Exercise extra care when wiring bathrooms because people often turn on lights and use electrical appliances with wet hands. To reduce chances of electric shock, all bathroom wiring must be grounded. The steps described here for wiring bathroom lights will include measures to ensure that the wiring is safe.
Initial discussions will focus on a new installation of lights and not a replacement of existing lights. Replacement of lights will be addressed at the end of the article. An assumption is made that there is accessible attic space above locations selected for the new lights.
To install a light fixture in the ceiling, place an outlet box with cable clamps at the desired location. With a stud finder, locate the rafters nearest that location and trace the fixture's outside shape with a soft-leaded pencil. Drill holes at the corners of your traced outline, large enough to insert the blade of a keyhole or a reciprocating saw. Using either saw, cut a hole in which to insert the outlet box. Repeat for each fixture.
Attach a hanger bar to an outlet box with screws for each light you plan to install, take them into the attic and locate the freshly cut holes. Adjust the hanger bar to a length that fits snugly between the two rafters. Position the hanger bar so the outlet box fits through the hole, its top edge flush with the bottom edge of the ceiling. Attach the two ends of the hanger bar to the rafters with nails or wood screws.
If the lights are heavy, use a 2-by-4-inch piece of lumber as a box hanger to make a sturdier mounting. Cut the 2-by-4 to fit between the rafters with the outlet box nailed or screwed to it. Then nail the 2-by-4 to the rafters.
Unless the ceiling lights have attached switches, they will need to have a wall switch installed. Standard height for wall switches is 48 inches from the floor. Place the front of your switch box against the wall where you want to put the switch. Trace around the box, drill holes at the corner of the outline and saw between the holes.
Ask a helper to stand at the hole for the switch box and tap on the wall. Go into the attic and move to a position directly above the tapping. Drill a hole through the top plate of the wall and push the end of 12-2 (for 12-gauge, two-wire) Romex cable, with a ground wire, through the hole into the void between the two walls. Keep feeding cable into the wall until your helper sees the end.
The helper needs to grab the cable end and pull it through the hole. Sometimes it is necessary to straighten a coat hanger and use its hook to catch the end of the cable. About one foot of cable needs to be pulled into the bathroom.
Punch out one of the knockouts in the outlet box and ask your helper to loosen the corresponding cable clamp. Cut the Romex cable to length with a pair of electrician pliers and push the loose end into the outlet box, leaving about 6 inches hanging into the room.
If additional outlet boxes were installed, connect them with 12-2 Romex cable in the same manner.
Go back into the bathroom, punch a knockout from one end of a switch box and loosen the corresponding cable clamp. Push the end of the Romex cable into the box and tighten the cable clamp. Fit the box into the wall with its ears against the outer surface of the wall. Insert the ends of a Madison hanger (drywall hanger) into the wall beside the switch box and bend the flaps into the box. Repeat the procedure on the other side of the box. The switch box should now be firmly attached to the wall.
Using a wire stripper or sharp pocket knife, peel the outer sheathing from the Romex cable hanging from the switch box. Remove the sheathing as far back into the box as possible. Remove about one-half inch of coating from each wire. Attach the black wire to one side of a single-pole switch, that has a ground, and the white wire to the other side. Attach the ground wire to the green screw on the switch. The ground wire will either be green or bare copper. Cut off any exposed ends of the peeled wire.
Push the switch into the box and fasten with the attached screws. Install a switch plate over the switch using the screws included in the package.
It is now time to determine the source of electrical power. Go to the breaker box and turn off a breaker. Check in the house to see what is on that circuit by turning on light switches and plugging a small appliance into the receptacles. Make a list of the items on this circuit. Repeat the process with every breaker or circuit.
Calculate the load on each circuit by adding separately the wattage of each light bulb and the amperes (amps) used by each item that is operated continually or occasionally from each receptacle. Each appliance shows the number of amps it uses on its nameplate. Calculate the total watts for each circuit and divide the total by 120 volts to obtain the amps. Add all of the amps together to derive the circuit load.
Major appliances such as refrigerators, freezers, air conditioners, heating systems and hot water heaters need to be on their own circuits and do not need to be included in this exercise.
Circuit loads should be no more than 80 per cent of the amp rating of the breaker. For instance, a circuit load for a 15-amp breaker should be no more than 12 amps, 16 amps for a 20-amp breaker and 24 amps for a 30-amp breaker.
Calculate the number of amps that your new light or lights will add to a circuit load and then determine if one of the existing circuits can carry the additional load. A new circuit my have to be added.
To use an existing circuit , the new light needs to be connected to a hot wire in the designated circuit. The easiest place to make this connection is usually an outlet box above an existing light fixture. Run 12-2 Romex cable from the outlet box, containing the cable from the switch, into the box where the connection to the existing circuit will be made. Do not make the connection at this time.
If a new circuit is to be installed, the cable needs to be run to the breaker box instead of connecting with an existing circuit. A new breaker will need to be installed to accommodate the new circuit.
Install the light fixtures by following the manufacturer's directions. Connect the black wire from the switch to the black wire coming from the existing circuit. Connect the white wire from the switch to the black wire from all new light fixtures. Connect all remaining white wires and twist all ground wires together. Use the appropriate-sized wire nuts to make the connections.
Connect all remaining light fixtures to the installed Romex cable by twisting black to black, white to white and ground to ground.
Turn off the breaker to the selected existing circuit and make sure you have turned off the right one. Connect the black wire from your new lights to the black wire in the existing circuit. Then connect white wires and ground wires. If there is not a ground wire available in the existing circuit, a bare copper wire will have to be run from the new lights to the breaker box and connected to the ground in the box.
After all connections have been made, turn on the breaker and make sure that it does not trip. If it trips, check all of your wiring connections to see if they were done correctly and that there are no exposed bare wires. Once the breaker is on, check to ensure that the newly installed lights are working. If everything works properly, your job is completed.
Bathroom lights are often wall-mounted. If this is the case, mount a switch box in the wall for each light. If the light fixture is large and heavy, it should be attached to studs with wood screws and not depend on the switch box to hold it. If new lights are to be installed where there are existing lights, cut off the appropriate breaker and take down the old fixtures. Connect wiring on the new fixtures in the same manner it was connected on the old lights. Make sure that the existing wiring is in good shape before connecting new fixtures and that a ground wire is used.
Always make sure your hands are dry and that you are not standing on wet ground or a wet floor when working with electrical wiring. When working with installed wiring, ensure that the power is off.