Bathroom exhaust fans with built-in light fixtures are fairly
common. If you have one of these, you may have wired the fan and
light to separate switches. Now, you’d like to operate both of
them from the same switch. The wires are already where you need
them to be, so you don’t have to run new ones. That detail greatly
simplifies the electrical procedure you’re about to undertake. If
the light and fan are separate fixtures, you’re going to have to
run a short length of wire through the attic to connect them.

Shut off the breaker controlling the light and fan circuit. This
is the single most important part of any home wiring procedure.
Even after switching off the breaker, it’s still a good idea to
use a voltage tester to test the wire connections before you work
on them.

In electrical work, a pigtail is a collection of wires spliced
together with a short length of wire. The pigtail allows you to
connect all the wires to a single terminal. The secret to making
an effective one is to twist the wires together clockwise – not
counterclockwise – before screwing on the cap. That way, the
clockwise threads of the cap make the connection tighter when you
screw it on.

Disconnect the two black wires from both switches. If you don’t
know which one supplies power to each switch, turn the power back
on and test the wires by connecting each to ground with your
voltage tester. The live circuit wires will give you a reading and
the others won’t. Turn the breaker back off when you know which is
which.

Make a pigtail with the two live wires and a short length of black
wire of the same gauge. Connect the short wire to the top terminal
of the switch you want to use. Do the same with the two other
wires, and connect them to the bottom terminal of the switch.

Unscrew the cap from the two white wires serving the switch you’re
no longer using, add a short length of white wire of the same
gauge and screw the cap back on to make a pigtail. You may need a
larger cap than the one that was on there. Splice the other end of
the short wire to the two white wires serving the new switch. If
you want, you can also simply splice all four white wires together
under the same cap, but you may find that difficult to do with
four wires.

Remove the ground from the out-of-service switch and the one from
the switch you’re using. Connect them by twisting them together or
using a crimp ring, then attach them to the green ground terminal
on the switch you’re using.

Locate the switch cable, which should be sticking through the top
or side of the box serving the fan or light. Using wire strippers,
strip an inch of insulation from the end of the black and white
wires. Splice the black wire from the fan together with the black
wire from the light and the black wire from the switch, and cap
them. Connect the white wires together in the same way, then
repeat with the ground wires. When you splice ground wires, you
don’t have to cap them, but it’s still a good idea to do so
because the cap helps keep them together. Mount the fan and light
and move to the switch box.

Connect the black wire from the fan to the bottom terminal of the
switch and the live circuit wire to the top terminal. Splice the
white wires together and cap them. Twist the ground wires together
and connect them to the green ground screw on the switch.