hot” by wrapping it with a black electricians tape. Recoding a neutral wire for a light switch is a common application.
The black hot wire to the vanity light and white/neutral wire recoded as black for a hot wire for the vanity light switch are twisted together and secured with a wire nut:
The remaining neutral (white) wires are twisted together with pliers and secured with a wire nut. Remember, “righty-tighty and lefty-loosy” when twisting and nutting wires.
Tightening the wire nut on the neutral (white) wires:
Gently fold the wires into the metal junction box:
Metal Junction Box Ground Wiring
Almost forgot to install the #10-32 green ground screw and ground wire in the steel electrical junction box (see the red arrow). Metal junction boxes must be grounded per the National Electrical Code (NEC). The purpose of grounding a metal junction box is to prevent the metal box from becoming energized with electricity and a shock hazard should a wire come loose or damaged and contact the metal box.
The junction box ground wire is taken from a scrap section of NM-B 14/2 cable, looped around the #10 ground screw, then twisted and nutted to the other ground wires. The #10 green ground screw is not included with the junction box and available in a package of 10 for about $1.
The junction box steel lid is fastened with the two screws provided with the box:
The new wires are fastened within 12 inches of the junction box with insulated wire staples as required by the NEC. The staples should be hammered in just enough to prevent the wire from slipping, but not so tight that it crushes or damages the cable. Always staple cable flat:
Look back at the finished basement crawlspace above the basement bathroom drywall ceiling. I fastened a piece of plywood to the ceiling joists so I’d have a place to sit while wiring the new junction box:
Bathroom Sink: GFCI Electrical Outlet Wiring
With the crawlspace wiring complete, work moves back to the basement bathroom. The GFCI ground connection is made with a “pigtail” (short piece of wire) to the line- and load side ground wire then nutted together:
The line side hot and neutral wires are backwired to the GFCI outlet. I prefer backwiring when possible because it’s a much better connection. Backwiring means the outlet is designed with small holes for the wires to enter through back of the outlet that are captured by a screw-down clamp. I recommend backwiring only on quality outlets that use the screw and clamp system as shown here:
The vanity light and wall switch are wired in How to Finish a Basement Bathroom – Part 15.
Thanks for reading,