Debugging Electrical Light Switch Wiring for
New Ceiling Fan Installation
By Mark J. Donovan
QUESTION: Mark, I'm getting
ready to install a ceiling light and I want to run power off an existing light
switch. The dilemma I have is I am not sure the cable in my attic is supplying
power to the light switch or actually coming from the switch. Do you have any
advice on figuring out the electrical wiring of a light switch?
ANSWER: AD, figuring out the electrical wiring of a light switch is
similar to piecing together a puzzle. This said, I would first suggest tracing
the electrical cable in your attic to see where it goes. If one end is dropping down into a
wall that is in the general area of where the main circuit panel is below, then
chances are it is a power feed from the panel.
Next, check the other end of the
electrical cable to see where it drops down. If it appears to be dropping down into a
wall bay area that is in line with the switch, then chances are this is the
feed cable to the switch.
If on the other hand one end of
the attic cable appears to drop down into a wall bay area that is in line
with the switch and the other end drops down into a wall bay area that is
associated with a switched outlet or light(s) already associated with the
switch, then chances are this is a control feed from the switch that
supplies power to the switched outlet and/or other lights.
Two Lights Operated off 1 Switch
Figure by Mark Donovan
Once you have a reasonable opinion of
what the attic electrical cable is doing you can confirm your assessment at the
switch and outlet boxes. However, extreme caution has to be exercised when doing
this. Electricity can kill so use the right tools and shut off power at the main
circuit panel whenever working in an electrical box.
Figuring Out
Electrical Wiring of Light Switch
To confirm your assessment start by
removing the switch cover plate. With the switch in the off position, use a
multimeter to carefully determine what side of the switch is powered and
un-powered.
While holding the multimeter leads
between Ground and the powered side of the switch, have someone turn off power
to the switch at the main circuit breaker box and confirm that the multimeter
shows a loss of power to the switch. For safety, check both sides of the switch.
Then flip the switch to the on position and check again to make sure power is
indeed off.
Next, with the power still shut off to the switch and controlled lights and
outlet, remove the switch from the box and disconnect the cables. Separate the
power feed cable from the switch controlled cable. Also make sure all the wires
associated with each cable do not touch each other.
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Now go over to the closest outlet or
light (relative to the switch) and check for voltage across the two contacts, or
three contacts if there is a ground contact. You should not see any voltage. If
the switch is feeding a light you will need to remove the light to expose the
wires. Again, you should not see any voltage.
Next, go back to the switch and twist together the black and white wires
associated with the cable that feeds the outlets and switch.
Now set the multimeter for an Ohm reading. Again, go back to the outlet or light
electrical box and see if you measure a short circuit between the two main
contacts associated with the outlet. You should, as you twisted the black and
white wires back at the switch. Go back and untwist the wires and then recheck
the outlet or light fixture to confirm that the short has been removed.
Once you have figured out the electrical wiring of the light switch, label each
of the cables in the switch box with a marker and tape.
Wiring Ceiling
Light Fixture
To wire up the new ceiling light on
the switched circuit, run a new cable between one of the other existing outlets
and the new ceiling light electrical box. Make sure you use the same gauge cable
wire for the new electrical box. Tie the corresponding black and white wires
together in the existing outlet or light box. Also tie the bare copper ground
wires together. Finally, attach the black wire at the new electrical box to the
black wire on the ceiling light. Then attach the white wire at the new
electrical box to the white wire on the ceiling light. Finally connect the light
fixture ground to the ground wire and fasten the ceiling light to the electrical
box.
Turn power back on to the switch at the main circuit and test the switch and new
light fixture.
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- Figuring Out Electrical
Wiring of a Light Switch -
